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How do you power a rocket engine?

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Re: How do you power a rocket engine?

#21

Post by panick21_ »

cubefox wrote: post #12
panick21_ wrote: post #8

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. ------- Another t…

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-...

Maybe I wrote it wrong. Closed expander cycles are heat limited.

Areospike require more cooling then traditional bell designs.

So an areospike epxnader cycle could produce more thurst.

See here for some research:

https://www.phoenix-int.com/wp-content/uploads/2017/05/Param...

Re: How do you power a rocket engine?

#22

Post by mLuby »

hydrogen7800 wrote: post #19
jacksonkmarley wrote: post #2

Wouldn't the pressurant/backfill gas dilute the propellant as it was added?

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…

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?

Re: How do you power a rocket engine?

#23

Post by panick21_ »

master_crab wrote: post #18

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.

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.

Re: How do you power a rocket engine?

#26

Post by master_crab »

panick21_ wrote: post #23
master_crab wrote: post #18

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.

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…

“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).

Re: How do you power a rocket engine?

#27

Post by Toutouxc »

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.

Re: How do you power a rocket engine?

#29

Post by gameswithgo »

master_crab wrote: post #18

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.

No post body was provided.

Re: How do you power a rocket engine?

#30

Post by WalterBright »

master_crab wrote: post #18

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.

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.

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