Homework Help: Rocket accelaation at the start of fuel burn

1. Oct 12, 2011

nag

Hi Guys this is the problem I am stuck at:

The solid fuel of a 420.0 kg rocket traveling at 18100.0 km/hr is ignited to correct the rocket trajectory in mid-flight to Mars. 5.00 kg of fuel is burnt in 13.0 s. If the exhaust velocity of the fuel, relative to the rocket, is 3650.00 km/hr, what is the acceleration of the rocket (in m/s2) at the start of this burn?

I tried momentum conservation and unable to end up with correct accl of Rocket.

Thanks a lot

2. Oct 12, 2011

Andrew Mason

Use F = dp/dt = vdm/dt.

The rate of change of momentum of the rocket exhaust gives you the net force on the rocket. So you should be able to determine the acceleration at the beginning of the burn. Why is it (slightly) different at the beginning than at the end of the burn?

AM

3. Oct 12, 2011

nag

Thanks for the help. It is obvious that backward thrust imposes forward force on rocket. In this case as the fuel start to burn I guess rocket gains a little acceleration. Please correct me If I am wrong or missing any.

4. Oct 12, 2011

nag

Thanks for the help. It is obvious that backward thrust imposes forward force on rocket. In this case as the fuel start to burn I guess rocket gains a little acceleration. Please correct me If I am wrong or missing any.

5. Oct 12, 2011

nag

Hi,

I followed your reply and net force on rocket = (1013.89 m/s)(5/13)kg/s = 389.96N

Since F=ma => a = 389.96 / 420(the initial mass)

Is this right way of doing. Thanks.

6. Oct 12, 2011

Looks right.

AM