Should overheating cause increased specific impulse? Also supersonic vacuum

[Mastikator]

I'm asking scientifically, the heat of the thruster increases, would that cause the temperature of the exhaust to increase (or not decrease) thereby increasing the pressure and exhaust velocity?

Also, when you're going supersonic you're creating a small vacuum behind you because the air isn't fast enough to rush in behind you, shouldn't that essentially give you vacuum specific impulse, potentially even at sea level?

I'm not asking game mechanically, just in real life, is that a thing with rockets?

[Guest]

No.

10 chars.

[jwenting]

indeed. You're NOT creating a vacuum behind you at all. You're creating a bubble of slightly lower air density at best.

But in reality the main thing you're creating is a shockwave of compressed air at higher than normal density in front of you.

[AbacusWizard]

If you were creating a vacuum behind you, I'm guessing that would just make things worse--any increase in I_sp would be balanced out and beyond by the fact that you've got way more air pressure in front of you pushing you back than air pressure behind you pushing you forward.

[wossname]

Thermal expansion of the exhaust bell would cause the engine to become fractionally under-expanded, to the detriment of ISP and the stickers would also peel off.

[KerikBalm]

The temperature inside the combustion chamber is much higher than the rest of the rocket engine. When other parts of the rocket engine get hotter, it doesn't affect the temperature that the combusting fuels reach

-nor is there significant energy tranfer of thermal energy from the exhaust to those parts of the engine (it may be significant for engine temp, but not for exhaust temp... thing about the mass of the engine, vs the total mass of the fuel passing through it)

[wossname]

The temperature inside the combustion chamber is much higher than the rest of the rocket engine. When other parts of the rocket engine get hotter, it doesn't affect the temperature that the combusting fuels reach

-nor is there significant energy tranfer of thermal energy from the exhaust to those parts of the engine (it may be significant for engine temp, but not for exhaust temp... thing about the mass of the engine, vs the total mass of the fuel passing through it)

So... no stickers?

[KSK]

For that matter, do rocket engines fail through overheating in real life? Turbopump failure, combustion instability, busted engine bell - I can think of other more-or-less likely failure modes. But overheating? My (not-really-a-rocket-scientist) gut tells me that if you can get your engine going at full blast, then almost by definition, it can handle the heat its throwing out. KSP style steady-build-up-of-heat-then-boom never seemed that likely to me.

Although I'm probably missing something obvious.

[AbacusWizard]

Thermal expansion of the exhaust bell would cause the engine to become fractionally under-expanded, to the detriment of ISP and the stickers would also peel off.

And worst of all, it would void your warranty.

[NathanKell]

Combustion chamber burn-through? Oh yes. But engines are rated at a given chamber pressure, so they're not expected to overheat at all.

[Iskierka]

The issue in-game is that the engines neglect the actual method of cooling used, which is to pump fuel or oxidiser round the engine and absorb the heat, which is then burnt and thrown out the back, meaning that even though the engine is absorbing a lot of heat from its fuel, the net heating effect on it is negligible as the heat energy is expended from the vehicle. In KSP, engines simply produce a fixed amount of heat power as they run, rather than reaching a self-sustaining equilibrium, and this (very small, considering) amount of heat must be absorbed by other parts, or conducted, convected, or radiated away the same as, for instance, electrical heat, even though they're two rather different kinds of heating being experienced due to their cooling mechanisms.

Now, real components -can- be slowly burnt through, but this is because of design or manufacturing flaws, not just a gradual build-up during operation. It's also specifically burning through the flaw - such as how on Challenger burning solid fuel forced its way out through the O-rings - not simply a heat build-up leading to gradual failure. Most materials, simply being heated up, won't suddenly and abruptly fail after being exposed to a certain amount of heat, they'll lose strength as they approach a failure point, and so these components will be designed to not move towards that failure point to begin with, as you don't want to play with how close you can get to failure on a weakening component.

[peadar1987]

The effect of the air not being able to instantly fill the space behind a supersonic aircraft is more that the aircraft "drags" a volume of air behind it, rather than creates a vacuum.

[tomf]

Were you thinking of the drop of of performance of KSP jets as they get hotter/faster? I believe thatJet engines are normally limited by the temperatures of the turbine blades in the exhaust. This means that as the intake air heats up (due to the compression effects of increased speed) the amount of thrust you can get from heating it further by burning fuel becomes limited.

[SomeGuy12]

The issue in-game is that the engines neglect the actual method of cooling used, which is to pump fuel or oxidiser round the engine and absorb the heat, which is then burnt and thrown out the back, meaning that even though the engine is absorbing a lot of heat from its fuel, the net heating effect on it is negligible as the heat energy is expended from the vehicle. In KSP, engines simply produce a fixed amount of heat power as they run, rather than reaching a self-sustaining equilibrium, and this (very small, considering) amount of heat must be absorbed by other parts, or conducted, convected, or radiated away the same as, for instance, electrical heat, even though they're two rather different kinds of heating being experienced due to their cooling mechanisms.

Now, real components -can- be slowly burnt through, but this is because of design or manufacturing flaws, not just a gradual build-up during operation. It's also specifically burning through the flaw - such as how on Challenger burning solid fuel forced its way out through the O-rings - not simply a heat build-up leading to gradual failure. Most materials, simply being heated up, won't suddenly and abruptly fail after being exposed to a certain amount of heat, they'll lose strength as they approach a failure point, and so these components will be designed to not move towards that failure point to begin with, as you don't want to play with how close you can get to failure on a weakening component.

Actually, ablatively cooled rocket engines are a thing. That's a lot closer to the way they are portrayed in the game. With an ablatively cooled engine, you aren't pumping cryogenics through the engine bell, and are just relying on the thing taking longer to fail than the amount of fuel you have.

[Iskierka]

Even with ablatively cooled engines, we kind-of end up in the same situation, however, as the ablative material conducts heat very poorly, so the heat is absorbed into the outer layer, that layer breaks off, and then the heat is gone, leaving none added to the engine itself. Also, the ablatively cooled engines still generally use regenerative cooling in the combustion chamber and fuel pump, which are the two parts which you'd actually expect a failure in - particularly ones related to heat.

[arkie87]

I'm asking scientifically, the heat of the thruster increases, would that cause the temperature of the exhaust to increase (or not decrease) thereby increasing the pressure and exhaust velocity?

Also, when you're going supersonic you're creating a small vacuum behind you because the air isn't fast enough to rush in behind you, shouldn't that essentially give you vacuum specific impulse, potentially even at sea level?

I'm not asking game mechanically, just in real life, is that a thing with rockets?

I think the way rocket engines work is much more complicated than assuming efficiency increases with temperature from Carnot cycle. Rocket engines and their nozzles are designed for a given flowrate, pressure and expansion ratio etc... so increasing temperature will likely reduce ISP is flow is overexpanded, no?

Also, you question about rocket engines exhausting into a vacuum at supersonic/hypersonic speeds i think is a good one. For detached shocks, the Mach number is actually below 1 on the craft side of the shockwave, so the flow could feasibly get around to the rear. For oblique shock waves, i think you have to look at tables, since flow can be sub sonic or supersonic on the craft side of the shockwave. Theoretically, pressure should be significantly reduced behind a blunt object. However, you need a velocity of ~411 m/s to pull a vacuum at sea level assuming Cp = -1.

In reality though, i am sure the flow field is more complicated if you are exhausting hot gas, since you have a mass source on the rear of your craft which can prevent detached flow from occurring, which should further reduce pressure drag....

Thus, it seems like drag itself should be reduced when thrust is applied (since pressure on the rear of the craft is not negative); though i suppose, this might be two ways of looking at the same thing...

[karolus10]

I don't like the idea of engines overheating at all, most of them use fuel flowing trough them to cool the engine (especially NERVA's also they exhaust temperature would be lower than normal chemical rockets) or they designed to work at 100% capacity and not melt or malfunction for certain duration.

I would like to see starting/restarting(with cool-down to arm again) of the engines with it's different capabilities to do so for different types of engines (from smaller orbital engines that are as easy to control as RCS up to massive behemoths(size 3 ones ?) that you can start just once unless it's their special feature) and fuels (RCS-like hypergolics vs cryo fuels).

If anyone would add some balancing factor to NERVA it would be adding more gradual start/shut-down of the "nuke" engines as instant ON/OFF shut-down means stopping the coolant as well.

Edited June 26, 2015 by karolus10