Question regarding rockets. (1 Viewer)

[independantz]

If a rocket had a mass of 32000kg, of which 85% is fuel, and a thrust of 400000N determine:
a) the rate of acceleration and g force at lift-off
b) the rate of acceleration and g-force just prior to exhastion of the fuel, Assume it is travelling horizontally and acceleration up to orbital velocity.

I can do a, but unsure about b.

The answer for b is:
a: 83m/s/s
g=9.5

 

[Mark576]

(b) Assuming you're asking for acceleration in the horizontal direction;

f (net) = T = ma (this is the net force acting on the rocket to produce a horizontal acceleration)

Hence: a = T/m where m = 15% * 32000 = 4800kg

a = 83.333... = 83m/s/s

g force = apparent weight/true weight = (mg + ma)/mg = m(g + a)/mg = (g + a)/g = 9.5

 

[independantz]

How come you neglected the weight? , that is the formula is meant to be
T-mg/m

 

[Mark576]

At lift-off, this formula is applicable because we are simply summing the forces and calculating the acceleration upwards, however when we are in orbit, as suggested by the question, the two forces acting on the rocket are the thrust force, produced by the rocket engines, and the gravitational attraction between the rocket and the Earth which is provided by the centripetal force between the two bodies, F = mv^2/r. When we want to calculate the horizontal acceleration, we can use T = ma because we neglect the centripetal force. This is because the centripetal force doesn't affect the horizontal acceleration of the rocket, it only changes the direction of the velocity, but not the magnitude.

 

[samwell]

use the followin formula a= (T-mg)/m it will help wit ua qsn. Where T is thrust n a is acceleration. for g force us (mg+ma)/mg.

 

[Mark576]

use the followin formula a= (T-mg)/m it will help wit ua qsn. Where T is thrust n a is acceleration. for g force us (mg+ma)/mg.

You can't use this formula because this formula is derived as the sum of all forces acting on a rocket at the point of lift-off. There are different forces that act on a rocket once it has reached orbital velocity.

 

[samwell]

At orbital velocity the main force acting on the rocket is F=ma and centripetal force
(xplainz y g-forces r zero durin orbit). U r rite but i just gave the general formula.