Do Spacecraft Really Have To Endure The Hazards of Reentry

[@baxtermullins1842]

There was one professor in the early days of Apollo that explained you could use rockets to overcome gravity to slowly deorbit but the amount of fuel was more than what it took to get to orbit. So, a heat shield was more efficient, less cost, less massive. His sketches showed multiple rockets pointing towards earth center and rockets slowing the spacecraft down to match earth’s rotation, then allowing the ship to slowly descend! Fuel requirement was enormous.

[@madigorfkgoogle9349]

or in clear text, the weight of the fuel needed for slow descent is way higher then the weight we are able to decelerate by this method. Or to put it in some illustrative numbers, to use a slowed down re-entry of 50t module you need 75t of fuel, which means the module cant weight 50t since it has the fuel on board that is adding weight that at the end needs more fuel to slow down the module. (the numbers are strictly illustration, not any mathematical/physical equation)

[@DanTheisen]

This is a better answer than the “you can’t” Scott has in the video. Tell us what it costs. Obviously heat shields will be around for as long as we have chemical rockets. Maybe in some fictional future we’ll have fuel that doesn’t weigh as much. Note I said Fictional.

[@molybdaen11]

What if we give them a giant Ballon full of hydrogen just before reentry?

[@armastat]

Just use an orbital Tug System. Its a space structure u dock at or connect to, that tug then de-orbits u by using its own propulsion to slow u down very quickly (so that you dont descend very far into atmosphere before coming to - say - mach 2), you then detach and descend on your own. (That solution I am not going into yet). The tug then accelerates itself back to orbit. You can argue the practicalities of that if you want, but you can't say its not possible. Its just an engineering problem not a physics problem.. Heck just look at a space elevator, no heat shields required there either.

[@armastat]

Incidentally the reverse is also true. build smaller launch systems to get you to a very low earth orbit where the tug picks u up and then lifts you to a much higher orbit. Heck it could take u all the way to the moon, refuel your spacecraft and then de-orbit at the moon and release u a kilometer up so your tiny spacecraft could get the rest of the way. it then speeds back up and returns to earth carrying ships on a return trip.

[@dannypipewrench533]

That reentry footage was terrific.

[@HansMilling]

What if the spacecraft is nuclear powered? Then you could break using less mass/fuel.

[@CheradenZakalwe]

​@@HansMillingexplain. Nuclear power is just creating something very hot and radioactive. How would that be used to slow down a spacecraft.

[@slome815]

@@CheradenZakalwe Nuclear rocket engines are a thing, And they are very efficient, at least when it comes to specific impulse, the few one tested (all on the ground), like NERVA archieved a specific impulse of more then 800s, thats double that of normal hydrogen rocket engine. Ofcourse they still use reaction mass (hydrogen), but the heat for expansion is provided by the nuclear reactor.

[@dufkers]

@@slome815did you know that the Chinese did experiment with using dense wood like oak as a heat shield and it did work. So, just to summarise some of the options for reentry: a nuclear engine, a lump of wood. I think the KISS principle favours the lump of wood.

[@glennbabic5954]

I like how it airbrakes once then skips off the atmosphere like a stone and then plunges back down

[@Zeecontainers]

That reentry video always gives me a strong sense of relief and appreciation for the safe embrace of earth. Even compared to ending up alone in the middle of the ocean, which is normally considered an exceedingly horrible and deadly situation, it's a warm, protective and comfortable bosom compared to space's sheer hostility to life.

[@viarnay]

And showed us that the starship is a. tough one

[@benjaminhanke79]

You make watching KZbin more efficient by presenting two videos at the same time.

[@privacyvalued4134]

And then speed up the video playback to 2x to get a 4x overall efficiency improvement.

[@teyton90]

@@privacyvalued4134 I can't compile all the information even on 0.5x speed

[@nukesrus2663]

Ah, the TikTok ADHD technique.

[@mindfornication4funn]

@@nukesrus2663 is that a thing ??!!

[@exentrikk]

​@@mindfornication4funnyes it's called the Subway Surfers technique

[@scottwatrous]

I feel like I'm in a capsule returning from the Moon and Scott Manley is on the intercom just rambling on and on about re-entry physics while I'm trying to enjoy this moment.

[@Miata822]

There is a full length video w/ ambient capsule sounds. It is mesmerizing. I'm surprised Scott didn't link to it. I can't.

[@nutsackmania]

um

[@logarhythmic6859]

I know the sounds in the background are the thrusters firing, but I like to think it's just Scott controlling it via keyboard while casually talking about reentry.

[@casualbird7671]

​@@logarhythmic6859it is lovely to think of it like a KSP video

[@rahmirahmiev2195]

i feel like he is the type of guy who would totally do that 🤣

[@schmodedo]

Although I had seen it before, I appreciate you leaving the re-entry video up as you narrated. The vortex of superheated gases behind the capsule is mesmerizing.

[@renedekker9806]

Ah, that is what that is? I already wondered about that.

[@weseehowcommiegoogleis3770]

I just came from the bath room with the same effect.

[@danwile5973]

The thrusters are scary sounding. Keeping the capsule right on the knife edge of catastrophe.

[@beckydoesit9331]

Amazing. Too bad it's fake. Space is fake and the Earth is flat. NASA lied to you. Sorry to tell you.

[@dallasangler]

Being utterly mesmerized by the parachutes interplay at the moment of splashdown "sparked joy" in this heart. Thanks Scott.

[@tissuepaper9962]

Isn't it interesting that they don't seem to touch each other and instead partially deflate when they get too close to one another. I imagine those vents around the outer edge are forcing some air out to the sides to keep the parachutes apart but that's pure speculation on my part.

[@L33tSkE3t]

⁠@@tissuepaper9962I believe you’re right. The parachute system for soft touchdown after the terminal stages of atmospheric reentry is something I probably know the least about in terms of space hardware but, I believe those vents both around the edge and sometimes on the top do multiple things and one of them is to help provide stability as allowing some of the air through that parachute I believe helps to stabilize it by allowing for a less turbulent stream of air to flow through and this helps to keep the parachute inflated and the flowing air helps to keep it from oscillating violently, preventing the introduction of unnecessary structural stress into the lines and on the stitches of the parachute while being pulled down by the weight of the craft attached. I’m not 100% sure but, I believe that this at least part of their functions.

[@thinkingoutloud6741]

The inflatable heat shield idea led you to mention the idea of inflatable zeppelins. I kept waiting for you to go to inflatable wings on a space plane. Or, extremely large Kevlar parachutes. At the high altitudes, they could be ver thin and compact before deployment.

[@ianmangham4570]

DONKEYSMELL

[@wagnerrp]

@@thinkingoutloud6741 The high altitude parachute doesn’t really work. There’s not enough air density to keep the parachute reliably inflated. Instead you have a “ballute”, with an enclosed volume and a ram-air scoop to pressurize it.

[@homeopathicfossil-fuels4789]

I want more of these refutations of common "Why dont they just do X" I love your content Scott, followed you since the early days of KSP, I remember being hyped every single time you released a "100% reusable space program" video because your solutions to things were so creative.

[@DominikPlaylists]

Why don't they just install a really large parachute very high in the atmosphere during the skipping phase? With arbitrarily large area the parachute can fully stop the rocket to terminal velocity and radiate the heat away faster. It's essentially the same principle as this inflatable heat shield but parachutes are cheaper and simpler.

[@briankale5977]

@@DominikPlaylists Cue the meme "HAHAHAHAHAHAHAH ........... Oh wait, ..you're serious? ........... HAHAHAHAHAHAHA."

[@matthewrberning]

Yes, this would be a great series/service!

[@DrDeuteron]

people with these rejected ideas should try to figure it out for themselves.

[@declandougan7243]

@@briankale5977Damn dude, he’s just asking. Do you actually know the mathematical analysis required to shoot down that idea or just have an ego?

[@theevermind]

Is reentry really necessary? If I went to space, I would insist on coming back, so yes, it's necessary.

[@chrisView]

😂

[@dimitar297]

What if your return somehow promoted vaccine hesitancy, would you consider staying out there to protect Granny?

[@dmitryshusterman9494]

Is it your granny or my granny?​@@dimitar297

[@JimmyRussell-zd5qo]

I shared the idea of running refrigeration tubes through the epoxy that holds the tiles last week. Look what their doing now. Removing 18,000 tiles and epoxy. I expect they will probably follow a modified version of my suggestion. This would also resolve the oxygen freezing and shutting down engines by using oxygen for coolant.

[@sevenismy]

@@JimmyRussell-zd5qo not sure if the added complexity and weight of the pipes makes it worth it. Maybe if we have refuelling on the moon, we can do a less complex more fail safe active cooling system, which can waste more fuel

[@brucewatt1032]

I never saw that re-entry footage before - my goodness, how amazing is that?!?! Thanks for going through the details of re-entry Scott, you answered all my questions on that topic in one short, concise and easy-to-understand video.

[@Alarix246]

I think this footage (original with sound) was a first released to public (if I ain't mistaken).

[@MrGrace]

I was blown away watching that!

[@crewsgiles9499]

Is there a version with telemetry displayed?

[@JarrodFLif3r]

I am amazed by the 'skipping' of Orion. The calculations to figure that out are truly incredible.

[@iitzfizz]

I came to comment the same, I've seen the video before and never even realised it was doing that; though now it seems obvious. Amazing indeed. Also the little flip manoeuvre it did too.

[@TraderDan58]

Totally agree. I thought the same thing. I’m also amazed that the capsule can be “steered”. Apparently the capsule is designed where the center of gravity is slightly offset. This causes the capsule to fall at a slight angle. By rotating it in the direction you want to move it you can steer. Amazing.

[@CarlosAM1]

​@@TraderDan58 that is a pretty common thing with capsule desgins, still pretty cool

[@sciencecompliance235]

Numerical analysis

[@robertmiller9735]

The Russians worked that out long ago: that's how they'd have brought back a lunar Soyuz. Several Zond probes demonstrated it.

[@Rick-the-Swift]

0:01 before I even watch this video or read a comment, I'm going to predict that the answer is no, it's not necessary for spacecraft to endure the hazards of re-entry- it's just a hell of a lot cheaper for them to do it that way. Can't wait to watch the video now and see what others think 😄

[@StreuB1]

A skipping rock on a pond is one of the best visual analogies to reentry that I ever heard. Really helped me understand and visualize it after that.

[@Knofbath]

I've had a few KSP re-entries like that. Come in too hot and need to spend a few orbits bouncing off the atmosphere while trying not to violently explode. Eventually the "water" has absorbed enough of the energy that the rock can land and sink to the bottom.

[@lostbutfreesoul]

@@Knofbath I was thinking the same thing, so many close calls in that game!

[@longsleevethong1457]

More like shooting a bullet at a flat calm surface of water. At certain angles it’ll bounce off or it’ll penetrate.

[@liquidsnakex]

@@longsleevethong1457 Both are awful analogies. Every contact with the atmo is slowing the craft and bringing it lower, it never really skips off anything.

[@jonsteensen7706]

@@Knofbath yeah the issue is that it works best in The Kerbal Space Program, as the Kerbals can go on living forever, without you having to consider caring for their basic needs. E.i. they won't die of starvation, oxygen or water running out or the spacecraft getting full of "Kerbal waste products". In real life going halfway back to the Moon, because you did not slow down enough the first time you entered the atmosphere, isn't really a doable thing.

[@jeromethiel4323]

I remember watching a documentary about the Apollo program, and they went to an aerodynamic expert and asked, "how do we stop our capsule from melting on re-entry." And he told them "make it blunt." The reason being, as Scott said, the bow shock of air formed by a blunt object pushes the super hot air away from the capsule, and actually insulates it from the hot air. Allowing a relatively small ablative heat shield to protect the capsule from the small amount of heat that gets through. The space shuttle used the same concept, that's why it was all blunt shaped curves on the leading edges. And even then, the heat tiles were essential to insulate the interior of the craft from the extreme heating of re-entry.

[@ivekuukkeli2156]

Scott has also presented this phenomenon very deeply. I was surprised of his explation: the shape is optimised for a pattern, where the hotest region is some centimeters (cm) from the spacecraft surface ! Not on the surface.

[@DrDeuteron]

so if I take 1/2 mv2 = 3/2 kT, for mach 24 and O2, I get T = 86,000K ...so I think it's not the air temp, but the air speed. It has much more kinetic energy than thermal energy.

[@patreekotime4578]

​@@DrDeuteronwhen two objects are travelling at different speeds, the interaction becomes heat. The energy has to go somewhere, and typically it becomes heat.

[@deanlawson6880]

@@patreekotime4578 One of the things I notice as a lay-person (ie.. non-scientific but still technical person), is that when you have any kind of excess energy and you try to convert it to any other kind of energy you get a whole bunch of *Heat* in addition to your net result of energy in the final form you're working toward. I'm sure there are relevant laws and complex formulas (thermodynamics and physics) that can show this and predict and model this accurately. Just an observation from watching this and other videos on topics like this.

[@DrDeuteron]

@@patreekotime4578 and what is the conversion between velocity and temperature? I mean how do you get from meters per second to kelvin?

[@dgkcpa1]

Talked to a person who worked on the US X-15 rocket plane program (1959-1968). They considered putting the X-15 into orbit, but none of their pilots could fly the re-entry profile on the simulator without burning up. A non pilot member of the program asked if he could try re entry on the simulator, and they let him. He succeeded, and was able to repeat the manuever again and again. Everyone wanted to know how he was able to succeed where others could not. Simple, he said, he watched the temperature guage. If the X-15 got too hot, he pulled up; when it cooled down he let the plane descend. He did this over and over, and showed that winged reentry from orbit was possible. The X-15's glide ratio was about 4 to 1. Constructed of inconel X alloy. An ablative coating was tried on the X-15, but was found to be unsatisfactory, and actually interferred with the plane's natural ability to disapate heat.

[@MarKeMu125]

I knew his reasoning for why a winged design wouldn't work was flawed. Heating being a factor of speed and drag would means you don't need to dissipate as much heat should the speed be controlled.

[@elpelicanojiji]

That was an extremely raw simulator I guess considering the computing power at that time. I bet it didnt consider all the variables

[@rapid13]

Neil Armstrong flew the X15. Everyone who did was an engineer. I’d like a source for this story.

[@chrisView]

This is a very pragmatic way of doing things. It's good to try outsiders since they are not bound by any concepts. Sometimes experts get enslaved in their thinking. Ask the special forces.

[@rapid13]

@@chrisView So…the county should have me try to design the new bridge they’re going to build? I’m not an engineer, so I’m not “bound by any concepts.”

[@erdngtn9942]

Dude, thanks for being you. This is one of the dopest videos during your commentary I’ve ever seen. I’d have never sought this out but we’ve got you to show us something special while learning the best of human exploration

[@erdngtn9942]

Ps, I was all about it till they hit the water. Imagine a failure and survive space only to sink into the darkness and being killed by pressure or cold.

[@MrTonaluv]

​@@erdngtn9942they float you know? Capsules? All the Apollo missions landed in the ocean...

[@raf530i]

Congrats Scott for explaining complex thermoaerodynamics without having to display a single equation on the screen 👏🏻

[@brianjuelpedersen6389]

What’s wrong with equations? The laws of Nature are written in the terms of math(s). Perhaps you do not like that because math is hard. Which is is. But noone promised you Nature chose to make things simple.

[@Longwing70]

I like the equations because they help me understand what's going on especially contemplating the gliding scenario between space and aerodynamics. It makes sense that if you have enormous rockets burning tons of fuel to put a little payload into space and escape Earth, then that little payload must dissipate all that energy somehow to return to Earth.

[@zenithperigee7442]

@@brianjuelpedersen6389 , maybe OP wasn't suggesting "equations are bad" but rather they meant that Scott explained things so well, it was simple enough to understand wherein "utilizing equations" would've made it less understandable for the masses whose strong points are NOT "solving equations" because as you said "Math is hard." It's like the difference in hosting "a discussion with the general public" versus "a technical presentation for a body of professional peers."

[@blackghost87]

Well I definitely missed those equations, I kinda got lost midway through the explanation without any visuals. I'm not saying it should be equations, but at least having some graphs or sketches would have helped a lot.

[@breakfreak3181]

​@blackghost87 This is a video aimed at the laymen masses (such as myself) and thus of more value without equations. I understood what was being talked about throughout, and I'm not scientifically, or mathematically, minded *at all.* I'd posit that if you could understand the equations / graphs etc. underpinning what is described in the video, you can easily understand this video *without* them, as you are probably a lot more advanced mathematically / scientifically than the 'average joe'. I think this is, in essence, what the OP was getting at. The video described the reasoning for re-entry methods in an accessible manner that did *not* require complex maths to be shown (or rather maths that would be complex to an average person).

[@thomassutrina8296]

Great talk! As a 1972 Aeronautical Engineer BS degree I knew the conclusions for both including the Blimp or Zeppelin solution. And even looked into it. So the lighter then air embodied rocket would float to a high atmosphere elevation with zero velocity effectively. Heating say hydrogen as the lift gas would help get higher but not significant. Then collapse the blimp as the hydrogen lift gas and LOX/LH2 is burned would save almost nothing in fuel considering you have to lift the blimp also. So going into orbit is at best a wash but floating to altitude and being in the correct spot and surviving turbulence etc. in the air is a higher risk then flying or rocketing to that starting height. Blimp would be so large that you would expend energy just to keep it from skipping or it would have to be a lift body that may actually be pushing towards the earth. Lift body means more weight and control surfaces, more weight. Trade off. And the gas inside the blimp couldn't be hydrogen or oxygen. Needs to be something that will not burn or be the oxidizer. Skylon fly to space with inflatable heat shield for reentry that is discarded or retracted to finish by flying for a landing. Now that is maybe the best combination since the large wings already need to be there to fly at a low mach number early in the climb into space.

[@HappyBeezerStudios]

I always wondered why nobody made a launch pad in the Andes. You save about 4 km of atmosphere and are at the equator so a launch eastwards will be most beneficial. Add to that the bulging and you can launch from the point furthest away from earth's center, the inactive volcano Chimborazo with about 2 km further out than Everest. You get less distance to space and less gravity to overcome.

[@Keithustus]

*than Then is for sequence. Than is what you need for comparisons. ‘If it’s cheaper to A than B then be ready for C.’

[@NA-hi7lx]

Its not about gaining altitude. Its about getting enough speed to maintain orbit. Orbit is achieved when you fall back to earth at the same speed as the earth moves away from you. The ground moves away because you are flying sideways as you are falling. (to visualize this, draw a canon ball being shot from New York to Tokyo) You need to get to about 20,000 mph to reach low earth orbital speed. This is why there's little to be gained by Blimping a craft high up into the air first. Launching from the tip of Mt Everest will give you an extra 5 miles of radius relative to earths 4000 mile radius. The extra speed gained is a rounding error. Its far more effective to launch from closer to the equator

[@KENARDO]

Interestingly, I recently finished re-watching Mobile Suit Zeta Gundam (1986), which features inflatable heatshields for various spacecraft entering Earth's atmosphere. For a show about giant robots from the 1980s, they sure did their homework on ballutes.

[@OsirusHandle]

Old anime were written by massive nerds, of course they were filled with good scifi! Recent stuff has gotten pretty terrible by comparison :/ You dont get these crazy high quality OVAs for example anymore.

[@randomnickify]

Not only old Gundams, even modern Gundams are surprisingly good at the science and laws of physics once you get over big robots - and even big robots have proper inlore explanation. You also have to remember Gundams are the franchise that has multiple entries meant for different ages from little kids to adults.

[@andersbackman3977]

⁠@@randomnickifyIt would be really interesting to learn that in-lore explanation for giant robot shaped war machines.

[@Mute_Nostril_Agony]

In the film 2010, the US-Soviet space ship uses a ballute as a speed brake as it slingshots through the upper atmosphere of Jupiter no

[@AsbestosMuffins]

reentry heating was a serious plot point of one episode of the original show

[@AerialWaviator]

One thing missing from this video is an overlay of velocity and altitude. It would be cool to see the initial entry, climb out of the atmosphere, and reentry to final decent as plots in parallel with the video. (there's likely some national security restrictions to include, but just wishing) Regarding the L/D (Lift to Drag) ratio of space capsules. Apollo had a L/D ratio of 0.52:1 (or ~1:1.92), and a Dragon Capsule has L/D of 0.18:1 (1:5.6). Apollo reentered at 11 km/s, while a Dragon reenters at a 7.5 km/s. Huge differences in amount of kinetic energy, as notes is a factor of mass*velocity^2. Starship will be the largest (reusable) spacecraft to undergo reentry. It too will reenter at ~7.5 km/s.

[@DrDeuteron]

or it's just proprietary. but idk.

[@tjthill]

Coming back from the Moon unless you've got fuel to burn you're going to reenter at 11km/s-ish. From Mars it turns out it's not that much higher, iirc it's less than 13km/s. It's not the craft that matters, it's the trajectory.

[@arthurzettel6618]

​@tjthill Trajectory and especially Velocity that matters because the higher the Velocity the narrower the window of reentery and the more likely that the vessel will not survive reentry.

[@MiltonCedeno-l5x]

One thing missing is how stands the Dream Chaser space place for reentry. From what Scott said it should be less fiery than the capsule?

[@Keithustus]

@@arthurzettel6618humans suck. Sure, we can send things out of our solar system, but so far, we’ve only ever returned craft as far as….the moon.

[@Bloodgt3]

I remember trying to watch the reentry video and getting bored and didn't finish it but , just having you talk over it made it so much more enjoyable for my brain to watch , you should do this format more often. It really works. Ty

[@CIinbox]

As beautiful as the re-entry footage is, it wouldn't hurt my understanding of the video to show some of the calculations and concepts you're explaining on the screen (maybe in a corner). Thanks for the interesting vid!

[@zebastianjohanzen3865]

I love using the ballute mod in ksp. Shoot for a periopsis of around 56 Km, and deploy the ballute as soon as I'm in the atmosphere. It's so much more gentle landing.

[@rogerphelps9939]

KSP does not correspond very well to reality. My son used to play with it a lot and the scaling factors were all wrong.

[@richardbloemenkamp8532]

@@rogerphelps9939 That's why you need to install the Realism Overhaul and Real Solar System mods. There is a community of people trying to get close to reality.

[@SaviorTheBurn]

​@@rogerphelps9939well kerbin is 1/8 the size of earth and atmosphere out to 70km. It's not meant to be like earth.

[@daves6213]

periApsis

[@nukesrus2663]

@@daves6213 perryplatypus

[@tlrmatthew]

This just blows my mind. I already understand that re-entry is a rough experience to go through but the mathematical knowledge that is understood about it is just beyond me. The way Scott talks about it, although i don't understand the most of it leaves me in awe of how much understanding there is about the subject. How much Scott must put in as regards research & actually understanding then making it into such high quality videos is amazing.

[@RobertDeloyd]

Nice to watch the re-entry all the way to the ocean!

[@brucehemming9749]

Hi Scott the science is fascinating and the video of the Artemis re-entry was really good thanks for sharing! Happy new year 🎉

[@pirojfmifhghek566]

As you were talking, I was thinking about inflatable or temporary heat shields that spread out and create more surface area. I hadn't heard about that at all. It's super cool that it's already been developed and put into testing. I'm excited to see where that technology goes.

[@BloonWhisp]

JPAerospace is a leader in alternate ways to achieve orbit. Their design seems to be safer up and down as Scott seemed to suggest. G forces much reduced, and the orbital airship never has to land. That task is given to a more suitable and smaller craft when the Dark Sky Station is built.

[@nickasdf]

Excellent video. Setting it over the full reentry video, with some tie-ins, was a great idea. I didn't have the patience to watch the full Orion video when it first came out, but had no problem sitting through it this time. The topic of discussion is fun, and is the sort of thing that gets people interested in space and physics, without being too esoteric.

[@gregbailey45]

Plus it was sped up...

[@theafro]

Scott talking about rocket science to a backdrop of stunning footage. I love this format!

[@KCadbyRacing]

Awesome explanation Scott. The sound(s) are truly amazing (on the original, without commentary)...

[@soffici1]

Fantastic footage and an excellent explanation for what’s going on. Thank you, Scott A bit of trivia about the L/D ratio for airliners: the B767 has 12 (they discovered it during the Gimli glider incident, go check it out), while the Airbus flock tend to have around 15 (yes, even that monstrosity known as the A380). The B747 and 777 also have around 15, while the original B737 was supposed to have 17 (highly doubt that, but hey). The B787 and B777X are at 20! Except the B767’s, which was actually found out by accident, the rest are all theoretical, so I wouldn’t count on them if were to have a total loss of power on all engines anywhere far from an airport safety cone. Gliders are on another planet. The first plastic gliders of the 1960s had around 32-35 at relatively low speeds, while more modern ones like the Nimbus 4 have a manufactured-declared L/D of 60+. Recently manufacturers have stopped publishing the “polar curve” of the gliders they make, so we don’t exactly know their design performance with the seams level of detail, but I’d guess is not very far from 50 to 55. The major improvement on previous iterations lies in the speed at which they obtain those L/D ratios, given by the much higher wing loading of modern gliders (55+ kg/sqm vs 30-35kg/sqm for the 1960s ones) Still not useful for atmospheric reentry Happy 2024

[@rogerphelps9939]

Easy to determine lift to drag ratio. Put the plane into level flight. You know what the fuel load is so you know the total mass of the aircraft. You also know what the trrust of the engines is because there will be known relationships between fuel consumption, rate ambient pressure, temperature and airspeed etc and thrust. Given that the lift must equal the current mass of the aircraft, lift to drag ratio is easily determined.

[@soffici1]

@@rogerphelps9939 theoretically you’re absolutely right, and aircraft manufacturers know all of this, but good luck finding those known relationships! I reckon the fuel burn curves of engines and the related thrust ones are among the best kept secrets in the industry, so we’ll have to rely on what the aircraft manufacturers say

[@rogerphelps9939]

Indeed. I am sure engine makers will tell airframe makers only as much as is strictly necessary.@@soffici1

[@FourthWayRanch]

It's called a sailplane

[@soffici1]

Glider, sailplane… different names for the same object In the USA there is the “soaring society of America”, in the UK the equivalenti is called “British gliding association”

[@ILikeDoritos456]

A G-Force indicator would be a very fascinating addition to the Falcon 9 telemetry displays of both stages.

[@gottfriedheumesser1994]

I think we will not have to add one as the rocket has at least three of them for inertial navigation.

[@HashtagBirdyy]

I thought it displayed acceleration? That's basically the same thing right?

[@ILikeDoritos456]

@@gottfriedheumesser1994 the rocket almost certainly has G sensores since they say they limit acceleration to 3G when carrying astronauts by throttling the engines.

[@ILikeDoritos456]

@@HashtagBirdyy No they don't display acceleration. They only show altitude speed and time elapsed. I suppose if you wanted to do the math, you could figure it out, but for quick reference G rate display is what I meant.

[@gottfriedheumesser1994]

@@ILikeDoritos456 No modern rocket or spacecraft can work without acceleration and rotation sensors regardless of whether manned or unmanned.

[@markschoenberger7825]

I have always enjoyed your depth of knowledge in these videos ... this one in particular with the step-by-step math included was insightful. Thanks!

[@frankgulla2335]

Scott, what a great talk. I don't know how many stayed with you, but since I teach an engineering Thermo-Science lab,I was with you every step of the way. Great Job.

[@bzakie2]

It was terrible. I understand reentry and why you can’t stop and then re enter, but I didn’t understand a word Scott said.

[@johnpaulvanson5170]

Little bit surprised you didn't mention (unless I missed it) for the second part, the Japanese project back in 2008 to drop paper airplanes from the ISS. It didn't go anywhere, but the paper planes would've enjoyed a relatively low velocity, survivable-to-paper reentry courtesy their large surface area for drag (and some lift) against their very low weight.

[@brandyballoon]

Maybe it didn't go anywhere because you can't "drop" things from the ISS. They'll just end up orbiting with all the other space junk.

[@Simon-ho6ly]

@@brandyballoon actually you can, the ISS is low enough there is a somewhat significant amount of atmospheric drag, so much it has to boost its orbit up on a regular basis

[@ianallen738]

while at the same time securing the guinness world record for longest paper airplane flight (time and distance) for probably the rest of recorded history. very sneak, japan. very sneaky.

[@watvannou]

@@brandyballoon Nothing just stays in orbit forever, all those satellites still experience gravity and they will eventually fall back to earth. There are also varying distances of orbit and things closer to the Earth will of course come down sooner.

[@dragonmaster1360]

@watvannou It's not the gravity that will pull them down. Well, it will, because that's what gravity does, but it won't technically be gravity doing it. If they were high enough to be put of our atmosphere ENTIRELY, they'd essentially orbit forever, because they're going fast enough to counter Earth's gravity. That's how they orbit in the first place. No, it's the air resistance that will pull them down. Once they run out of any fuel on board used to boost their orbit and counter the drag from the extremely thin atmosphere in their orbit, the air resistance will eventually cause them to slow down enough to de-orbit. So no, it isn't gravity that will pull them down, it's the air resistance that will slow them enough that they can no longer counter gravity like they normally would.

[@GeoffBlackmore]

A simpler way to explain it is that as soon as you begin any form of braking in space, you will drop below orbital velocity and begin to fall into the atmosphere. The two suggested methods by non-rocket scientists to slow down before re-entry would only result in the same immediate re-entry that they say they want to avoid.

[@Jolly-Green-Steve]

You could position space craft vertically and fire engine to increase height of orbit while slowly pitching engines forward to 45 degree angle and if you were in high enough orbit you could slow down to under 3k mph before hitting atmosphere to not need a heat-shield. this would kill your payload capacity though requiring tons more fuel.

[@Phootaba]

Love the content. Loved watching the parachutes and their aerodynamic effect on each other. Scott!? Can you do a video on how parachutes are calculated with regards to size, passtrough vs letting air pass outside it? In the video you can see the air passing outside the chute interacting with its siblings, was mesmerising to watch

[@gcewing]

I wonder if they've fixed the parachutes in KSP2 so they don't act like bosons and pile on top of each other.

[@agustinbs]

I WANTED THIS QUESTION TO BE ANSWER FOR SO LONG, THANKS SCOTT FOR THIS. Very interesting and the ratio of lift and drag explained everything

[@bruceaurand32]

Great video.. I have often wondered about this and this easily answers the questions that I had about it though I had pretty much already come to the same conclusions. This just presents it so well and clarifies some questions I had about confirming it. One thought that I had was the very idea of engine braking to slow the craft enough for a simple and safe reentry but that idea included having an abundant amount of fuel on board in order to accomplish that goal and that is a very easy one to dispel as being impractical as your video suggests. Thanks for producing this. It was very well done.

[@schr75]

Hi Scott. The best gliders are now more than 70:1. My own 40 year old glider does 45:1 without even fitting the tip extentions.

[@CaseyDuBose]

Amazing

[@BillysFingers]

That's incredible! I did about 250 hours in a wooden/fabric ES-60 Boomerang back in the late 70's and it had a glide ratio of about 32:1. I gave up flying in 1979 but a 70:1 ration would be a dream!

[@schr75]

-Modern High performance gliders are a dream, but you can get just as much fun out of an old Ka-8 with a L:D of 25:1. You are still alone in the air like a bird. @@BillysFingers

[@arturoeugster7228]

Diamant 18 has that L/D ratio, 60 has been achieved, with boundary layer suction near 80, I have the report by the University of Delft. Will send a copy to any one interested.

[@gabrielcoelho2346]

​@@arturoeugster7228 I am interested. How can I find that report?

[@JLange642]

Thank you Scott! I have often considered why we did things the way we do and why not try X- you explained a lot of it in this video.

[@robertobruselas3952]

A remarkable analysis of the heating phase during reentry. A fantastic approach has been taken to address the issue. Well done.

[@prjndigo]

Short answer: it keeps space clean and free of as many bodies as possible. Long answer: the amount of thrust it would take to actually bring an object back in without a _Dynamic Entry_ is roughly equivalent to half the thrust it took to put something up there but then you have to add thrust to putting the object and its return rocket up there. So for what's basically two tons of refined high-tech terracotta you can cut the price of the trip in half and look ballsy doing it.

[@Stickleback]

yet someone can go into space and fall back to earth with a parachute.

[@Doctor_Glados]

@@Sticklebackthat someone was not in orbit. Being in space and being in orbit are indeed not the same.

[@liam3284]

given how much energy is avaliable, seems like an opportunity to use it. Thermo'electric or some high pressure turbine generator?

[@tenalpoen]

What about landing on a planet or moon with effectively no atmosphere like the earth's moon? Then you have no choice but to bring extra fuel for deceleration, right?

[@crabbcake]

they aren't orbiting or were ever at a high speed@@Stickleback

[@larrywalsh9939]

Just think, if you brought along enough spare fuel that you could decelerate back down without heat re-entry, every ton of that extra fuel you're carrying means 1 ton less of actual payload you can take. If you needed an extra 40 tons of fuel to do this and your payload capacity was like 45 tons, your actual effective payload becomes only 5 tons, and that turns it into an incredibly cost-inefficient vehicle, since your cost/mass ratio goes WAY up.

[@plektosgaming]

It's worse as it's not 1 ton but more like 50+ tons of fuel for 1 ton of payload. The Saturn V rocket burned 20 tons a SECOND for its first stage.

[@DrewReynolds]

I guess you could refuel for reentry; especially if we start living in a world where many space craft are not designed to ever surface after they are launched like ISS.

[@prjndigo]

and you'd still have to carry the goddamned heat shield in case your return rocket didn't light off right...

[@Munakas-wq3gp]

I have to correct him a little though: It IS possible to slow down on re-entry, we just don't have the technology for it yet. The 40's era rocket technology just doesn't cut it. Possibly in the future we will have more efficient sources of propulsion.

[@Munakas-wq3gp]

@@DrewReynolds We could just drag a fuel line to space. You know how a hose will extend out when you spin around with the hose in your hand? Just mega size that using earths rotation :P

[@Zuckerpuppekopf]

Moving from space faring speeds to earth faring speeds requires a huge change in momentum, and that is only controllable by the input of opposing energy. Either you have to carry that energy yourself, as with a huge fuel cost, or just use the frictional energy that naturally occurs as the space object contacts the atmosphere. Free energy vs costly energy. That's the equation. If you try to slowly work against gravity to gradually dissipate momentum, and spread the frictional energy over a longer span of reentry time, you still need to carry more fuel to control that longer reentry. The easiest way is ultimately just to let atmospheric friction do its thing for "free", just using the cost of heat resistant tiles.

[@langdons2848]

I like the inflatable heat shield idea. I was always intrigued by that style of system being deployed by the ship Leonov to aerobrake in Jupiter's atmosphere in 2010: The Year We Make Contact (Space Odyssey II).

[@dr_jaymz]

Isn't there a design concept for a personal inflatable heat shield for one man escape pod. I saw it on vintage space. Its much easier to shed energy from 75kg of human going at 17500mph than 105 tons of space shuttle bricks. I just feel the deceleration may be too intense if you're very light.

[@langdons2848]

@@dr_jaymz I think I've heard of that too. Talk about an extreme sport...

[@CapinCooke]

Holy moly! That’ll be one hell of a ride 😱. “You go first. I’ll watch”. 😂

[@Tuberuser187]

@@dr_jaymz Not sure if we saw the same one or even if there was more than one but the one I saw was a giant mylar bag with individual cells on one side and several large cans of expanding foam, the idea being they filled the cells to form the parabola and then become the ablative layer for the Astronaut on the other side, which was clear and would look like a hopefully uncooked foam and cellophane packed cut of meat from the supermarket.

[@chrispeoples4606]

Like anything in aviation and space travel, the tricky part is not getting up there but rather figuring out to get back on the ground in one piece! Scott great work here, I plan to use this video in my physics classes for work and energy unit and my thermodynamics unit. CP

[@notyourroad]

Yyyeesssssss!

[@As3th8r]

Good Video. I once read about the aerodynamics of reentry capsules and their testings back in the days. It was great to see the different designs and their 'surprise' that the now normal design would be better than something pointy.

[@nozrep]

that is fascinating. As deadly as the atmosphere can be against us, it is so frikkin thick and it protects us. Just fascinating to listen to. I love to learn stuff. But I am also bewildered because I ain’t no physicist or mathematician. Thank you for re-learning me this concept that I remember learning in high school but here, in a much more detailed manner!

[@gadlicht4627]

What if you attached butter toast to cat, with butter side being side not attached to cat and other side attaches to back of cat. Since cat always lands on legs and toast butter side up either it spins super fast or levitate which both would provide necessary lift (this is joke)

[@samuelgarrod8327]

That's debatable.

[@Keithustus]

Landing…fine, but no source of delta V for deceleration. :)

[@thaddeuszukowski4633]

Thank you for a clear explanation in simple terms. I have always wondered why we would go back to, what seemed like, less sophisticated technology after the master piece that was the shuttle.

[@EstorilEm]

I can’t believe how complicated and precise the reentry maneuvers are for Orion. You can say what you want about SLS, but the Orion capsule has been pretty damn impressive (and RELATIVELY on-time and on-budget) since the start - which is incredible given its size and complexity vs. the Apollo capsules. It is literally the only spacecraft in the world capable of doing what it’s doing.

[@michaeldeierhoi4096]

Orion is the first vehicle since Apollo that is capable of carrying humans to the moon and back. I agree that is a big deal. And it was done more efficiently and safer then was done with Apollo.

[@teebob21]

I might have the scale slightly wrong, but if the Earth was the size of a basketball, the safe re-entry corridor is the thickness of a piece of notebook paper.

[@brookswoolson909]

@@teebob21Someone just watched Apollo 13, huh? They use that same analogy in the movie!

[@teebob21]

@@brookswoolson909 Well, it's been a hot minute since I watched that movie, but I also worked at NASA between 1989 and 2007. (Mars Global Surveyor, mainly)

[@brookswoolson909]

@@teebob21 Very cool! I actually just rewatched Apollo 13 the other day, so that’s why it was top of mind. I bet this video is especially poignant for you because of MGS’s aerobraking technique?

[@TheMrBigJeff]

I’ve had this question since I first learned about rockets existing - never had a career in any scientific field so never really quested to find the answer but I am ever so grateful to finally get it. Thanks Scott 🙏☺️✌️

[@c.ladimore1237]

2010 showed that inflatable airbreak manoeuver (in cinema), but it still nearly broke the ship apart. really we just need a space elevator if we can get something with enough tensile strength. read an article recently that they had a new design for one in the works

[@pauljs75]

In the general scheme of things, the resin composite used for ablative heat shields is the same as that of most automotive brake pads. So it's relatively inexpensive compared to other possible approaches of slowing down. Although it may be modified a bit, it's not like some super exotic material either.

[@placeholdername0000]

Heck, wooden heat shields have been used. These are however only useful as a single use item obviously.

[@OsirusHandle]

Huh, I heard they used aerogel or something. TLI.

[@placeholdername0000]

@@OsirusHandle The shuttle used ceramic tiles which were fairly exotic.

[@DavidEsp1]

@@placeholdername0000 Not significantly ablative AFAIR (or did they have ablative coatings?)

[@placeholdername0000]

@@DavidEsp1 True, they weren't meant to ablate.

[@ceejay0137]

A futuristic solution is an orbital spaceport plus a space elevator to get back down to the surface. Maybe in a century or two, if we haven't blown the place up by then . . . sigh. All the best for 2024, Scott. Fly safe!

[@UltraNoobian]

Just lie and tell them space elevators make it easier to blow up other places.

[@DarkNightDreamer]

Assumingg we ever can figure out how to build a space elevator :/ We could totally do a station in geo synch. orbit though. Will we? Prob. not in the next 40 years unless its a private company cause NASA has a shoestring budget and everything they wanna do has to get the okay of our wonderful congress which never agrees on anything :/

[@midtskogen]

You would still need fuel/propellant to dock with the port and elevator. The atmosphere basically is free propellant, so a bit wasteful not to use it. Maybe your argument is to dock a large spacecraft to a port in an orbit favourable to your incoming trajectory, then switch to a tiny vehicle which is much cheaper to take to the elevator, but you would still need tonnes of fuel to do either. So atmospheric breaking it is until we have warp drives...

[@AmirDarkOne]

space elevator for big planets like earth is just a pipe dream any civilization who can build a space elevator , is advanced enough to not need it.

[@pan2aja]

The US just bombed the Nordstream 2 pipeline... So probably No space elevator anytime soon

[@georgetheofanous6792]

You've forgotten more about math and science than I'll ever know. I still appreciated what little I did understand of your explanation. Thank you.

[@BreakingBarriers2DIY]

Love these thought experiments. We should be careful not to say that alternatives are “not possible” when we actually mean not economical or practical. ;)

[@jameswest4819]

Or not thought of...yet.

[@fimbulvntr]

It's certainly possible to use rockets to decelerate. I've done so multiple times (by accident) when I tremendously overbuilt my rockets on KSP 😂 Economical? Oh gods no. Possible? Definitely.

[@GantryG]

I would say it like this: Our (chemical rocket) tech is currently barely energetic enough able to have enough energy to get things into space, not to keep it there without using the magic of orbit paths and then get back on earth by using very little energy by using aero braking. When we have more energetic means (fusion, antimatter, Star Wars tech, etc.) then sure, one can spend the energy to slow down in space and come down as slowly as you want. Like in Star Wars, the ships are depicted as having very energetic propulsion systems and the ships are depicted as not orbiting planets, because they don’t need to.

[@jameswest4819]

@@GantryG I was curious about a video I watched earlier that talked about SpaceX and Nasa as well as Boeing. Supposedly they have built aircraft that may fly to satellite altitudes up to 350 miles. Is that still within an area that requires aero braking? Maybe they haven't tested them yet.

[@roaringsheep977]

Here among other ksp players as Scott is one of us anyway there is also this way of adding not fuel but life support stuff because in ksp base game we can ignore it and we have the time speed up function so I avoid violent reentry by having some small wings on my return craft. (and it may take too much regular astronauts time) but in the game I would set my Periapsis inside the Kerbin atmosphere and I strife the atmosphere without heat shield to lower my Apoapsis by just a bit each pass and align the craft so that the small wings have maximum drag and at first I strife only the thinner atmosphere and carefully I go into the deeper parts of the atmosphere with more heating. And while at the Apoapsis I would raise my periapsis for minimum fuel mostly just monopropelant if I feel the heating is getting too much at that depth of the atmosphere where the periapsis is. And then when the orbit is getting circular I still try to keep it a bit eliptic to get some time to cool the craft. But I feel that mostly I almost don't burn at all so no heat shield. did not try with the life support mod. But hey It was just my aversion to my craft burning and it is just a game and my poor astronauts spent some time between the Mun and Kerbin. So do I say if the NASA astronauts took this idea seriously it would solve all problems? Well no but it may be another possible Variable to consider maybe one day it will come in handy for some mission. So technically you can avoid a lot if not all the burning during reentry at the cost of more time for the astronauts which may be absolutely too big a price to pay and totally useless in real life but it exists. If ksp did not make a huge miscalculation like leaving something out of the equation. And this idea came to me during early stages when one of my first attempt at a stable orbit did fail and I started to burn up but then suddenly the craft came out of the atmosphere because even thou I could not get my Apoapsis high enough in time I did get the peariapsis out of the atmospere on the other side so I found out against my intuition that once back in the atmospere does not mean forever back in the atmosphere. That is where the idea was born to gently reenter. Maybe someone can refute me let's have fun and try this for more refuting potential @Scott Manley

[@bobmarley3594]

I tried that in KSP 1 a few years ago, and I came with the same conclusion, using the heatshield is the best solution.

[@shanent5793]

I recently started playing that. A miscalculation returning from the smaller moon meant I had to use up all the thrusters and push with the jetpacks just to get my orbit down to 65km, it still took dozens of passes to get back. The first few passes completely consumed the ablative part of the heat shield, whatever was left seems to have held up. Is the ablator just a placebo?

[@bobmarley3594]

@@shanent5793 KSP1 parts are very tolerant to heat. To have a more realistic situation (and probably still not enough to be similar from a real Earth reentry), try to land on Eve.

[@OsirusHandle]

I remember playing on Real Solar System and good lord, a few Km too low and you exploded, a few too high and you just shot through the atmosphere. Real difficult. I tried a winged reentry vehicle and it was really difficult to land too.

[@Runiat]

​@@shanent5793 for landing a single capsule on Kerbin, from orbit, essentially yes. If you want to bring home more than just the capsule or come straight in from an interplanetary trip, whether on Kerbin or somewhere else, that's when you're more likely to need the ablation.

[@nikolaideianov5092]

Thats why when i do anything flying in rss i use A LOT of parachutes​ @@OsirusHandle

[@Lktravel1]

12 minutes to go from so space to ground level is incredible

[@AtomicOverdrive]

Scott did an excellent job of explaining why its not practical to slam on the breaks and slowly drop back into the atmosphere. However once humanity moves past the whole needing a rocket to get into space issue and can travel up out of earths atmosphere like seen in most scifi movies, then yea you can just fly down to earth at slower speeds. But currently, the way re-entry is done is the most practical method.

[@Runiat]

Just one slight issue: we don't have enough handwavium to make reactionless thrusters capable of lifting a significant payload into orbit.

[@mikebridges20]

@@Runiat "Handwavium". I gotta remember that!!

[@tma2001]

you mean like a space elevator ?

[@mikebridges20]

@@tma2001 Yeah, that's a good example of "handwavium". The only place a space elevator works is at geo-stationary orbit.

[@AtomicOverdrive]

@@tma2001 Space elevator is another one of those stupid invention ideas that by the time material science has developed to the state that it can be done, the whole issue with overcoming gravity will already be solved using solid state mechanics..

So, if plasma is what is formed at the front of the vehicle, can you push against it with some EM fields? That way the vehicle stays clear of the plasma and you get the slowing effect.

[@RiversJ]

In theory certainly yes, in practice that would require a Lot of energy to push against it at such high energies and since you already have a plasma formed around the ship trying to use an electromagnetic tether could be a tad.. problematic shall we say. But yea if someone solved the engineering issues yes

[@u1zha]

The pushing is quite hard... We do it in fusion reactors (with huge magnets and induced currents and still barely manage to give it the desired ring shape) and Earth does deflect incoming charged particles with its magnetic field, but AFAIK there's no technology to take chaotically forming wildly varying masses of plasma that form near the front of our vehicle and ask them to take a step back.

Not a physicist. Plasma is positively charged. If you have some grid 1m ahead of the ship made from a heat resistant material, and charge it positively, would that not push against the plasma? And i dont think you need to control the plasma. Just push against the whole of the front wave. I presume the net effect would be decelerating...

[@shanent5793]

Wouldn't that be like moving a conductor near a magnet? The magnet induces a current in the conductor and the electrical resistance causes drag. I think that would slow the plasma down which is less desirable than letting it quickly go around the spacecraft

[@esecallum]

*instead of messing with thousands of tiles which adds weight and complexity why not simply carry dry ice and heat it with the high reentry temperature so that it flows out thru nozzles at the FRONT at high pressure and encases the vehicle in cold CO2 gas for those few minutes of reentry? the cold co2 gas will act as a shield.*

[@EmergentStardust]

I've been wondering about the lift to drag ratio for re-entry. Great video!

[@glidingnick]

Another great video. The best gliders have an LD of 65 or 70. It would be great to see your take on sailplane performance and I'm sure you'd love the experience of flying a Nimbus 4D.

[@peoplez129]

Problem with gliders is you would have to descend way earlier in order to get where you wanted, and you'd have a big chance of overshooting or undershooting the landing by a lot. You obviously need spacecraft to come down somewhere remote and safe, while also being specific. There's also the cost of recovering the craft...the more you're off course, the higher the cost, and not a trivial cost either. So heatshields just makes everything simpler and more predictable. These craft are also a lot heavier, because they need to be structurally strong for space, while maintaining an airtight atmosphere, and withstand things like flexing.

[@trevormarsh8987]

Scott, this episode was fantastic. I loved that you ran the video through your whole segment. It was a great idea and worked well. Bravo 👌

[@phil20_20]

If you have anti-gravity, you can go whatever speed you want.

[@carlousmagus5387]

To a point.

[@irri4662]

Happy pre new year everyone.

[@1_2_die2]

SpaceX Starship would be a good example for the blunt way to do re-entry, using the biggest surface you have to hand. Happy New Year🖖 and thanks for your work, time and passion.

[@Prich319]

This is also the reason why lifting bodies are considered the optimal design for a reentry vehicle. The design allows you to reenter at a gentler angle, so while reentry takes longer, you get more time to bleed off speed in the upper atmosphere. If I remember right, the Venturstar had it worked, would have been able to use a titanium heat shield instead of silicon tiles because the shallow angle meant it would never reach the temperatures that space capsules and even the shuttle would.

[@brucehansensc]

Thanks! Just spent some time reading about Venturestar. You are correct, no heatshield! Surprised Scott didn't mention it. What's also interesting is its more viable today then when originally worked on due to advances in materials.

[@jeffcox4538]

Love this one! Thank you for reminding me of some basic physics!

[@chris.dillon]

I think this kind of stuff is what KSP teaches in a certain deep way. It lets you try your original thoughts and fail. It makes you *feel* it.

[@plantstho6599]

This was an enlightening video. I was trying to come up with some esoteric designs for smoother reentry and the zepplin concept is not far off from what I came up with. I was actually thinking of something like the ships from Arrival, which are zepplin shaped, but are oriented perpendicular to the ground, rather than parallel.

[@tonycosta3302]

The movie 2010 showed their ship using an inflatable balloon to slow it down when it reached Jupiter. It was a nice depiction of how it would work on a real ship.

[@DUKE_of_RAMBLE]

I'm probably alone on this island, but I found *_"2 0 1 0"_* to be a *much* better and more interesting movie, than *_"2 0 0 1"_* was... Was a real shame they didn't make the third.

[@rand0mn0]

@@DUKE_of_RAMBLE They called it a "Ballute". There's a wikipedia article about the concept and it's actual applications.

[@DUKE_of_RAMBLE]

@@rand0mn0 I'll assume quoting me was on accident... 😅

[@joseacuna3239]

It’s so cool seeing the capsule coming from orbit to ballistic re entry in a snap, I was trying to wrap my head around the amount of energy shed in this maneuver but still out of my league.

[@YagiChanDan]

Scary when you think about the kinetic energy in a crash involving a pickup truck at motorway speeds (0.02 kilometres per second)....then think about these capsules travelling at 11 kilometres per second.

[@joseacuna3239]

@@YagiChanDan you’re right, this is why for me at least it’s so hard to wrap my head around it. The sole image of an object moving that fast is incredible considering that the fastest accelerating object I’ve seen is a top fuel dragster.

[@RealBelisariusCawl]

I love the fact that the footage starts to look like an old ‘60s reel from the distortion. Sick

[@benmarteinson48]

Hullo, Scott Manley here... makes my day. would love to see a video of you describing the physics of skipping off the atmosphere. great vid and happy new year sir. fly safe

[@iitzfizz]

Around 4x faster than a rifle bullet, what a crazy ride it would be. I would _really_ love to experience it one day.

[@sciencecompliance235]

It's faster than that. What rifles do you know that fire at 2.75 km/s?

[@dudeinanofficechair7662]

​@@sciencecompliance235it's probably a unit's confusion. 2.7 kilo-feet per second (worst unit ever) is pretty reasonable for rifle rounds.

[@bighammer3464]

What a normal person would think is that you basically come to a stand still in space, let gravity start pulling you down and just fire rockets to slowly set you back down for miles.

[@rotorfamily]

Very interesting! You talk about the L/D ratio, but those numbers sound very similar to the glide ratio (a GA plane has around 10:1 very good glider has 50:1 etc.); would you say as a rule of thumb they can be treated the same?

[@Modellflypappa]

It can be proven that these numbers are exactly the same. So not merely a rule of thumb, but a rule.

[@niconico3907]

Lift/drag ratio is glide ratio

[@alexbowman7582]

You can either come in X15 style like a wasp waisted dart (narrower at the wings to reduce vibrations) or like the Shuttle with a surface area to cause drag and reduce speed. Either way if you get your angles slightly wrong you’ll be in trouble.

[@parrotsticks]

It's amazing enough already that there's such a simple and effective answer to a problem involving so much energy

[@keithmoore5306]

hey Scott ever consider looking at ships from sci fy series as to their true viability? i had space 1999 pop up in my recommended a while back and got wondering how the eagle in that would work as a space station to moon ship as well as a do all on the moon. the original battlestar galatica and babylon 5 ships might be fun too!!

[@Edino_Chattino]

Those ultralarge spaceships would be built in space - and remain there. There's no way it would be economical to put such things in orbit and land them on other planets. This way, they don't need to be aerodynamic, and thus they would probably look like a bunch of random Lego pieces put together, much like the ISS.

[@2fathomsdeeper]

The Eagle is basically a flying truss. The head is a control/escape module, while the legs are landing gear/fuel tanks. The payload section would give a capacity of about two 53' semi-trailers. Overall size is about 100 feet long. It would be a space only vehicle that would only be able to land on the moon or lesser bodies. Only the head would be able to reenter an atmospheric planet. Being a flying truss, you can attach just about anything to it, so the system is a very good design. I look at it as a flying semi-truck. That brings us to the need for a space station that can handle the role of transport hub/shipyard/quarantine facility. Sorry Elon, your starship is only good enough to be a Earth to orbit shuttle.

[@keithmoore5306]

@@2fathomsdeeperwell i said space station to moon nothing about landing on earth!

[@keithmoore5306]

@@Edino_Chattinono the smaller ships fighters shuttles and so on!

[@barryon8706]

IIRC Space 1999's Eagles had "gravity shields" or some such. That probably makes launch to orbit a lot easier. 😊

[@SolraBizna]

My dad once asked me if a "gentle glide" reentry was possible. I tried to explain why it wasn't, but he said he wanted to hear from an expert about it. Wish he were still around, so I could watch this video with him.

[@Keithustus]

Air. It’s still a fluid. You need it to get out of the way. Moving that is not easy! A spacecraft entering an atmosphere is like an animal entering water. Slow? Easy. Fast? Nope! Look at the engineering of automobiles increasing speed from 150 mph to 200 or more mph. F1 stuff. At lower speeds the air resistance is minor, but as vehicles have gotten faster over the previous decades, A LOT of the energy needed to reach top speed is to move the air out of the way, even for sleek aerodynamic cars.

[@AndTecks]

You have to spend so much energy getting there. it should make about the same coming back I think. I am sure there are some orbital factors to consider like drag and lift

[@andoconmax]

Who here from ift 3

[@SassInYourClass]

If I’m on a return trip from Mars, then I’m firing my boosters to slow down my orbital velocity around the Sun to eventually match Earth’s. By the time I reach Earth’s atmosphere, I should effectively just be traveling right alongside Earth with little velocity compared to the surface. With the right planning, I feel like I should be able to get to a point where my ship is effectively just stalled at the top of a free fall curve, yes? And we have an entire sphere to work with, so I could come from the outside or inside of Earth’s orbit, and use it’s gravity to slow me down as I pass around it, or something. Like, it just feels like there should be some clever way to return from beyond Earth’s gravity well such that my speed when hitting the atmosphere is less than LEO velocity.

[@jamesfrankiewicz5768]

Inflatable heat shields are an old idea, at this point… heck, the original Mobile Suit Gundam anime series even featured them back during its run in 1979. It's actually kind of mind-boggling that no one tested the idea until recently.

[@GoldenCroc]

I was about to say... must have been a relatively early depiction in entertainment media.

[@Ryan_Christopher]

_2010_ film had the Soviet Spacecraft deploy an inflatable heat shield for atmospheric braking above Jupiter to enter orbit around Europa.

[@tissuepaper9962]

I imagine it took quite a lot of research to come up with a textile that is pliable enough to be packed up tightly, airtight enough to be inflated to high pressures, and robust enough to withstand the extreme temperatures of re-entry. Most heat shield materials are hard and brittle.

[@carltheshivan]

They've probably had the idea for decades, but material science and funding needed to catch up.

[@AndrewinAus]

@@Ryan_Christopher Yes the 'Bal-oot' as they called it 😀

[@janphilipp86]

Could you theoretically fill up a SpaceX Starship via orbital refuelling and use the engines to break to almost zero, shut down the engines, wait until you reach a speed which leads to too much heat and restart the engine again? So basically plenty of engine re-starts, plenty of stop and fall, to get through the upper layers of the atmosphere.

[@oneoveralpha]

For any craft launched from Earth, the simplest and cheapest way is probably a heatshield or just built tough to survive the heating. But for a craft built in space, that - for some reason - needs to land on Earth, and if you have plenty of fuel from asteroids, then you could probably do a prolonged powered landing. Maybe someday an ultra-delicate instrument built on the moon needs to go to Earth, but it can only handle up to 1.05g and too many starts and stops would jostle it too much. Although, you might need like 100 tons of fuel to land a kilogram of cargo.

[@laurencehoffelder1579]

Yes but in which way would that be more usefull than a heat shield?

[@Iain31313]

You wouldn’t need to go to almost zero speed. You could regulate the engines so that your speed stayed below a point of renter heating. The issue is, like Scott touched on in the video, is that you’d need a lot of fuel to remove the speed. To fully refuel a starship using the lowest figures is 6 tanker ships (nb this number has also been suggested to be very low and it could take up to 20). The cost alone to send up these ships is insane when they can use existing heat tile technologies at a fraction of the cost. Not to mention now you have 6+ extra tanker ships which need to reenter, unless these aren’t reusable.

[@panda4247]

The problem is the same as Scott already described - you'll need too much fuel - regardless of whether you took it up on the same rocket or another and refueled, you need to get the fuel up somehow (which required even more fuel....) Dry mass of starship is 85t, so let's say you have 100t of what you want to re-enter. So according to Scott's calculation, you need around 500t of fuel to slow it down... payload capacity is 150t, so you would need another 3 starship launches to get that much fuel up for the refueling... Nah. Tio expensive

[@janphilipp86]

I know that it wouldn’t make financial nor logistical sense, but I am curious wether this work in physically in terms of fuel capacity. Maybe the ship would run out of fuel and still be high in the atmosphere.

[@ayulin9577]

I think for the first question it is important to mention that this is basically what the falcon 9 booster does during reentry burn. It's not completely removing reentry, but rather just making it a bit easier on the booster. Now, the reentry speed of the booster is of course much lower than orbital reentry or even lunar, but it is the same basic principle.

[@satoshimanabe2493]

Special case of scenario #1: what if SpaceX wanted to return HLS to the earth? (As one would expect, the renders don't show TPS tiles.) If fully refueled in LEO, would it have enough delta-V to propulsively slow down to avoid adiabatic reemtry?

[@gasdive]

Yes. It's very close to a single stage to orbit vehicle. So it would have enough Delta V to cancel most of its velocity.

[@Sundablakr]

Yes but then you're spending a lot of money on more launches to get that fuel up there in the first place. There would have to be an exceptionally good reason to want to bring it back down, and I can't imagine one.

[@stereoroid]

This is one of the long-term hopes for a Moon colony: the ability to refuel ships there so they can use retrorockets to slow down before Earth re-entry.

[@sandramiller7972]

They do not show a TPS because they want to test the empty steel starship to see if it can survive by using lift with repeated bouncing to shed enough energy for a de-boost and sky dive. The TPS would then only be needed if the lifting controls failed, like it sometimes does with Soyuz. F. Miller PhD P.S. They can make fuel in orbit. They call it rocket science because it is not easy.

[@sotthapana]

This explanation mainly demonstrates how much easier it is to use conventional knowledge to rule out big ideas, rather than to do the engineering to actualize them. The 20 seconds you spent at the end on the idea of a "zeppelin" that slowly accelerates and decelerates was the closest you got to answering the questions.

[@scottcarr3264]

That is one of the most complete and informative explanations I have heard on this, well done, I have wondered about the problems involved.

[@samfunderburk5176]

Got me chortling: "Now for the space shuttle: it was famously not a great glider, and on final approach it would have lift to drag of about 4.5 to 1 … which is not bad considering it’s a brick."

[@linuxgeex]

Great analysis Scott. I'd add one thing. The atmosphere boundary, if you can call it that, isn't exactly uniform. Things which affect the shape and density include the Van Allen Belts (dependent on current magnetic pole position and axial tilt relative to plane of orbit/insertion) large weather systems, ie low pressure regions are rising which push the stratosphere towards the thermosphere which pushes the exosphere, which is what you make make contact with. Then there's solar winds, which push the atmosphere towards the Earth on the sunny side and away on the dark side, such that the exosphere can be over 100km higher on the dark side, depending on the recent strength of the solar wind, which we can't always predict though we are getting better at it. An example of this was when SpaceX lost a large number of Starlink satellites during a launch due to solar wind pushing too much atmosphere in their way during the first few hours after deployment where they don't use their thrusters to avoid damaging each other... I'm going to assume their firmware has been adjusted to shorten that waiting period if they detect delta-v while not operating thrusters!

[@Wayn0z]

It won't be long before the TR-3B is declassified as top secret, and this space technology is history.

[@heathb4319]

Before i watch the video, this is a question i have had for probably 25yrs. And you are the first person i have seen bring it up other than myself to one other person about a year ago. So i am curious on what you are about to show me. On to the video... Ok, im back... Thank You Sir. I now understand this even though im not a rocket scientist or engineer and hate hard math :) Very good explanation. I had figured some of it out like more weight for reentry fuel so more fuel and weight and kinda zeros out the idea and the slower you come in the harder the drop without powered entry. But i never thought to DIP in and out like skimming a pond to have time to cool off for a minute and then back in again and so forth.

[@recumbentrocks2929]

Now it all makes sense, thanks for explaining it Scott.

[@drockjr]

Scott, You add so much value to the world! Thanks

[@michaelwilson9449]

I have absolutely no idea what you talked about. I was too transfixed by the re-entry footage. Amazing!!!

[@sabrewolf4129]

It always amazes me with these physics type videos, how MUCH the presenter loves the sound of his own voice. Slowing down the vehicle in orbit so it will descend, IS THE WHOLE POINT. The Space Shuttle has been doing this for decades. It does a de-orbit burn, allowing it to descend and fly back. If the de-orbit is longer than normal, she will come back the same as always, just at a slower speed so re=entry isn't as hot as what we call normal.