…aren’t done yet. I wonder if transonic combustion could improve the performance of rocket engines as well?
15 thoughts on “Internal Combustion Engines”
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…aren’t done yet. I wonder if transonic combustion could improve the performance of rocket engines as well?
Comments are closed.
I wonder if the atomization is similar to what Pursuit Dynamics does with their water atomization systems, that can allegedly atomize water to the point that 3 gallons can cover a football field. Although, obviously, there’s also heating involved in this process.
Long live the I.C.E.!
I would imagine the question is covered in L*.
In an Otto cycle engine you have a very short time period and a very
short combustion length. in a rocket engine you can increase L* the
problem is you tend to run Fuel rich to keep wall temps down.
If you run Ox rich, the wall temps get excessive.
We already know how to improve rocket engines 1000x. It’s called an external combustion engine. Me=broken record.
How in the world would *external* combustion improve a rocket?
And, no, “transonic combustion” wouldn’t improve a rocket either. In a steady flow situation, high subsonic Mach numbers are the source of the bulk of the performance losses. That’s why air-to-air missile designers use blast tubes only as a last resort, for packaging purposes.
The combustion in a rocket is as efficient as it can get, but it is only that way because it is constrained to happen at low Mach number…
In a steady flow situation, high subsonic Mach numbers are the source of the bulk of the performance losses. That’s why air-to-air missile designers use blast tubes only as a last resort, for packaging purposes.
As a non-rocket scientist I require clarification as I obviously lack whatever critical mass of knowledge it is that makes this apparent shorthand comprehensible (I assume) to initiates. So, questions:
1. High subsonic Mach numbers of what? Fuel entering the combustion chamber? Is this the inside-the-liquid-rocket-engine equivalent of the scramjet “keeping the candle lit” problem? If so, how would supercritical fuel or oxidizer, for that matter, make the problem worse?
2. Performance losses relative to what absolute or ideal baseline? And of what, exactly?
3. Why are air-to-air missiles relevant in any way to the discussion at hand? So far as I know, every extant air-to-air missile uses a solid fuel motor. What am I missing here?
4. What, pray tell, is the ominously-named “blast tube?”
5. “Packaging purposes?” What? Are air-to-air missiles as difficult to remove from their wrappers as, say, the typical frozen breakfast burrito?
I humbly await the boon of wisdom in these matters.
A blast tube is a connection between the solid rocket motor exhaust and the exhaust nozzle proper. They are used purely for packaging considerations; for example, you may need to put the solid rocket motor more in the middle of the missile body (lengthwise) in order to keep the static margin (a measure of aerodynamic stability) in the controllable range; you would use a blast tube in this situation to connect the solid motor and the nozzle, instead of just extending the SRM all the way to the nozzle throat, because you need space to put control surface actuators, for example, between the SRM and the nozzle. I believe that the PAC-3 surface-to-air interceptor uses a blast tube for this exact reason, but I’m not 100% sure.
How in the world would *external* combustion improve a rocket?
Why the stars, MfK? External means not melting the combustion chamber. External means a simpler than internal combustion engine. External means high efficiency and high thrust. External means lifting cities rather than closets into orbit.
Two examples: NSWR or Orion. The first is really only for in space, but Orion is the best way to get anything to orbit. Nothing else comes close.
Neither of those is an external combustion engine. One is a nuclear steam engine (and uses completely internal heat release), and the other uses atomic bombs going off behind a pusher plate.
cthulhu explained blast tubes and the allusion to air-to-air missiles very well. As for the rest of it…
1. High subsonic Mach numbers of the reacting flow in a rocket combustion chamber are bad. Heat transfer coefficients go up (and maximize at M = 1, i.e. a the throat), and thus the flow loses thermal energy that would ordinarily go into kinetic energy. Regenerative cooling recovers some of it, but uncooled (or radiatively cooled) chambers do not. The stagnation pressure also drops due to friction, which means that the ability of the nozzle to turn heat into kinetic energy is reduced.
2. Performance loss compared to what one could get compared to what one would get with low subsonic Mach numbers before the nozzle entrance.
Neither of those is an external combustion engine.
Technically it’s not combustion, but it’s definitely external of any combustion chamber. The point being heating gas for expansion inside a chamber is limited by the chambers ability not to melt. Remove the chamber and it’s no longer a problem. If you want high efficiency and high thrust these are two of the best options. Orion is positively old fashioned.
When I think of NSWR and Orion, “external combustion” isn’t exactly the defining feature that sticks out in my mind…
But it is, exactly the defining feature that makes them work so well. If we ever want wealth we need to embrace E=mc^2. It’s just stupid not to. We live in a radiation rich environment. Couldn’t live without it. Abundant low cost energy is wealth. We’ve let the wackos commit generational theft and now we’ve put them in office.
To my mind the interesting tidbit in the story is that the problem with raising combustion temperatures, which would a priori improve your thermodynamic efficiency, is generation of NOx. So a serious problem here is the presence of nitrogen in the oxidizer feed, so to speak. I wonder if there are plausible ways of reducing the nitrogen content of air? I vaguely recall there are some nifty filtering materials that may be able to separate O2 and N2.
Major props to cthulhu. As MfK also acknowledges, you nailed number 4 and put numbers 3 and 5 in the pockets as well. Nice trick shot.
As to my numbers 1 and 2, MfK, in your second comment you refer to “High subsonic Mach numbers of the reacting flow.” By “reacting flow” do you mean the stream of hot combustion products that results once the fuel and oxidizer have actually mixed and burned? These are the gases that exit via the combustion chamber throat, correct? If that’s what you mean, then I guess I have an implicit answer to my original numbers 1 and 2.
There’s a great deal else touched on here that I seem to understand poorly or not at all. Thanks for the help so far, but I think I’ll see if I can find a copy of ‘Liquid Fuel Rocket Engine Design for Dummies’ and read it off-line. As I’ve discovered about other aspects of physics, getting past normative notions of common sense and “intuitiveness” is often essential for true understanding.
“By “reacting flow” do you mean the stream of hot combustion products that results once the fuel and oxidizer have actually mixed and burned?”
I think the problem here is that you think that the propellants somehow mix instantaneously, react to an equilibrium condition instantaneously, then just continue to travel through a needless finite length of chamber until they come to the nozzle entrance (the discussion is not limited to liquids).
After a liquid engine or solid motor starts, there is hot gas touching every surface – including the injector in a liquid, or the grain surface in a solid. Propellant mixing occurs over a finite distance within that hot gas, but the reactions go on all the way to the nozzle entrance. A good design will have the reactions reach “completion” as nearly as possible at the chamber conditions at that point.
I recall a proposal for an automotive engine cycle using methanol, where a methanol/water solution is reformed to CO + H2 by engine exhaust heat before being injected into the cylinders. Methanol reforming in this “chemical recuperation” can be done on copper catalysts at fairly low temperature.