Some physics

[DrDress]

Hi

 

I'm planning a Duna return mission, and I would like to try some calculations beforehand. But I'm a little stuck. I want to calculate my total Delta-V, but I can't find the exhaust velocity anywhere (I'm using a NERV). I try to calculate it from the engines thrust, specific impulse and fuel consumption, but I not getting anywhere. Any help?

[Gaarst]

Effective exhaust velocity is a measure of your engine's efficiency, just like specific impulse.

It is given by:

v_e = I_sp * g₀ with the Isp in seconds (800s for the NERV) and g₀ the standard gravity acceleration at Earth (always 9.81 m/s² no matter what body you're orbiting).
So for the NERV, you'll get v_e = 7848 m/s

Note that Isp is sometimes expressed in m/s and is then equivalent to the effective exhaust velocity.

Edited February 15, 2016 by Gaarst

[DrDress]

What if I have three engines. Are the delta-v just added together for each engine?

[mythbusters844]

Nope, just use the formula as normal.

Edited February 16, 2016 by mythbusters844

[pincushionman]

4 hours ago, DrDress said:

What if I have three engines. Are the delta-v just added together for each engine?

If they're identical engines then your Isp is that of the engine type. If you mix and match (i.e. you use Thuds or Twitches off the side of a tank for extra thrust, in addition to a central engine) you need to do a weighted average against flow rate.

[AbacusWizard]

More engines (of the same type) increases your thrust but doesn't change your exhaust velocity--if one engine can shoot exhaust out the back at, say, 1000 m/s, three engines don't shoot exhaust out the back at 3000 m/s; they just shoot out 1000 m/s exhaust at three times the rate (i.e. three times as much fuel used per second).