Nozzles

As far as I understand it, a rocket engine works by expelling mass to the opposite direction. So, a rocket engine nozzle is there to direct the exhaust molecules / atoms in the optimal direction (backwards), not to generate forward force by having those molecules push against it.

Seen like that, why not get rid of it, and have the air (if available) function as a nozzle, or (even better) try to modulate the exhaust like a laser?

That might be doable in multiple ways:

1. Create an enclosed, circular chamber around a spike (that extends outwards), of which both inner surfaces are ribbed, thereby creating a resonance through a succession of higher and lower pressure folds. That's how they build laser diodes on chips. And, fortunately, the resonance needed for the exhaust is of a much lower (and thus larger sized) frequency. The main problem with that, is that throttling will decrease the efficiency a lot.

2. Varying the fuel / oxidizer supply at high frequency. That's like PWM. It has the same effect, but it requires either semi-solid-state valves, or many small and very fast ones.

3. Layered burning, like film cooling and turbine engines that redirect part of the intake air around the engine, to have the exhaust heat it. The simplest way to do that is probably by spraying something like many small jets of water at high pressure at the sides of the exhaust.

Would any of the above work?


Edit: a fourth way might be to beam PWM microwaves at the exhaust, but that requires an additional power supply.

Most rocket engines operate near maximum theoretical efficiency. The biggest desgin problems are about not melting, exploding or vibrating apart. If it isn't right on the edge of doing one of those three, you have made it to heavy and nobody will buy your engine.

1. Aerospikes use air, and bell nozzels use metal to match the resonance of the engine while the exhaust is exbanding. Slight mismatches can cause dramatic losses in thrust. A laser is not producing thrust, the idea here is to null out reflects and resonances.

2. You are going to cause severe water-hammer in you engine and your tanks. You are going to add vibrations and generally mess with the clean and stable combustion of the engine. (Ka-Boom)

3. The oxidizer (in russian engines) is already piped through a jacket around the nozzle, both to heat the fuel/oxidizer, but more to stop the whole mess from melting. In an expander cycle engine the boiled of liquid is used to run the turbo pumps and increase the chamber pressure without burning fuel.

A lot of thrust is gained from having a correctly designed nozzle. If they didn't weigh anything they would be longer and larger. However their weight does start to count against the added thrust after a certain point.

Yes, but a (cluster of) single rocket engine(s) with a fixed nozzle is also an important reason why SSTO is so hard.

Variable geometry seems harder than some kind of aerospike idea. But that doesn't work in a vacuum, so you need some kind of mechanism to have the particles going in the right direction.

Which one doesn't work in a vacuum?

As a different idea: how about an ablative nozzle, that slowly expands? But that gives problems when you want to brake when coming down.

I'm not following here what are the issues? What are we trying to do?

Monroe

Make an engine that would work well for a SSTO.

That's a tough one. I'll watch this one.

Monroe

http://en.wikipedia.org/wiki/Skylon

Using one type of engine for SSTO is probably that hardest but this space plane is pherhaps the closet thing yet to coming close to it.

Interesting design. Neat idea.

Yeah, but it's a paper-only design with no functioning hardware and no appreciable funding. They need $10B.

Well, a fuel combination that burns cool with a good Isp would help. Maybe Methanol/H2O2?

Using anything other than Hydrogen/Lox is going to make the numbers for an SSTO very very hard.

In fact the only thing that would make life easier would be finding out how to manufacture cyclic Ozone.

Frankly, if you want a chemically propelled SSTO, the numbers are hard, period.

Are you talking about a nozzle that burns away making a larger volume for the exhaust gases to expand into at higher altitudes? This would improve overall Isp as it would better match the expanding exhaust gases and not have to have a "one size suits all" approach during assent.

If you are talking about a mechanically expanding nozzle, I think something along those lines was tried on an upper stage using an RL-10 engine, although I'm not sure about the details.