LV-Ns vs Aerospike Altitude

[Just1ChanceFree]

Anyone know the exact altitude on Kerbin where LV-Ns become more fuel efficient then the 388-390 of aerospikes?

[RFK128]

Around 10,000 m lol.

[Scotius]

Unfortunately NERVAs are also heavier and have lower thrust. Whichever way you cut the cake, SSTO are hard to build, pilot and not really effective compared to rockets. But still they are cool as heck, and everyone wants to build one that works.

[Luxmaster-CZ]

You can open context menu of active engine to read actual specific impulse of a running engine.

Once it reaches above 400, read altitude and you got it.

If I am right, IPS of aerospike changes from 370 to 390 only, so you can take it like flat 400 and be happy

[tavert]

Unfortunately NERVAs are also heavier and have lower thrust. Whichever way you cut the cake, SSTO are hard to build, pilot and not really effective compared to rockets. But still they are cool as heck, and everyone wants to build one that works.

You're really not answering the question here...

OP: The way rocket Isp works in KSP right now is a linear interpolation as a function of atmospheric pressure. So for aerospikes we have: Isp_aerospike = 390 - 2*p

For LV-N's we have: Isp_LVN = 800 - 580*p

Setting the two equal to each other and rearranging, we get 578*p = 410, or p = 0.709 atm.

Now to get altitude on Kerbin for a given pressure, the formula is altitude = -5000 * ln(p). So LV-N's are more fuel-efficient than aerospikes for altitudes higher than 1717 meters.

[Just1ChanceFree]

Tavert, thanks. Those were the formulas I needed!

[Francesco]

ya, LV-N's are very cool: you can turn them on at 1 km of altitude and improve the overall efficiency of your ship.