Should the Rapier have a more constant thrust output?

[AeroGav]

Since most of us would agree that the RAPIER is the Kerbal equivalent of the SABRE engine, I'm wondering if the thrust and velocity curves are realistic in view of what i've read about the sabre elsewhere.

As we can see, the RAPIER produces more than 8 times it's static rating at mach 3.8.   

However from this site (http://www.astronautix.com/s/sabre.html ), I read that

 

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To maximize the core engine thrust in air-breathing mode the engine was operated at constant chamber pressure, which resulted in nearly constant core airflow.

The intake capture area was determined by the requirement to supply this airflow at the maximum air-breathing Mach no, notionally M5 @26km.

The compressor flow capacity was determined by specifying that the engine should achieve full chamber pressure at Mach 0.6 sea level.

This specification yielded the required intake recovered pressure to achieve full chamber pressure along the trajectory (nearly constant at 1.3bar).

 

So if the chamber pressure and flow rate are constant,  from mach 0.6 to mach 5 , and sea level to 26km,  then there should not be  much variation in thrust.  Perhaps the multiplier should get no higher than 2x at the very most?

 

In addition -

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to allow the engine to operate over a range of speeds, the reduced incoming air enthalpy at low Mach no required 'topping up' by combustion heat release in a preburner to ensure constant turbine inlet temperature. The resulting engines ran at a nearly constant turbomachinery operating point and fuel flow over the whole air-breathing Mach no range.

Basically in a SABRE engine the air compression turbines and turbopumps are driven by a closed loop helium turbine.  It uses the hydrogen fuel as a cold sink and the heat of the air that's got compressed by the intake at high mach number, to drive this cycle.  Except at low speeds that heat won't be sufficient to boil the helium at a rate to drive the turbomachinery, so a preburner makes up the difference.   Elegant...   

[panzer1b]

I know there was a mod (cant seem to find it right now) that actually made all the jet engines act more realistically.  If you are interested in engine realism then i strongly encourage trying it out (ill edit this post if i come across it anytime soon).

Otherwise, realistic or not, i actually like the stock rapier because it offers a different thrust curve then most other engines and it requires some engineering to make the best out of it.  Right now the rapier is all about getting it above mach1 (easier said then done when you have a 40t SSTO pushed by a SINGLE rapier engine), but once above that it actually has the perfect thrust curve for what its intended for: SSTO craft (thrust skyrockets once you get past mach2 and really helps with pushing the velocity up to save fuel by not burning rocket mode the entire way up).  That said, im no expert on the sabre engine so i cant quite say if it makes sense the way it is or not.  All i do know is that the majority of real world jet engines have more thrust when in motion then stationary and that they also loose thrust as you increase the altitude because you the mass flow rate drops due to a thinner atmospheric density.  While it probably doesnt match real world numbers, the performance of stock jet engines isnt what id call absolute bull, since they do kinda act as a real jet would (and given how the stock KSP universe is scaled and such, it makes more sense to use different values for thrust/ect to make it balanced gameplay wise).

[Wjolcz]

The RAPIER is really odd when it comes to thrust output. I often find myself wasting a lot of fuel climbing my SSTOs just to dive, gain a lot more speed, which makes the thrust output higher, and only then go out of the atmosphere for the orbital injection. And I'm not the only one who experiences it.

I guess I could just add more engines, but more engines = more fuel and I want to keep my part count as low as possible. I know the RAPIER was made kinda awful just to not appear that OP engine "to rule them all", but come on, do I really need to perform the rollercoaster everytime I want to get out of the atmosphere?

Edited November 23, 2016 by Veeltch

[juanml82]

38 minutes ago, Veeltch said:

The RAPIER is really odd when it comes to thrust output. I often find myself wasting a lot of fuel climbing my SSTOs just to dive, gain a lot more speed, which makes the thrust output higher, and only then go out of the atmosphere for the orbital injection. And I'm not the only one who experiences it.

I guess I could just add more engines, but more engines = more fuel and I want to keep my part count as low as possible. I know the RAPIER was made kinda awful just to not appear that OP engine "to rule them all", but come on, do I really need to perform the rollercoaster everytime I want to get out of the atmosphere?

Try adding a bit more wing. I was having trouble breaking the sound barrier with a design of mine and adding more lift and rotating the wings some 5° upwards did the trick. Now I just point 15° up and head for the stars (ok, planets. Or moons. But you get the idea)

[AeroGav]

5 hours ago, Veeltch said:

The RAPIER is really odd when it comes to thrust output. I often find myself wasting a lot of fuel climbing my SSTOs just to dive, gain a lot more speed, which makes the thrust output higher, and only then go out of the atmosphere for the orbital injection. And I'm not the only one who experiences it.

I guess I could just add more engines, but more engines = more fuel and I want to keep my part count as low as possible. I know the RAPIER was made kinda awful just to not appear that OP engine "to rule them all", but come on, do I really need to perform the rollercoaster everytime I want to get out of the atmosphere?

Maybe what they were trying to acheive, was have the thrust output fall by a factor of 8 as you go from sea level to 26km (sounds reasonable) but have that cancelled out by the 8x growth in thrust from 0 to mach 5, so that over a realistic steady climb scenario, output remains constant.  If you were to somehow do 100 m/s at 26km, output would fall off severely, but if you were going at mach 5 by this altitude it will be close to sea level power.

Unfortunately this means people can just accelerate to mach 4 or 5 at sea level and enjoy 8x static thrust rating without the altitude penalty.

I thought there was supposed to be a "choke limit" mechanic as well, which stops the engine thrust multiplier rising above a certain value no matter how fast you're going  at low altitude, reflecting that the engine compressor can only handle so much air flow through it?    Not to mention that exponential drag buildup should prevent this or destroy any vessel that tries.