# The Naked Scientists Forum

### Author Topic: Why do we need to light the rocket fuel for a rocket to take off?  (Read 2282 times)

#### GlentoranMark

• Full Member
• Posts: 98
##### Why do we need to light the rocket fuel for a rocket to take off?
« on: 28/05/2011 15:36:17 »
After watching this great video:

I seen the fuel flying out of the rockets before it was lit by sparks. The rocket then proceeded to take off.

But why do we have to light the fuel? Shouldn't the rocket just move in the opposite way of the fuel as in Newton's equations regardless if the fuel is lit or not?

#### imatfaal

• Neilep Level Member
• Posts: 2787
• rouge moderator
##### Why do we need to light the rocket fuel for a rocket to take off?
« Reply #1 on: 28/05/2011 18:59:00 »
Mark - yep liquid squirting out will produce a reactive force (not nearly enough to move a huge space shuttle); but you can get a much better one if that mass is moving very very fast.  By burning it and making vast amounts of very hot gas which is moving out at great speed we get a much better reactive force.

The thrust is calculated as follows:
Thrust = mass per second * velocity of exhaust
[ Newtons = Kg/s * m/s = Kg.m.s-2 ]

As you will be pumping out the same mass per second the best way to get more thrust is to make the exhaust move very quickly - and that means getting it very hot and expanding very quickly, which is why we light it

#### GlentoranMark

• Full Member
• Posts: 98
##### Why do we need to light the rocket fuel for a rocket to take off?
« Reply #2 on: 29/05/2011 14:07:27 »
Thanks Imat and a good answer but why should lighting it make it behave any different?

• Neilep Level Member
• Posts: 1451
##### Why do we need to light the rocket fuel for a rocket to take off?
« Reply #3 on: 29/05/2011 15:18:04 »
It was indeed a good answer, which already explained why it behaves different?

The gas moves out faster when it ignited. The faster it moves, the more kinetic energy it has. The more kinetic energy it has, the more kinetic energy the shuttle gains from the equal and opposite reaction.

#### GlentoranMark

• Full Member
• Posts: 98
##### Why do we need to light the rocket fuel for a rocket to take off?
« Reply #4 on: 29/05/2011 16:07:05 »
Matt gave a clear and concise answer but I just can't get my head around why lighting it makes a difference.

#### Geezer

• Neilep Level Member
• Posts: 8328
• "Vive la résistance!"
##### Why do we need to light the rocket fuel for a rocket to take off?
« Reply #5 on: 29/05/2011 17:44:41 »
It's not so different from firing a gun. If you watch what happens when a field gun fires, you'll see that the gun barrel moves in the opposite direction from the shell. This is often called the recoil.

That's a bit strange, because the gun barrel is many times heavier than the shell, so how can a small thing like the shell move a rather massive thing like the gun?

It's because the shell accelerates very rapidly to reach it's muzzle velocity. That rapid acceleration produces a force that pushes the gun with a force that is proportional to the acceleration of the shell. You can figure out the acceleration of the shell fairly easily if you know the muzzle velocity and the length of the barrel, but it's obviously quite large!

The propellant burning in a rocket is accelerated by a large amount too, and the force produced is proportional to the rate at which the mass of the fuel is accelerated.

If you Google F=ma you'll find lots of examples of this.

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### Why do we need to light the rocket fuel for a rocket to take off?
« Reply #6 on: 29/05/2011 23:23:58 »
The important thing about lighting the fuel is that it liberates a lot of energy and the gas expands pushing it out of the combustion chamber with a lot more velocity than if it had not been lit this vastly increases the thrust of the rocket motor

#### The Naked Scientists Forum

##### Why do we need to light the rocket fuel for a rocket to take off?
« Reply #6 on: 29/05/2011 23:23:58 »