Rocket Thrust Question...

[Austin]

Ok guys...

... this has been bothering me for a while so I am going to put it out there and see what happens. Before I do, realize that I like designing and building and am familiar with Barrowman and other formulas as I enjoy math very much, however, I am NOT a physicist! I am hoping one of you all out there can help.

Now, think about this a while before you answer...


Senario:

Rocket weighs 100lbs at the pad and is using a 500lb average thrust motor, giving it a 5 to 1 thrust ratio. Rocket is expected to pull 10 "G's" once off the pad.

Question:

Once the rocket lifts off and begins to feel the G-force, why doesn't it slow down, since with 10 G's the rocket now weighs essentially 1000lbs and using the 500lb motor, now has a 1/2 to 1 thrust ratio?

You would think Momentum would play in here a bit, but wouldn't force due to thrust-to-weight has severly deter acceleration?

Inquiring Minds want to Know...

Carl

 

[rstaff3]

The simple answer is that the mass of the rocket stays the same (except for burned propellant of course). The 10g's is just a way of describing the acceleration in terms of a known entity. Remember a G is 32 feet per second per second, and is thus accelleration.

 

[kgholloway]

Carl,

To amply the previous reply note that the formula for acceleration is:

A=F/M

Or acceleration is equal to the applied force divided by the mass of the object being accelerated. When we say something "weighs" 100 pounds we are actually saying that the acceleration of gravity is causing an object to exert a force of 100 pounds perpendicular to the surface of the earth. Therefore the mass of an object is equal to:

M=F/A

Therefore an object that weighs 100 pounds has a mass of:

M=100 Lbf/32ft/s/s or 3.125 "poundals"

A poundal is defined as that amount of mass which will exert 1 pound of force when accellerated at 32ft/s/s.

Complicated as heck due to the English system of measurement but I'm not even going to try to convert the above argument into metric units.

In any case since your rocket "weighs" 100 pounds at rest it therefore has a mass of 3.125 poundals. If it accelerates at 320ft/s/s (10 * 32ft/s/s/) it would therefore be exerting a force of 1000 Lbf on the motor, motor mount, centering rings, etc. So long as your motor was producing 1000 Lbf your acceleration would remain constant at 10 gravities.

Ken