Aerospike engines

[nikolay-spb]

I am trying to understand how do they achieve more ISP than ordinary engines. There are alot of explanations but i still dont understand very well. Because air flows down the spike there are some variations between pressure which makes exaust gasses to flow with more speed and engine get more thrust in atmosphere?

[FishInferno]

I think it is that since there is no engine bell, the exhaust plume can freely expand as it gets higher, But I am not exactly sure how this works

[SargeRho]

They don't, not really. They do however have a more "stable" ISP that doesn't change as much with altitude as that of a bell nozzle.

[sgt_flyer]

Actually, they don't have more ISP than other engines.

Ex : SSME vs XRS-2200 linear aerospike :

XRS-2200 :

Sea level ISP : 339 seconds, vacuum ISP : 436.5 seconds

SSME :

Sea level ISP : 366 seconds, vacuum ISP : 452.3 seconds

Now, keep in mind that the XRS were only prototypes (and not with the absolute best materials possible to handle higher temps / pressures) (and compared to other aerospikes, somewhat underperforming in the sea level ISP)

The advantages of Aerospikes compared to standard nozzles is that 'air pressure' acts as a variable shape nozzle. (Most delaval nozzles are optimised for specific phases of the flight - except the SSME's, who have a 'trick' to improve ISP at sea level pressures - but it made the nozzles heavier)

So the same well engineered aerospike engine is not supposed to outperform ISP wise standard nozzles - the best aerospike will end up having more or less the same ISP when firing at the same altitude a dedicated nozzle operates.

Now, the nozzle will underperform at other altitudes / pressures than what it was made for - while the aerospike will continue to be as efficient as the nozzle made for this altitude.

Now, while you save on structural weight thanks to an aerospike not needing a huge nozzle, they have a much more complicated plumbing - so more costly to devellop and build (as you need an even fuel burn all around the spike) - and that would only help for a mere 1 to 2 minutes while the rocket is operating while in dense atmosphere. (So, let's balance a costly engine which will help lower a bit your fuel consumption VS a vacuum optimised nozzle during the early stage of flight, towards the cost of a bit of additional fuel to keep your vacuum engine fire long enough (for rockets using solid boosters around vacuum optimised engines, like Ariane V or the space shuttle )

[nikolay-spb]

sgt_flyert thanks for good explanation, so the pressure was key. I was always wondering why engine has higher ISP in vacum, it is because on ground level pressure negate pressure in chamber so you have less thrust? In vacum pressure in chamber is same but outside pressure is 0 so total pressure is much larger than on earth, so you have more thrust.

[Kryten]

Which incidentally means that the advantage given by an aerospike nozzle varies massively depending on chamber pressure. Hence there being little interest in the architecture in Russia, where engines generally have very high chamber pressures, and hence the only group actively trying to fly an aerospike (Firefly) being one that's focused on low-cost low-pressure pressure-fed engines.

[shynung]

I was always wondering why engine has higher ISP in vacum, it is because on ground level pressure negate pressure in chamber so you have less thrust? In vacum pressure in chamber is same but outside pressure is 0 so total pressure is much larger than on earth, so you have more thrust.

Pretty much. Though, it had to do more with exhaust expansion than back-pressure, since pump-fed engines have chamber pressures at thousands of atmospheres.

[cicatrix]

I am trying to understand how do they achieve more ISP than ordinary engines. There are alot of explanations but i still dont understand very well. Because air flows down the spike there are some variations between pressure which makes exaust gasses to flow with more speed and engine get more thrust in atmosphere?

They do not have 'more ISP' it's that their efficiency does not change with atmospheric pressure, i.e. they will have basically the same thrust along the whole ascent curve.