# Back of envelope calculations for thurst, specific impulse, etc.

1. Dec 11, 2007

### engineer23

"Back of envelope calculations" for thurst, specific impulse, etc.

I am working on a project that is really out of my realm of experience, so if you could give me some basics, that would be much appreciated.
How do you determine the amount of thrust, power, etc. needed to travel a distance of 630 million kilometers in space? What if the payload is 2000 kg and I want the journey to take 1 year? Don't worry about trajectories and orbital mechanics....I just want a simple calculation. The thrust calculations I am familiar with are those in which the thrust is the force necessary to overcome drag, but I'm assuming there's a more general form? How do I get the specific impulse? Power?
Suppose I'm launching from high earth orbit as well. I guess I need escape velocity for HEO?
I really have no experience with propulsion, but I just need some ballpark figures to base a design on.

Last edited: Dec 11, 2007
2. Dec 11, 2007

### mgb_phys

Really it all depends on the gravity, if you not near any bodies and not in orbit then it depends on how fast you want to go - and what speed you want to be doing when you get there.

Simplest case would be to accelerate the 2000kg to full speed quickly, coast for a year and then spend a similairly negligible time slowing down.
Lowest energy case would be to accelerate constantly for 6months, then turn around and decellerate at the same rate for 6 months.

s = ut + 0.5 at^2
s = 630e9 m, t = 32e6 sec
a = 2 s/ t^2 = 1.2 e-6 m/s^2
So a constant force of 2mN would do it.

Last edited: Dec 11, 2007
3. Dec 11, 2007

### engineer23

Thanks!
What is the power required for the journey then? Is it just mass*acceleration*distance/time?

4. Dec 11, 2007

### mgb_phys

Or you could use ke at the midpoint divided by the time to midpoint.