How do you power a rocket engine? • Hacker News Board

Re: How do you power a rocket engine?

#51

Post by bombcar » Sat, Apr 30, 2022, 8:10 PM UTC

namibj wrote: post #32

Toutouxc wrote: post #27

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 diverg…

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…

This made something I never considered thinking about perfectly clear - thank you!

Re: How do you power a rocket engine?

#52

Post by pontifier » Sat, Apr 30, 2022, 8:23 PM UTC

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.

Re: How do you power a rocket engine?

#53

Post by phkahler » Sat, Apr 30, 2022, 8:28 PM UTC

phkahler wrote: post #45

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.

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

Re: How do you power a rocket engine?

#54

Post by ncmncm » Sat, Apr 30, 2022, 8:29 PM UTC

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.

Re: How do you power a rocket engine?

#55

Post by melony » Sat, Apr 30, 2022, 8:31 PM UTC

ncmncm wrote: post #54

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 i…

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

Re: How do you power a rocket engine?

#56

Post by numpad0 » Sat, Apr 30, 2022, 8:31 PM UTC

showerst wrote: post #17

speedylight wrote: post #15

Earlier quoted context omitted.

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 th…

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.

0: "My recreation for Yamanote line trains(E231-500/E235-Series) using self made VVVF inverter" https://www.youtube.com/watch?v=i0B2bvd9rFQ

1: "I put a biped robot on a bicycle" https://www.youtube.com/watch?v=SqBw7XapJKk

Re: How do you power a rocket engine?

#57

Post by phkahler » Sat, Apr 30, 2022, 8:35 PM UTC

panick21_ wrote: post #46

phkahler wrote: post #45

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.

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.

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.

Re: How do you power a rocket engine?

#58

Post by jacksonkmarley » Sat, Apr 30, 2022, 8:42 PM UTC

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…

Awesome detailed answer! Much appreciated.

Re: How do you power a rocket engine?

#59

Post by NickNameNick » Sat, Apr 30, 2022, 8:51 PM UTC

phkahler wrote: post #45

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.

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.

Re: How do you power a rocket engine?

#60

Post by ncmncm » Sat, Apr 30, 2022, 9:12 PM UTC

melony wrote: post #55

ncmncm wrote: post #54

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 i…

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

Yeah, but they always want to go boom. That's what makes it rocket science.

I was taken by the the idea of a lead screw as a fuel pump.