1. Only first generations of "Soyuz" used H2O2 propellant, because it have very limited time before use (for "Soyuz", guaranteed 6 months), and because it have relatively high melting temperature.
First chose H2O2, because "Soyuz" planned as independent ship, to fly relatively short missions around Moon (it is near impossible to withstand even 6 months in so small volume).
When "Soyuz" primary role become companion ship for space station, it switched to hydrazine type propellant.
2. In pressure fed engines used almost all possible propellants and oxidizers.
This is not error just clarification.
3. Exists three-propellant engines. For example, exists soviet engine for "spiral" system, which used kerosene+LOX+LH. First it run on mostly kerosene+LOX, with small percent of LH, than switched to pure LOX+LH (sure, LOX share also other).
4. Exists simpler bipropellant engines than mentioned, unfortunately with worse efficiency. - First Britain satellite flown on H2O2+RP1 Black Arrow rocket.
> I've frequently read in news articles that the lifetime of the Soyuz spacecraft on orbit is limited by H2O2. You appear to be saying that's false
May be I misunderstood, what said by insiders. As I remember, first "Soyuz" spacecrafts was maid with H2O2 system, but than they made lot of changes, including new reaction control system.
But looks logically, system used in space separated from system used on atmosphere.
And because "Soyuz" landing on dry land, and because accuracy of landing is its weak side, it is also logical to not use toxic components.
I think you remember, how long astronauts stay inside "Dragon" once, because sensors shown hydrazine fuel vapor near ship.
- American ships
> Perhaps you can point at a source?
Sorry, it is not easy now.
It is not secret. Before approximately 2005 I talked a lot on Russian forums, and there was specialists from Russian space industry, and they shared extremely interest info, and I even considered to go work there.
Plus books, mostly from Russian authors.
After ~2005, appeared serious tension in talks, so I avoid them, to save my own psychic health.
I cannot be sure, but looks like, it was another step in preparing for big war.
And you are right, that open information on Russian technologies is very limited. This is partially, because they considered nearly all space technologies as semi-military, so some information just classified. Other part is that Russians extremely conservative in business behaviors, and one thing, they inherit form Soviets - very big love to close nearly all information, even harmless and useless, just to show only good info, to look better, to look winner.
What I mean about Soviets - typically, they made launches without announces, and only talk about them if success.
Because of this exists "kosmos" satellites, some of which was planned to flight with this designation, but mostly these are unsuccessful satellites, which reach orbit (and appeared in NORAD list) but does not function from beginning.
If satellite not reached orbit (for example because of malfunction of rocket), and NORAD don't catch it, nothing said at all. If reached some orbit, but malfunction - appears new "kosmos". Because of this behavior, in late USSR was about 1500 "kosmos" sats, and now this number more than 2000.
One thing I always wanted to see was a close expander cycle areospike. The reason is that expander cycles are heat limited. Areospikes biggest negative is that they require more cooling but if you are heat limited that is an advantage.
You could make the highest possible thrust expander cycle and it would be high efficiency upper stage for early staging as you don't lose efficiency by staging early.
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Another thing that jumps out at me is that its really sad development on the F-1 and J-2 stopped. The mentioned J-2S never flew. The US had this amazing technology stack, F-1 + variants, J-2 + variants and Apollo stack.
Instead of keeping around a Saturn 1B type vehicle with the Apollo on top they threw literally all of that stuff in the trash and started new with Shuttle.
But non of that new Shuttle stuff is better when thinking about it end to end. Because of Shuttle SkyLab could not be saved and because of Shuttle SkyLab 2 wasn't launched.
The US with the Apollo stack and incremental updates could have dominated in space.
A complete and utter mismanagement of the investment that was made during Appollo.
Regarding the first point, I'm not sure whether aerospike engines can still be considered heat limited. Even in the 90s with the X-33 aerospike spacecraft, heat was not cited as the reason for cancellation. And nowadays material science as progressed by a lot, which resulted in more heat resistant materials, e.g. this one:
https://www.nasa.gov/feature/glenn/2022/nasa-s-new-material-...
Can someone ELI5 to me the physics behind the divergent part of a de Laval nozzle? I know that in a straight pipe subsonic flow tends to accelerate towards M=1 and supersonic flow tends to slow down towards M=1, I know what choked flow is and generally how the convergent-divergent design works, but for the life of me I can't find anywhere an understandable, non-hand-wavy explanation of WHY supersonic flow in a divergent nozzle does what it does.
Basically, it would already work if you left out the diverging part.
However, the exhaust has still excess pressure.
You want to make it do work while expanding to ambient pressure, so you provide a ring around the throat where the exhaust pressure pushes against to direct the exhaust momentum vector for every part of the exhaust plume to be as close to retrograde as possible.
Sideways momentum is wasted momentum.
To actually get it to push against your ring, the diameter can't increase too fast, because the sideways flow velocity of the exhaust inside your bell needs to be subsonic. So you first increase steeply, because it's still high pressure and thus hot and thus has a high speed-of-sound, and gradually reduce how fast you increase (gradually reducing the taper).
Eventually you have expanded it to a pressure equalling ambient pressure, and won't get more thrust from further expansion (doing more also causes flow separation at the edge where ambient air pushes the plume radially inwards and separates it from the inside surface of the bell. The supersonic shock effects can break your bell if you're not careful.).
Also at low pressure you get little thrust per additional nozzle area, while the low temperature requires a low taper that requires a lot of surface for the additional nozzle area. That is why even vacuum engines don't go down to extremely low exhaust pressure.
IIRC you can only ever double your momentum with an expanding nozzle, even in the asymptotic limit of an infinite bell in perfect vacuum and compared to a knife edge throat.
Fundamentally, rocket nozzles are weird open-cycle heat engines subject to the Carnot limit. Thrust times (exhaust) velocity is power, and the chamber (combustion) temperature your hot side.
If your exhaust has a different molecular mix than the atmosphere, there is some additional energy to theoretically gain from there. Otherwise, you should have IIUC reached outside temperature when you reach outside pressure. Or maybe you would need to expand to outside temperature regardless of pressure to scratch at Carnot-efficiency...
Quote: On the other hand, if the converging section is small enough so that the flow chokes in the throat, then a slight increase in area causes the flow to go supersonic. For a supersonic flow (M > 1) the term multiplying velocity change is negative (1 - M^2 < 0). Then an increase in the area (dA > 0) produces an increase in the velocity (dV > 0). This effect is exactly the opposite of what happens subsonically. Why the big difference? Because, to conserve mass in a supersonic (compressible) flow, both the density and the velocity are changing as we change the area. For subsonic (incompressible) flows, the density remains fairly constant, so the increase in area produces only a change in velocity. But in supersonic flows, there are two changes; the velocity and the density. The equation:
- (M^2) * dV / V = dr / r
tells us that for M > 1, the change in density is much greater than the change in velocity. To conserve both mass and momentum in a supersonic flow, the velocity increases and the density decreases as the area is increased.
That's why I was asking for an ELI5 explanation. I know that the equation holds and that compressibility turns everything upside down, I just haven't been able to figure out an intuitive explanation.
I have this idea that in supersonic flow, a pressure wave can't move backwards against the flow, right? Which would mean that any single molecule inside the flow has no way of knowing what's in front of it (because the information simply can't get there), but it feels the pressure of the molecules behind it, so it accelerates towards the void.
The "Fanno flow" article on Wikipedia says that "... For a flow with an upstream Mach number greater than 1.0 in a sufficiently long enough duct, deceleration occurs and the flow can become choked ... Conversely, the Mach number of a supersonic flow will decrease until the flow is choked.", which means that supersonic flow behaves differently in a diverging nozzle than in a simple straight pipe. This is the part that I don't understand. Is the friction inside the nozzle somehow inhibited by the walls of the nozzle gradually moving out of the flow's way or something?
Well, the slow-down won't happen until after it exits the nozzle (c.f. Mach diamonds in rocket exhaust... they are the shocks where the supersonic flow interacts with the ambient air).
It is a great resource to learn about all different kind of rockets and rocket engines. And quite amazing to see how 3d printing can allowto build so many of miniature versions.
Integza is one of my favorite channels of all time, his videos are always really informative and fun to watch. The only problem is he’s completely careless about safety—mainly by not wearing any protective gear for the exhaust fumes which include burned plastic.
I’m really torn on Integza. It’s a great channel and I love his spirit, but guys like that (and the teenagers that follow them) end up getting hurt or burning down their houses, which makes the news and gets new restrictions passed on all of us.
Just a _little_ more nodding to safety would make him an awesome resource.
Maybe safety culture in Portugal is different, but in the US he does tons of stuff that violates the model rocket safety code, or is outright stupid.
There is something that separates Integza, colinfurze et al, and Applied Science, Huygens Optics et al, and another in guys making thesis/paper worthy developments like [0],[1]. They are technically all amateurs, in the sense that their performances do not involve organizational fundings or decisions, and I have to admit that entertainment value decreases in the order I mentioned these channels, but it seems to me as if there is a sloped ceiling for technical excellence of YouTubers, proportional to their popularity, and it isn't calming to think about it.
I wouldn’t consider the rocketry code itself a good argument. The model rocketry code, frankly, is biased hard against any kind of liquid (especially non-hybrid) rocket engines, so there’s not even really a way he could follow it with a liquid in any reasonable way.
(Which isn’t to say he couldn’t follow better safety common sense.)
The model rocketry code is for model rockets, not high power amateur rocketry. Though the HPR safety code also isn't friendly to liquids, but partly because it still dictates commercially developed motors.
+1 for Integza. You should probably include a disclaimer that it is backyard engineering at its finest. He doesn't really do a lot of hard science in his videos. He has great ideas and the persistence to get them at least semi-functional.
Tim Dodd is a world treasure when it comes to space education. The material he's produced are timeless. Thanks Tim for the quality and down to earth content you've made and all the behind the scenes interviews with Musk and other rocket companies.
Has anyone ever tried to capture outside air and mix it in the rocket exhaust? The reason would that more mass expelled at lower velocity can produce the same thrust with less energy.
Momentum is MV while kinetic energy is 1/2 MV*2. what a rocket really needs is change in momentum, so high energy exhaust.
Maybe too small an effect, but what if they put 8 NACA scoops on the sides of Falcon 9 and just fed the air in between the center and outer engines. It would get accelerated out with all the regular exhaust, and I suppose would cause a small back pressure on the center engine in particular, but they could throttle it slightly if that were a problem.
This approach is often called a thrust augmentor, e.g. https://doi.org/10.1023/B:FLUI.0000045678.92653.98. Basically you're using excess energy in a high-speed jet exhaust to accelerate ambient air, increasing the total momentum at a lower exhaust speed.
I wonder about screwing a solid slug of aluminum down through the top of the combustion chamber during the first few seconds of launch, to get more massive exhaust particles (Al2O3) when you need that most. The top of the slug would be something refractory to seal the hole when the top end hits the stop.
The energetics of burning aluminum are pretty favorable, enough so that there are solid rocket boosters that use it. 1500 C is hot enough to liquify aluminum pretty vigorously, but not boil it.
For all the talk of SpaceX and a new race to Mars, no one seems to remark on the fact that rockets haven’t remarkably changed much since Goddard’s day.
I remember asking one of my profs in college (an early researcher of the ramjet) what’s holding jet and rocket technology back. He said: melting point temps.
That's very debatable :) - everything is everything else, if squinting hard enough. Just an injector head of F-1 was a serious R&D topic, with whole methods of experiments invented.
Obviously there was a lot of refinement and development going on in those engines.
For another example, the Ohain axial flow turbojet is quite recognizable even up to modern jet engines. The Whittle radial flow turbojet was a dead end.
Innovations 1..3 are all revolutionary, not evolutionary, advances in rocket engines. 4 maybe is.
I mean one uses the main propellant to drive the turbine the other has a sodium permanganate/hydrogen peroxide. That seems to me a pretty significant difference in how the engine works.
To be fair, technically one could call both gas generators but its still significantly different.
I'm sorry, but picking a different source of hot gas to drive the turbopump is not a conceptual difference. Just like adding nitrous injection to your car doesn't make it a conceptually different engine.
Well, they are all chemical engines. We have no chemical engines that would work well for launching.
In space we do use a lot of solar electric propulsion and lots other things that Goddard knew nothing about.
Nuclear thermal propulsion could potentially be used but that has a whole host of issues where its not clear that its actually worth it compared to chemical.
SpaceX Raptor is approaching pretty much the peak of what is doable with chemical and if its fully and rapidly reusable it can bring the price to orbit down.
What really matters is not what method you use, but how much does it cost to go to orbit, or from LEO to Mars. From that perspective something like Starship is on a totally different level then anything that came before.
“Well, they are all chemical engines. We have no chemical engines that would work well for launching.”
I mean that’s what chemical engines are for. You aren’t launching off the ground with any of the electric based systems, and accidental radiation concerns have always hobbled nuclear engines.
But, yes point taken on using them outside the atmosphere. However, they don’t improve our ability to launch manned missions greatly (at least not for the foreseeable decades).
I've often wondered if "gravity feed" might be an option. In a tall liquid fueled rocket, Gravity, then thrust, could create arbitrarily high pressures at the nozzle if the column of fuel is tall enough.
Expert here. I'm a longtime lurker on HN, but registered just for this comment. I design aerospace pressure vessels for a living. As showerst alluded to, there are often things called "propellant management devices" or PMDs. There are a few reasons these might be used. In zero G, you need the propellant sump to remain wetted with propellant; it would be bad to ingest the pressurant/ullage gas into your engines. Some PMDs allow the pressurant and propellant to occupy the same volume, and will use surface tension devices inside the tank to direct liquid to the sump. These can be screens, vanes, channels, etc. In other cases, the pressurant and propellant are kept separate by a bladder. These bladders can be rubber, or even metal. The propellant mass is usually a large proportion of a space vehicle's mass, and you can't have that much sloshing around when you need dynamic control. A metal diaphragm keeps the propellant more or less static, and its center of mass in a predictable location.
As for the pressurant gas dissolving into the propellant in non-PMD tanks, I don't know enough about that. I imagine the solubility of He (it is usually helium) in these propellants is either accounted for, or negligible.
Edit: P.S. Software engineering is mostly foreign to me and much of HN content is over my head, but I like he level of discourse here. So, when a topic came up that I could actually contribute to, I jumped.
Nice writeup! Former chemist here. Helium is indeed poorly soluble in most liquids. It's actually used to sparge (purge by bubbling through) solvents to drive out other dissolved gasses.
This is because Helium has very weak intermolecular forces due to its electronic symmetry. For that same reason, it's also as close to an ideal gas, giving you the most pressure/volume bang for your mass buck (only hydrogen is better, and that's bad for oxidizer tanks for obvious reasons).
But this also limits the ability to cryogenically condense helium, which would improve storage density. But you really don't need much in turbopump fed engines.
Love how much expertise there is here; thanks for adding yours!
When you say "metal bladder/diaphragm" is that a sliding wall with propellant on one side and pressurant on the other? (I can't imagine how the seals in that would work.) Or do you mean the metal actually deforms in place?
Yeah, the metal deforms. They are single use devices. Imagine a spherical metal tank, with a thin metal hemisphere welded inside. The hemisphere would invert as the propellant is expelled. Hard engineering and manufacturing problems here. I think there have been piston tanks (which would be reusable with sliding elastomer seals. Not many elastomers play well with hypergolics), but they would be very weight inefficient.
Neat, that's one subject that's always bugged me as every article or video out there just handwaves away settling of propellant tanks in orbit with "ullage thrusters". No one ever mentions how you get a nice continuous feed to your ullage thrusters though.
Ullage thrusters are used exactly as you described, for tanks that do not have PMDs. For example, the large second or third stage main propellant tanks which need to be lit in zero G. On Saturn V, the ullage motors themselves were solid rocket boosters mounted to the aft end of the stage, and thus did not themselves need PMDs, nor access to the main liquid propellants. On some videos of stage separation, you can see these solids firing from the point of view of the jettisoned stage facing forward. (In fact, I think there were also retrograde solid thrusters at the fore end of the stages to aid in separation.) I suppose you could call ullage solid motors PMDs, too.
For an excellent deep dive into the systems, operations, and functions of the Saturn V, get a copy of "How Apollo Flew to the Moon" by W. David Woods.
Only if the pressurant gas dissolves into the liquid propellant. Even then, it doesn't even dilute it per se, it just makes something akin to rocket-fuel-soda which causes all sorts of flow problems in your feed lines. Naturally, if your propellant is stored as a gas, adding more gas to pressurize will of course dilute it as you say, but I've never heard of something like this.
Sure, if the pressurant gas dissolves into the liquid propellant.
But mostly chosen pressurant which is not dissolves, or used some type of separating membrane (or piston), for example, in Soviet space stations used
https://en.wikipedia.org/wiki/Metal_bellows
Not an expert, but fuel and oxidizer is stored as liquid (at cryogenic temperatures), so adding gas creates pressure at the "top" of the tank, pushing the liquid down into the engine (in addition to gravity).
1. Only first generations of "Soyuz" used H2O2 propellant, because it have very limited time before use (for "Soyuz", guaranteed 6 months), and because it have relatively high melting temperature.
First chose H2O2, because "Soyuz" planned as independent ship, to fly relatively short missions around Moon (it is near impossible to withstand even 6 months in so small volume).
When "Soyuz" primary role become companion ship for space station, it switched to hydrazine type propellant.
2. In pressure fed engines used almost all possible propellants and oxidizers. This is not error just clarification.
3. Exists three-propellant engines. For example, exists soviet engine for "spiral" system, which used kerosene+LOX+LH. First it run on mostly kerosene+LOX, with small percent of LH, than switched to pure LOX+LH (sure, LOX share also other).
4. Exists simpler bipropellant engines than mentioned, unfortunately with worse efficiency. - First Britain satellite flown on H2O2+RP1 Black Arrow rocket.