Rocket Engine Efficiecny

[REAPER7]

Hi in new to forums

I have been playing Kerbal for some time just noticed that all rockets have Specific Impulse (Isp) always lower for Above Sea Level (ASL) than (Vac.).

From what I know rockets are quite efficient and the only difference is mostly in the nozzle design.

https://en.wikipedia.org/wiki/Rocket_engine_nozzle

See ideal expansion. Hence it must be possible to have rockets that are designed to be more efficient in atmosphere.

Seems like this has not been implemented.

It would be nice to see some lower stage rockets/engines.

[Hcube]

7 hours ago, REAPER7 said:

Hi in new to forums

I have been playing Kerbal for some time just noticed that all rockets have Specific Impulse (Isp) always lower for Above Sea Level (ASL) than (Vac.).

From what I know rockets are quite efficient and the only difference is mostly in the nozzle design.

https://en.wikipedia.org/wiki/Rocket_engine_nozzle

See ideal expansion. Hence it must be possible to have rockets that are designed to be more efficient in atmosphere.

Seems like this has not been implemented.

It would be nice to see some lower stage rockets/engines.

Welcome to the forums !

Actually the specific impulse depends on :

-nozzle design

-type of fuel (very important !)

-outside atmospheric pressure.

The last one is very important too : the lower the external pressure, the more the gasses can expand, and the better the Isp. So the Isp will always be higher in a vacuum. But you can make nozzles that are optimized for low altitude and low pressure wich will have a good ASL Isp, and an almost identical, slightly higher, but still crappy Vac Isp. 

For example : there's the Aerospike, the solid rocket boosters, the mainsail... Mostly the big first stage engines.

 

I hope that my explanation was clear enough

Edited January 14, 2016 by Hcube

[REAPER7]

Ty for the info.

[regex]

I don't think there are any real life chemical rocket engines that have better sea level isp than space isp, for the reasons noted.  You can get atmospheric performance very close to space performance with good design but that also reduces space performance in the process.  It's a design trade-off.  Lifting engines tend towards excellent sea level isp versus slightly-better-but-still-anemic space performance while space engines tend towards terrible sea level performance (often not even listed) with excellent space performance.

It's worth noting that KSP's engines, in this day and age, are based on the theoretical maximums of Aerozine50/Dinitrogen Tetroxide (NTO) engine isps.  This combination, in one of several forms with varying fuels (MMH, UDMH, UH25) with the NTO oxidizer, fits KSP's generic "liquid fuel/oxidizer" mix very well considering KSP's fuels are near-infinitely storable and require very little special considerations for storage and use (no cryogenics, no ullage, etc...)

Edited January 14, 2016 by regex

[NathanKell]

In addition to the considerations above, also chamber pressure. Though even at very high chamber pressure you'll always do better when there's nothing pushing back against the exhaust (vacuum) than you will when something is (any kind of atmosphere).

[SpaceplaneAddict]

While your here, @NathanKell, why does the aerospike have such a drastic ISPS change? Isn't it supposed to by similar in performance in atmo and vacuum?

[NathanKell]

@SpaceplaneAddict I think we need to add this answer to a FAQ, it comes up every few weeks. :]

No, the aerospike is not a magic engine. It can't erase the fact that nonzero pressure (very nonzero pressure, a hundred kilopascals worth of nonzero pressure) exists at sea level. All an aerospike nozzle does is ensure that you have a nearly-optimal expansion ratio at any given pressure. In effect (tl;dr) that means an aerospike will do nearly as well as it can at any given ambient pressure.

So, here's the deal. A rocket engine works like this: propellants combust in a combustion chamber. That combustion has a pressure. The result is then fed through a narrow throat out into a nozzle (the thrust chamber), where it expands while going downwards. That creates thrust. Thrust is optimal for a given ambient pressure if that nozzle expands the exhaust such that, at the nozzle exit, the pressure of the exhaust is the same as the ambient pressure. However, Isp is governerned by how much expansion occurs.

This means for example that a combustion chamber with low pressure works acceptably in vacuum. This is because even low pressure exhaust can be expanded lots, since ambient pressure is zero and you'll never hit that. However, a low chamber pressure engine is crap at sea level, because you can't expand very much at all before you hit ambient pressure.

So that's one reason, intuitively, why rockets don't work as well at sea level: there isn't very much to expand. The second reason is even simpler (and ties into the first of course): at sea level, or indeed in any atmosphere period, the air is piushing back at the exhaust. In vacuum, nothing is pushing back, so you get the full effect of the exhaust. At sea level, there's a hundred kilopascals pushing back at the exhaust.

 

Now, why an aerospike? As I discussed above, in order to get the optimum Isp for that chamber pressure and reaction efficiency, you need to expand the results to ambinet pressure. However, if you do that with a simple bell nozzle, you only are optimal for a single ambient pressure; at all other pressures (i.e. all other altitudes) you will either be expanding too little (any pressure lower than the design pressure) or too much (any pressure higher than the design pressure). An aerospike gets around this by varying the expansion. It will never be as efficient as a bell at that bell's optimal pressure--the "fake" nozzle is not quite as efficient as a real one, so you take a slight loss across the board--but it will be rather more efficient than that bell at non-optimal pressure.

 

Finally, let's move to the aerospike ingame. You'll note that it basically follows the above: It is never quite as efficient as the optimal engine in any given regime. It's slightly less efficient than the Mammoth/Vector at sea level, and it's slightly less efficient than the Poodle in vacuum. However, its true advantage is that the falloff in efficiency at pressures higher than sea level (i.e. Eve) is much, much more gradual than bell engiens'.

 

To sum up: No, the aerospike is not a Magic Engine (tm) that can someone get around the fact that at sea level there's a shedload of air being shoved at you, making chucking high velocity gas out the back less efficient. Indeed, it is not as efficient getting around that as the best bell-nozzle engine, nor would it even be as efficient as a non-optimal bell-nozzled engine which had a higher chamber pressure than the aerospike. However, it does manage to minimize the losses from having a non-optimal bell no matter the ambient pressure.