Over-powered Engines

[Octagon]

I'm working on some engines and deeply studing rocket physics. But there is something that I want to discuss with you all: How do I know that my engine is over-powered?

Take this aerospike as an example:

MODULE
{
	name = ModuleEngines
	thrustVectorTransformName = thrustTransform
	exhaustDamage = True
	ignitionThreshold = 0.1
	minThrust = 0
	maxThrust = 200
	heatProduction = 600
	fxOffset = 0, 0, 0.25
	PROPELLANT
	{
	 	name = LiquidFuel
         	ratio = 0.9
		DrawGauge = True
	}
	PROPELLANT
	{
	 name = Oxidizer
	 ratio = 1.1
	}
	atmosphereCurve
 	{
   	 key = 0     600
         key = 0.5   500
  	 key = 1     400
 	}

}

[Decent Weasel]

I'm guessing that the only real calibration will be against other engines included stock, with "OP" being "significantly superior to an equivalent stock engine" rather than "an extrapolation that fits with the general performance curve of stock engines, but covers a regime stock engines don't." (Radial nuclear engines might be a good example of the latter, I guess.) For the aerospike specifically, I seem to have heard that aerospikes are generally 80-85% as efficient as equivalent bell nozzles, but hold that performance curve across the pressure spectrum.

[Tiron]

For the aerospike specifically, I seem to have heard that aerospikes are generally 80-85% as efficient as equivalent bell nozzles, but hold that performance curve across the pressure spectrum.

Specifically, what he's referring to is that aerospikes have a property that makes them automatically correct for altitude. Traditional rocket bells have to be tuned to a particular altitude, and at any other altitude their performance suffers. Aerospikes are less efficient at any given altitude than a rocket bell tuned for that particular altitude, but the aerospike has almost that same efficiency at any other altitude, whereas the bell does not.

You'd have to look at statistics on real aerospikes compared to the stock one to get a proper idea, but I can say right off that atmosphere curve is well out of whack. 600 seconds Isp is way too high. 400 is probably also a bit high. It also changes way too much: the whole point of aerospikes is that they have a more constant efficiency.

One thing to keep in mind though, is that because of the reduced sizes on everything in KSP, realistic items would generally be 'overpowered'. Reaching Low Kerbin Orbit takes around 40% of the delta-V that it takes to get to Low Earth Orbit...and not only is LEO higher, but it's still well within the atmosphere too.

As an example, let us take the J-2X, which isn't a particularly spectacular real engine:

It's 3 meters in diameter, a bit bigger than size 2 which is around 2.5m. It weighs about 99% what a poodle does, has 87% of the mainsail's thrust, and 112% the Isp rating of the LV-909 (or about 115% of the poodle's.) It's not even close to the most spectacular thing ever in real life, but in KSP, it would be TITANICALLY overpowered.

[Octagon]

For the aerospike specifically, I seem to have heard that aerospikes are generally 80-85% as efficient as equivalent bell nozzles, but hold that performance curve across the pressure spectrum.

Well... I'm basing my engine on this:

http://www.aerospaceweb.org/design/aerospike/compensation.shtml

Edit:

Specifically, what he's referring to is that aerospikes have a property that makes them automatically correct for altitude. Traditional rocket bells have to be tuned to a particular altitude, and at any other altitude their performance suffers. Aerospikes are less efficient at any given altitude than a rocket bell tuned for that particular altitude, but the aerospike has almost that same efficiency at any other altitude, whereas the bell does not.

You'd have to look at statistics on real aerospikes compared to the stock one to get a proper idea, but I can say right off that atmosphere curve is well out of whack. 600 seconds Isp is way too high. 400 is probably also a bit high. It also changes way too much: the whole point of aerospikes is that they have a more constant efficiency.

So in comparison with "MB-45 Rocketdyne lox/lh2 rocket engine. 200 kN. Isp=467s. Mitsubishi / Boeing" it would be something like this:

Maximum thrust: 200

atmosphereCurve

{

key = 0 420

key = 1 390

}

Edited October 3, 2013 by Octagon

[NoMrBond]

I made a stock-alike mathematical fit line for ISP/thrust/mass which might help

Since you have two parts (thrust and ISP), we can plug that in 600 ISP = 57.67 Thrust/Ton

Add +30% assuming 0% CVT/Thrust vectoring gives 74.971 Thrust/Ton

[74.971 T/t] / [200 Thrust ] = mass of 2.667698176628296 tons to be balanced (against stock)

You have a big gap between slISP and vISP which actually gives you more than 74.971 T/t, but I didn't have enough examples to model what sort of advantage that gives you

I can't see other stats which play into the equation so please excuse my guess