SSTO sea level flight path BAD ???

[@tadghie6193]

Gaining speed in the lower atmosphere comes for a number of reasons. You want to get as much speed in air breathing mode as possible because the engines are insanely more efficient, enough to offset drag losses significantly if your craft is aerodynamic. There's also the fact that the amount of thrust per engine increases with speed, and that means you're getting more thrust from the same engine mass. Rapiers increase their thrust massively past mach 1, so staying low and gaining as much speed as possible rather than height gets you into the higher efficiency range a lot faster. My final point is breaking your ascent into more steps. Start shallow to get past 400m/s, then climb into the thinner atmosphere to around 11km. Then it benefits you to level out again for a while, since your rapiers still give a lot of high efficiency thrust while in jet mode and can get you up to a significant fraction of orbital velocity in minimal drag conditions. You want to get as much speed as possible before finally switching into rocket mode, just to get that last little push to get you suborbital and finally into orbit. Thanks anybody who reads this rambling, and feel free to add or challenge anything incase I made a mistake. Love these research vids Vaos, I can imagine the duna colony dudes playing out these sims to build the most efficient rockets and planes for the base c;

[@mephisto8101]

Love the videos as well! :) That makes perfect sense to me. Rapiers have an efficiency of 3200 isp in airbreathing mode versus 305 isp (vac) in rocket mode. That is roughly an order of magnitude difference! With rapiers, I want to get around 1550 m/s at around 18-20km above sea level, before switching modes. My preferred flight profile for most SSTOs is the following: - take off at around 100 - 120 m/s at the end of the runway. - accelerate at sea level into the supersonic region and use the thick air for about 500-700 m/s. - climb gradually to 16km, whilst gaining speed. Usually crafts are about 1400 - 1500 m/s fast at this point. - use the thin air to accelerate to 1550 m/s. If needed, add Nervs here as well to accelerate faster. - speed is more important than height, so use rockets as soon as you start losing speed again. Fun thing is, for Je ne sais quoi, which rescales to a factor of 2.7x, I had to relearn building SSTOs, but the inital flight profile was more or less the same. If you accelerate at sea level too much, you lose out due to drag. If you accelerate under 16km too much, you can get issues with the temperature. My latest craft has over 41 tons per rapier and uses a combination of Rapiers, Nerv, and Wolfhounds to make orbit in JNSQ with over 3000 m/s dV to spare, enough for a comfortable trip to Minmus.

[@tadghie6193]

@@SSanatobaJR Oxygen levels seem to never really be a huge problem in KSP, but you are right that the rapiers lose thrust in the upper atmosphere. The thrust loss comes from a lack of air pressure, as jet engines function by expanding air with heat and expelling it quickly out the back the same way as a rocket engine works. Less air means less thrust, however if your speed is high enough, your rapiers will still maintain quite a high thrust that will still accelerate you at a decent pace while still in air breathing mode. You'll peak at a point, but the idea is to get the maximum speed from the jet mode before engaging any rockets, since any drag losses from being below about 20km are quite negligible compared to the ISP loss from rocket/nuclear engines. Definitely a really great point though, and I would've mentioned air pressure in my first post if I remembered! c:

[@SSanatobaJR]

@@tadghie6193 I'm sorry, I'm tired and didn't think everything through enough. I've deleted my previous comments. Air pressure and oxygen levels may be playing a factor, but it is more complicated than just that. I will come back to this after I have had a good night's sleep.

[@tadghie6193]

@@SSanatobaJR No need to apologise, we're cool! I appreciated your replies on the post, since I absolutely used to think jet engines gave out at high altitudes because there was too little oxygen. Seeing my comment get likes and replies is super cool since it's rarely happened before, and it was fun to have a little discussion about things with you c:

[@mephisto8101]

@@SSanatobaJR Hi Stephen! No worries, you raised valid questions which have been quite interesting. Long time ago, "air hogging" was a thing in KSP. Basically, you could overcompensate the thin air in higher atmosphere by using lots of intakes. As this resulted in some quite strange possibilities, it was patched out. I believe a hard cap for the altitude of airbreathing engines was implemented. You can test out the behavior of an engine at different speeds and altitudes in Kerbal Engineer, in case you do not have it installed. Kerbal Engineer is highly useful. :) You can display atmospheric behavior of rocket and airbreathing engines at different speeds and altitudes there, without the need for an actual flight. For Rapiers, the airbreathing ISP is fix at 3200, but thrust varies with speed and altitude. Rocket engines have a lower ISP in thicker atmosphere and a higher ISP in vacuum, but not airbreathing engines. So, efficiency stays constantly high, but thrust output varies. This means, your actual fuel consumption is a function of speed (mach number) and air pressure, as you can see here as well for various engines: wiki.kerbalspaceprogram.com/wiki/Jet_engine Maximum thrust of the Rapier is at Mach 3.7, which is then modified by air pressure. Have fun!

[@Carnasa]

I’ve never been one for ssto’s but watching all of these tests and seeing you push this style of craft to their limits has been really fascinating :p

[@skoovee]

hi mr rp1

[@vaos3712]

Loving your videos man!

[@XavierBetoN]

I'm more like a TSTO guy, but only one set of engine, not dumping any engines with the first stage, only tanks. Does it still count as a single stage engine-wise?

[@NickBDesigns]

Boom, notification = space cult meeting

[@PhazzeeYeehaw]

SPACE CULT! SPACE CULT! SPACE CULT!

[@guyman2674]

@@PhazzeeYeehaw Wait a minute, what have I gotten myself into?!

[@petersmythe6462]

It really depends on drag, TWR, and wing area. That being said, almost all of the most optimized SSTOs have extremely low drag, low wing area, and low TWR, with very high portions of the weight being payload and fuel, needing long runways to achieve sustained flight. These benefit tremendously from getting up to speed centimeters from the water.

[@SpecialEDy]

Exactly. I recently built an SSTO that can haul two full 5m long fuel tanks to LKO. It uses 50 something Rapiers and three vectors. You have to hold the brakes for the Rapiers to spool up, and then fire the vectors for the extra thrust and control authority to get off the runway. It ends up a few meters above the water, you absolutely have to skim the water all the way out till your even with the island runway and doing about 400m/s before the Rapiers are getting enough thrust to maintain a positive rate of climb. For massive SSTOs like this, I usually try go flat until I reach about 350-400m/s, then climb at about 10-15° depending on the craft. If you are climbing at the proper angle, you'll never have to change angle of attack and will starve out of air at about 1200-1400m/s.

[@magica3526]

every craft has a unique optimal flight path, basically determined by it's lift to drag ratio at varying velocities and altitudes

[@DistracticusPrime]

I do love this science-style format. Reminds me of the early days of KSP, back in my day before you were born, when we were distracting Orbiter players and making aerospace engineering students late for class. This experiment reminds me of Closette's "Goddard Dilemma Challenge". Do you remember? Take this OP rocket as high as you can by reducing thrust at just the right time.That similar dilemma Robert Goddard figured out was all about balancing the flight time fighting gravity against exponential drag due to speed. This seems similar to me, because I get the impression it's all about keeping the rapier engines near their performance "sweet spot" while avoiding excessive drag. Remember aerodynamic drag increases exponentially with speed. So, somewhere in the middle, the drag is moderate _and_ the engines are happy. Someone please kidnap a calculus student to help us confirm this?

[@21Trainman]

That *long time* comment made me realize I’ve been subscribed for going on 6 years now. It’s been great to watch your channel progress dude! Awesome content, keep it up!

[@hoffmankspengineering2034]

Maybe SSTOs with more drag than usual would prefer to ascend a bit first? I've had an SSTO that couldn't break past 400 m/s in the low atmosphere, but could reach orbit if it climbed first. It was probably a byproduct of it having too many wings. I had to accelerate to around 300 m/s, climb to around 12000m, then level out and accelerate again, which was kind of a pain, but in that case a necessity.

[@lewismassie]

It's funny that your smaller one failed to fly when over 20 degrees, since I flew it over a hundred different flight paths, and anything over 20 on that initial pitch up failed in exactly the same way. My going theory is that while it is _theoretically_ more efficient, the extra mass for the extra engines, and the drag loss as you pitch down removes all those benefits. If you're curious, I could make a modification to my kOS ascent program to log thrust, acceleration, lift, drag, velocity, altitude and similar and fly some ascents for you to compare

[@miltoska9708]

My theory is that the rapiers are bad for this profile I think a lower tech SSTO with separate rocket and jet engines would do better

[@lewismassie]

@@miltoska9708 The rapiers are supposed to be the ones that function the best in the upper atmosphere, second too the whiplash. Lower tech designs might be something interesting to test

[@MrQuantumInc]

I built an SSTO around the assumption I would need to stay between 20,000m and 30,000m for as long as possible but it has been very frustrating, unreliable, and inefficient. Changing your rate of ascent (angle of attack) is surprisingly difficult, so I would burn most of my rocket fuel while still around 30km. I was worried about air drag but really that itself explains why a constant rate of ascent is better. As you go faster you go higher making drag and lift roughly constant. It is also possible I don't understand how air intakes work, lol.

[@ChrisHovord]

16.9 subs! The space cult is growing 😆🤗🥳 Congrats VAOS 👏 your hard work is paying off

[@rouzbeakhlaghi3038]

Appreciate all your hard work. Been a subscriber for a long time and I appreciate you doing all the series that you do.

[@ravener96]

i dont think delta v is the be all end all measurement, since the rapier will only burn the oxygen in rocket mode. its tonnage that matters. basically if you knew your flight profile perfectly you should bring just enough liquid fuel for the air part of the launch, and then be perfectly balanced when in space. if your delta v is lower and tonnage is higher it actually just means your fuel oxidizer load is unbalanced.

[@minerscale]

yeah who cares about stoichiometric ratio, that can be calculated later!

[@h.a.9880]

It kinda depends. The delta-v in orbit is entirely based on LF and OX and there should be always more LF than OX unless you loitered too long in air-breathing mode. One could treat that plus in LF in orbit as a negative, since it's dead weight... but when you're going to land your plane at an airfield, that small rest of LF extends the coasting ability of your SSTO and allows you to correct small errors resulting from your approach. Delta-V might not be the be-all-end-all means to compare SSTOs with entirely different mission profiles (ie: a small unmanned 20 ton SSTO that's supposed to go to Minmus and back will always have more Delta-V than a huge cargo 200ton SSTO that hauls 50tons of fuel into low kerbin orbit), but you can compare the efficiency of launch profiles for one specific SSTO. Say, that 20ton Minmus SSTO: When you fly two different types of ascend profiles and one ends up with 200m/s more than the other, you now that ascend profile is better by a very specific and very relevant metric.

[@ravener96]

@@h.a.9880 no it doesent really depend. having too much liquid fuel or oxygen means you have dead mass that doesent contribute to delta v. if you want LF to land again you can add that in yourself, it doesent make delta v in orbit any better a measurement

[@h.a.9880]

@@ravener96 Are you aware you contradict yourself? First you call LF in orbit dead weight, then you point out that one can carry LF (ie: dead weight) if it is needed for landing. And how is delta-v not a good measurement in orbit, pray tell what are you doing in orbit that makes delta-v irrelevant? If you want to rendezvous with anything, you need delta-v to get there and back down again. What is a good indicator of a flight profile's efficiency if it isn't fuel consumption?

[@handy-capoutdoors4063]

I've been using the strain blitz or Matt lowne acent profile rule of thumb. For a small ssto I go for 30 to 40 degrees because the twr is plenty for that . And the bigger the ssto the more shallow my accent. 20 for medium and 5 to 10 degrees for large sstos. Since there is more fuel to burn you can sustain the low atmosphere longer to get up to speed for the rapier engines to get into their power curve around 400m/s then just tilt up a bit and let it carry you to space. A small ssto with high twr can take a steeper acent and it needs to as it can't carry as much fuel. You can conserve some by cutting thrust when you have an 80k apolapsys. You will slow down a little but you won't have to burn all the way up. Then when you get close to apolapsys burn towards circulisation. You should make it no problem... how much left though is the question. Dock and refuel in space. Put a monopropelant engine on it so you have a backup for maneuvering to a refueling station or enough delta v to get you back into a decent

[@kommandantgalileo]

Second SSTO: First Launch: 273 liquid fuel left Second Launch: 289 liquid fuel left Third Launch: 326 liquid fuel left Conclusion: third launch is better

[@benzobean5890]

One other thing to keep in mind is that jet engine thrust does not only change with altitude, it changes with speed. Engines like the Whiplash and Rapier, which are more ramjet style engines, gain thrust massivly at higher speeds, like at 450-500m/s. So when you start ascending right after take off, you can't gain as much speed, so the engines don't gain thrust. Then when you do level off, the higher altitude means less thrust, so less speed. So maybe try getting speed at sea-level, but not to much, like 500m/s, and then start your climb. Keep testing though, I feel like we could really be on to something!

[@VestedUTuber]

This is basically why I tend to like to just sit at 5-10 degrees upward and just let the craft accelerate for the first 1000m/s. The craft only ends up climbing once it gets into peak thrust.

[@bobc2636]

Nice video, i felt the same when i tested the same thing. Getting to and using the air intake feedback (about 450 when your twr starts increasing rapidly) is easier at low altitude. Even though the atmo is thinner and easier to move through it is also less air and thus less power as well. On a different note, you may find using the nuclear engines instead of the rapier's rocket mode may help to get a better delta V number from assent profiles due to only using liquid fuel for the whole flight and calculating one fuel left over at the end instead of both liq and ox ( that may not use all of one due to the lack of the other).

[@johnnyclik]

Very interesting, i like your approach to figuring this out. Havent played ksp in a while so dont know if changes were made to engine physics since then, but i believe you can reasonably break it down to thrust to weight ratios of the craft at different altitudes and speeds. Lower TWR in big crafts doesn't allow you to figt gravity ad gain speed without being efficient, as you conclude. For a smaller, more nimble craft, however, would the best ascent profile be similar in shape to the thrust curve that rapier engine in air breathing mode mapped on a chart? this would try to keep the craft in the engine's most efficient range as long as possible. (to the limits of the aircrafts ability i guess)

[@AD_SPACE_2024_...Aditya...]

12:37 basically you mean its not universal technique for the most efficient takeoff.

[@Anne-py1lt]

Space cult time

[@eekee6034]

It seems to me that if you want a heavy SSTO to climb early, you could choose between more wing area or more engine power. Once in space, that translates to more wing mass or more engine mass. I wonder what the best balance is? (Yes, I am hinting for another round of tests. :D ) That big heavy SSTO with the Mk3 side tanks looks great! You got the best numbers staying in the atmosphere with the dragless SSTO. I don't think the size was the main factor there. ;) Caps lock only helps if you make very brief taps, as I learned when I found my recent spaceplane build outrageously overpowered in its first test flight. While I was busy peering at drag triangles, it easily exceeded 1.5km/s at only 5km altitude. Obviously, I wanted to make it climb so it wouldn't burn up and so I could better monitor the drag, but I pressed too long, there was a bang, and suddenly I was only flying the fuselage. :) My excuse is the elevons weren't yet tuned. Roll was very weak and vague, I had to give it long presses. Pitch was the opposite.

[@aidan9411]

Day 2 of petitioning vaos to make the least aerodynamic air breathing SSTO Possible. Also, I like cake

[@Dakitess]

Well, i'm still a bit surprised to not see a full passive ascent in thoses comparisons. The thing is, with most part properly covered (i.e. proper SSTO Design even not using dirty fairing alpacas and co), drag comes from relative angle to airflow, your AoA. And for most SSTO (95% of them, being properly designed when it comes to rapiers to mass and wings to mass ratios), the ideal ascent is to push your rapiers at more than 420kN of thrust before climbing, which is only possible if you're about mach2.5 at sea level or about. And then, don't touch anything, kerbin curvature will make your nose up without needing even a touch on commands, avoiding any sudden AoA and drag, your craft will simply go from 0° at 1km to 15° at 20km, no need to pitch up, not even a little. By the time you hit 30km, follow prograde, your craft should be able to push its Apoaps forward without requiring to nose up. Do not nose up. Ever. If you have to do so, then just wait 35km instead of 30km for Prograde Locking. I look forward to see your comparison using that little rule, I quite never see it in video even among the one that claim comparison. Nothing to be rude here, sorry for my english that may sound rude, it's just about sharing tips and advices, i've been into SSTO for years and years :p Also, next time, might be cool to introduce PowerDiving to handle underpowered optimized SSTO that can't get past mach1 without a little prograde diving :D Good video as usual, of course :)

[@LeonardoDaVinci01]

Oh how I love the Jupiter Star.. I always used to build SSTOs like that when I first started playing the game back in 2015… man that feels so long ago

[@luism.5853]

If you install mechjeb you can record your launch with altitude, delta V and stuff like that. That maybe helps while testing.

[@romanr1592]

Hey, not sure if relevant or not, though I am definitely gaining speed at sea level - but I've made a 20t dry mass cargo SSTO that delivers orange tank to orbit with 1000 m/s delta v to spare. No clipping or fairing exploits, no craken drive, only 2 rapiers and 2 nervas.

[@romanr1592]

On a further revision I'm realizing that I'm using a quite unique sin curve ascend trajectory - I start on the green way back behind the runway (those lazy kerbals refuse to build a runway long enough), then fly meters away from the sea till I break the sound barrier. Once rapiers really kick in and give the thrust that they can I climb pretty rapidly to about 17 km where I level out and start to build up speed while climbing very slowly with the hope of reaching about 1600 m/s at about 21 km on rapiers alone before adding nervas into the mix. After that I keep climbing very slowly till I reach an almost orbital velocity at around 30 km and apogee of about 90. Then I just coast while facing prograde and circularize.

[@ValentineC137]

Spase kult

[@AlexandreLollini]

Look at your TWR : at what altitude it's maximal ? (different for each craft), then start a level flight at that altitude starting from 330ms minimum when orange ionisation starts, make 15° up. (in general you can't stay level for long my TWR can reach 2,9) That's how I get the most delta-V left in space. Personally I have problems with engines, the atomic shuts off so I have to manual trust it when in space don't understand why.

[@Mike-gy4mh]

Helpful. Nice work.

[@Albert-yu1cg]

what about the very low TWR of ions and LV-Ns in space? for very long range craft that don't have enough engines to circularise at 70km

[@vaos3712]

🤷‍♂️

[@sethdrake7551]

what if you take this further and use a sea level approach initally then pitch up toward the end?

[@currentcommentor8745]

Have u ever tried making low tech sites e.g. where you use Juno or whiplash engines.

[@vaos3712]

Yes. The videos are somewhere buried in my channel.

[@TheWanderfound]

Are you leaving the RAPIERS on auto-switching? You can get better results if you take manual control of that.

[@crashstudi0s]

I never knew when to do the manual switch, any tips for that?

[@mephisto8101]

@@crashstudi0s As soon as you start to lose speed again. Speed is more important than altitude in most cases.

[@TheWanderfound]

@@crashstudi0s In part, it depends upon circumstance. But, as a general rule of thumb, y'wanna switch the RAPIERs to closed cycle once you begin to lose speed. Which, if you've flown your ascent right, should be somewhere in the 25-30,000m altitude range, and your speed should be not too far below 1,500m/s.

[@kaminelson1277]

Can you test using the fearing trick with shock cones and release the fairing at higher altitudes so that they can get more air at the higher altitude with less drag in the lower altitudes

[@sebdapleb1523]

then it's not an ssto

[@kaminelson1277]

@@sebdapleb1523 yep forgot about that

[@GABRIEL-ej4ei]

Nice vid, keep it up and please get waterfall mod ❤❤🙏

[@matsv201]

I still want it 3 motor.

[@3DRC-707]

Shhhhhhpace cult.

[@techsbyglebbagrov7470]

Would a stair-like approach work? (Level speed up in thick part to climb to thin part, level, speed up in thin part to get up into space. Part as in part of the atmosphere)

[@MrrVlad]

yes and no. yes for lower tech engines like panther that may need extra time in 5-9km range to get to 800m/s. In this case you gently tap to loose a few degrees of pitch. for a 1Xpanther + 2Xterrier ssto you may want to stage around 10-12km at 800-830m/s with ~10 degree pitch and then make sure angle of attack won't exceed 6-7 degrees while you accelerate around 23-26km altitude. No - for rapiers, if you can get airborne, you usually have enough lift and thrust to use the sea-level constant arc with no inputs.

[@romanr1592]

I've made a video for that SSTO, I've been talking about (it's the only one on my channel). Pretty sure that is the most efficient design, or pretty close to it with payload fraction nearly 50%.

[@friedec3622]

Sea level route is great if you have low TWR but can build up speed. Using wing to lift the vehicle into thinner atmosphere.

[@patrickscott5605]

Hey Vaos im thinking of making a gaming channel with daily uploads of other games including ksp. do you have any advice ? i plan on doing a space news and concept designing lmk what you think!

[@vaos3712]

Never give up. Even if it looks hopeless. The KZbin algorithm will eventually pick you up. And the channel will begin to slowly grow.

[@adityagohil9353]

What does space cult mean?(i am new to the channel)

[@vaos3712]

It’s a fun in channel joke in a way.

[@adityagohil9353]

@@vaos3712 cool

[@nathangibbs89]

Sea lvl till 440 m/s to get more thrust from the rapier

[@generalcat7678]

I want you to continue the solar nation series

[@emqueue1h]

shadowzone again my beloved

[@krolon9786]

can't judge the stock but for FERRAM the path should have much bigger impact, because flying supersonic has incredibly higher drags, small planes have as much drag when supersonic at sealevel as big SSTOs at subsonic, so going high in FERRAM is very valid, but STOCK doesn't have that steep drag speed curve, it's exceptionally easy to go supersonic in stock, it just happens, with every design that isn't some big ytber big stuff, meanwhile for FERRAM you really have to go mo boosters route if you can't design a shit with good angles

[@crucialbeatle7935]

Why does this video make me want a vaos face reveal lmao

[@luisvega6580]

Cool vid

[@elektron2kim666]

You don't need oxidizer. A Nerv engine or 10 makes another story.

[@unidentifiedbiomass4106]

This is why I uh prefer the 2STO....

[@kommandantgalileo]

First SSTO: first launch: 2,365 liquid fuel left second launch: 2,949 liquid fuel left third launch: 2,777 liquid fuel left fourth launch: 2,899 liquid fuel left conclusion: second flight path is better

[@VestedUTuber]

You'd ideally want to run each path multiple times to get an average.

[@kommandantgalileo]

@@VestedUTuber true

[@harriwilen9445]

Cookie

[@AdamSchadow]

The reason you dont see much difference is because your sstos are not that well optimized the ideal ssto has almost no wing almost no engines and almost no drag while having a lot of liquid fuel and payload.

[@Nicolas_Gamer5476]

hey

[@theillegalgamer5298]

I like pancakes 🥞

[@miguelfranciscogonzalezmor5779]

The best SSTO, better than Matt Lowne ones

[@K.O240]

The point of an SSTO space plane is NOT to get out of the atmosphere as quickly as possible. What makes these style of craft unique? They use wings and air breathing engines. Both of which become useless once out of the atmosphere. With RAPIERs, especially on your example craft which have excessive TWRs, you should be achieving 1700m/s+ on air breathing mode. You're ~200m/s short of this which is a lot of delta-v to make up with the RAPIER's awful closed cycle efficiency. The purpose of air breathing mode is not to get as high as possible, but as fast as possible. Orbit is about velocity, not altitude. This brings us to the wings. Wings are an amazing tool that let you get into the air with a TWR

[@theillegalgamer5298]

I AM GROOT

[@andredeguemon]

sorb

[@JollyRogerAerospace]

Yes you are twisted. Lmao

[@40watt53]

>"Why am I losing subscribers?" >Doesn't do anything creative.

[@mussalo]

Could you leave out the constant clipping to edit the talk? It is very annoying and along with the jerking video takes away focus from the actual point you're presenting. Unnecessary and overrated relic of way to do yt videos.

[@aperson2294]

Hmm… Seems like another Passionate Review…