How do you power a rocket engine?

Soyuz rocket uses 82% H2O2 for all first and second stage engines.

Soyuz spacecraft uses H2O2 for capsule control during reentry.

Soyuz spacecraft uses a variant of hydrazine fuel for orbital maneuvers.

All of this is true for all versions of Soyuz rocket (except Soyuz 2-1v) and Soyuz spacecraft, since 1950-s to today.

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.

A while ago I looked at Wikipedia and saw that the Soyuz spacecraft article didn't mention what the reaction control system uses for propellant.

Perhaps you can point at a source?

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-...

Perhaps, being pragmatic, tweaking existing designs was not going to put more money into the R&D of Yoyodyne [1].

[1] https://en.wikipedia.org/wiki/Yoyodyne

Did you ever look into this? I've been wondering the same now for a couple of years.

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...

I have never really looked into it before myself, but this explanation seems to make sense: https://www.grc.nasa.gov/www/k-12/airplane/nozzled.html

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.

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.

+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.

Yes. See:

https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

https://en.wikipedia.org/wiki/Skylon_(spacecraft)

But there are lot of problems with this and it adds a huge amount of complexity.

Are you thinking of Air augmented rockets? Which capture and entrain surrounding air into their exhaust.

https://en.wikipedia.org/wiki/Air-augmented_rocket

I have the impression that they see periodic use in various categories of mid-size military missiles.

Missed the edit window. Does not need high energy exhaust, it needs high momentum exhaust.

Doable, but one mistake in modeling and the whole thing goes boom. I like the idea, it reminds me of detonation engines.

Haven't changed since Goddard? Oh my! Here's a list of the innovations the Germans made to Goddard engines in order to scale them up for the V2:

1. turbo-pump

2. building a jacket into the nozzle to both cool the nozzle and pre-heat the fuel

3. putting tiny holes in the jacket so the leaking fuel would form a boundary layer that would protect the nozzle from heat

4. baffles in the combustion chamber to damp out pogo-ing

There's a picture of Goddard looking at a captured V2 engine with his mouth hanging open in astonishment.

The Saturn V engines were scaled up V2 engines.

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, I guess is is possible, but to get 100 bar at 1g you need a 1km+ column. At higher accelerations though it's not such a bad idea.

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.

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.

So the short answer here is: no, it won't.

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).

I think it’s liquid oxygen/methane that goes into the engine. So backfilling with some other gas in the gas part of that should be ok.

No expert though :)

In some cases they use a bladder or a push plate.

How will we ever afford the colossal amounts of fuel that will use, and think of the pollution!