The Ullage Motor: Small but very important

A rocket motor with relatively little thrust is the key to restarting Spacecraft engines.
NOTE: See Pinned comment for some corrections

[@jimscarborough2568]

One nitpick: the horizontal velocity doesn't create microgravity. The fact that nothing's pushing on it does. The same microgravity is achieved in the KC-135 when it flies a parabolic trajectory. Something *IS* pushing on it, though: air. The engines have to exactly counteract the push of the air to keep the plane flying downward at the right speed to make it appear, from the inside of the plane, to be in free-fall. They are accelerating toward the earth, and have to pull out of it before hitting the ground or going too fast for the plane.

[@ReflectiveLayerFilm]

Correct. I was literally lost for words at the time of the writing when I realized that that part of the script would require a longer explanation. So now it's complete. Thanks.

[@james1234168]

GiantWaffle brought me here.

[@vaderbase]

I had no idea what these 'tiny' rockets are used for... thank you so much for this clip.

[@bepisbepi]

Just a little detail since i've seen a lot of people wondering about it. Ullage motors, such as the ones used in the Saturn IB and the Saturn V, are not liquid-fueled, but SOLID-fueled. They are self-contained and don't require ullage themselves. In the rare cases (unwanted complexity) of liquid-fueled ullage motors, they are turned on right before MECO so that the remaining thrust from the MEs acts as ullage for... the ullage motors themselves.

[@ReflectiveLayerFilm]

Thanks for adding additional detail to the video.

[@UrbanOdyssey]

You're content is underrated my friend. You have excellent explanations to this type of science and more people need to do this. If I could suggest a video for you to do is on the LEM. Specifically explaining why certain inputs where needed. For instance: ACA out of detent and Mode Control Both Auto when a few seconds later the AGS 413 command was enabled to power off the abort systems anyway. (That's always baffled me lol)

[@ReflectiveLayerFilm]

Yeah I really like the LEM. Was planning on doing a video about it a while back but never got around to it. I'll look into doing a style of video that's part history an part how things work.

[@MattH-wg7ou]

0:05 "This LITTLE rocket..." 🤣 Maybe I'm wrong, but I think the Saturn V is the most awesome thing humans have ever built.

[@ReflectiveLayerFilm]

Yeah I think Saturn V is the most awesome rocket by sheer size and its unique mission. What I was talking about at 0:05 was the little rockets attached to the second stage that fired during the intro. That's what the video is about, the Ullage motors.

[@MattH-wg7ou]

@@ReflectiveLayerFilm I know, I just thought it was funny, with a picture of the Saturn V with that comment. I was just being a smart ass 🙂

[@ReflectiveLayerFilm]

@@MattH-wg7ou That's cool. For some reason the emojis don't shown up correctly on my computer. They show up as boxes. That why I didn't get it.

[@willys4869]

These ullage motors were built and tested about 4 miles from my house in Neosho, Missouri.

[@darylcheshire1618]

Ullage is interesting, it comes up in so many ways. For example the Boston molasses disaster, it was claimed that the tank which ruptured was filled to the brim without ullage and on a very hot day it all expanded causing the tank to rupture. It doesn’t help that the tank was old and not designed for molasses. There was a rumour that the molasses was rushed in so the company can start making alcohol before Prohibition kicked in. I might be wrong, I thought it was OK to make alcohol for non drinking purposes. In Mt Isa in Queensland, the local taxi drivers have LPG tanks, they have defeated the ullage system so they can cram lots of LPG at the start of the day when it’s cool. They then drive around picking up passengers and creating this space. If this fuel is not used up, the pressure builds up and the safety valve opens spilling and wasting the LPG.

[@rorypenstock1763]

For regeneratively cooled rocket engines, ingesting a gas bubble would cause much more of a problem than a little lost thrust! The gas would reach the cooling channels first, and the lack of cooling would cause the engine to melt.

[@ReflectiveLayerFilm]

Good point and additional detail.

[@rorypenstock1763]

@@ReflectiveLayerFilm Your in-depth content is really underrated. You deserve more subs.

[@juni172]

Very good video. Thank you!

[@doodleboi7034]

Thanks!You Helped me Understand A simple but riddling One!

[@ReflectiveLayerFilm]

Awesome.

[@basile1286]

this channel is such a gem!

[@parveen7520]

I was always curious about these projections on the rocket, now i now what they are and what for. Thanks to you buddy

[@elishaafk5396]

just what i needed to understand. very well explained :)

[@angelleal3005]

Awesome video, great animation !

[@ReflectiveLayerFilm]

Thank you! Cheers!

[@easygoing2479]

Nice video, a good explanation of why ullage motors are needed in multi-stage liquid-fueled rockets. Just a simple point that may need attention in the animation: In the cutaway of the Saturn V staging process, when the first stage separates the animation correctly shows the second stage ullage motors firing but also shows the liquid fuels being forced to the _top_ of the tank rather than the bottom of the tank where it flows to the pumps and subsequently fed into the combustion chamber of the engines. When the ullage motors fire the fuel should be forced to the _bottom_ of the tank, towards the fuel pumps and rocket engines. Other than that, a good, informative video.

[@ReflectiveLayerFilm]

Thanks for the comment. In the animation the colored sections(green and blue) represent the liquid fuel and oxidizers. The many small gray circles represents gas bubbles and they move towards the top of the tank. I may have made the animation confusing when I stated that the fuels is floating around but shown it as a solid non-moving color.

[@easygoing2479]

@@ReflectiveLayerFilm Thanks! Sorry, I just saw the whole animation backwards, dark as liquid, light as gas. 🫣

[@ronaldtartaglia4459]

I want to lmow more about the flame trench.😊

[@dfdgfdgf0000]

Thx a lot

[@trabosci360]

This is a pretty neat solution. So this is how the SpaceX rockets land themselves back on Earth; by burning ullage motors retrograde to restart the engine? Same with the moon lander? Interesting to think about how the fuel inside the tanks remains in freefall while the outer casing of the tank is accelerated around it so that the fuel ends up at the bottom. Great video and animations!

[@nreuter]

Great vid! Thx

[@andrewnelson4386]

Great video

[@skateboardingjesus4006]

Great video. Just one thing though, and I'm not being pedantic; you said the ullage motors aren't to push the rocket forward, but to push the fuel into the bottom of the tanks? It does this only because it does push the rocket forward, in order to create an accelerating force. Still a well done video 👍

[@ReflectiveLayerFilm]

You are correct. I only said this to illustrate that its purpose is to settle the tanks not to provide useful motion. This is to contrast the other engines/motors which purpose is to move to rocket through a distance.

[@skateboardingjesus4006]

@@ReflectiveLayerFilm That's understandable. Again, great video 👍.

[@amirm3621]

So how RCS trusters work? They use hipergalic fuel

[@SergKiev87]

So, what will happen if you try to start engine with air pockets in the tank? May I remind you that engine underperforming for a few seconds is not a problem for that second stage. It once flew without two of it's engines and flight still was successful.

[@hubbsllc]

I figure it'd probably sputter, perhaps tear up the turbopumps or the engine.

[@sindhu9084]

Well it's best for 100 percent success rather than luck.

[@andrewhooper7603]

based ullage motor

[@NorthernChev]

If only you had mentioned how they are fueled, considering the video is on how rocket engines keep the fuel where you want it - which it isn't during staging - and ullage motors are little more than rocket engines. So, where does the ullage motor get its fuel and how it IT kept primed? Solid rocket fuel? Are they mini SRBs?

[@ReflectiveLayerFilm]

The ullage motor uses solid fuel. A liquid fuel ullage motor would have the same issue with fuel as the main rocket. Cold gas thrusters can also be used as ullage motors.

[@spacenout_6946]

Falcon 9 uses cold gas thrusters on its 2nd stage not ullage motors.

[@doxielain2231]

Those are still ullage motors, just using a different propulsion fuel.

[@leewilliam3417]

Mmmmm😊