ISP question

[Nich]

wouldn't ISP decrease as throttle goes down? Especially for atmospheric engines at low altitude.  For vacuum engines is it possible for gas to be over expanded at the nozzle exit or would that require a nozzle that extends to infinity?

[Ozymandias_the_Goat]

No. Specific impulse is the total impulse over unit of fuel consumed. But when you reduce your impulse by throttling down, you also burn less fuel, maintaining the ratio.

[Vanamonde]

Since actual space science is involved in this question, it has been moved to the Science & Spaceflight sub. 

[K^2]

For real engine, I_SP will, in fact, vary with throttle. And the max I_SP is not necessarily at max throttle, either. I could probably list nearly a dozen factors that I_SP generally depends on, and amount of fuel you inject into the chamber will impact, directly or indirectly, a third to a half of these.

I'm not sure if the impact of throttle position on I_SP will be higher with or without an atmosphere. The fact that the bell for atmospheric operations is shorter might actually help reduce the impact, at least, in some throttle ranges. This will probably depend a lot on the engine design.

[kerbiloid]

It will vary at least because temperature and pressure in the combustion chamber will vary with different values of mass flow.

[hms_warrior]

Ran into a site of a manufacturer of mini-monoprop engines for smallsats a while back. They had a throttle range for their engines listed and a performance loss at max throttle. (was somewhere around 5% loss).

Edit:

found the site: http://www.busek.com/technologies__greenmonoprop.htm

If you look at the second engine, it goes from 50 mn to 500 mn and the isp varies between 220 and 225.

Hm thinking about this... there is no clear indication this is related. Damn mind jumped to conclusions i guess.

Edited December 12, 2018 by hms_warrior

[Ozymandias_the_Goat]

16 hours ago, K^2 said:

 

For real engine, I_SP will, in fact, vary with throttle. And the max I_SP is not necessarily at max throttle, either. I could probably list nearly a dozen factors that I_SP generally depends on, and amount of fuel you inject into the chamber will impact, directly or indirectly, a third to a half of these.

 

But does this happen in ksp?

[Xd the great]

6 minutes ago, Ozymandias_the_Goat said:

But does this happen in ksp?

No

[Ozymandias_the_Goat]

1 minute ago, Xd the great said:

No

Ah

[kerbiloid]

It happens in KSPI-E mod.
Its engines have dynamic ISP and thrust, so you can vary this choosing what you want.