[GlentoranMark (OP)]

After watching this great video:

http://blogs.discovermagazine.com/badastronomy/2011/05/27/endeavours-eye-view-of-her-last-launch/

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]

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 (OP)]

Thanks Imat and a good answer but why should lighting it make it behave any different?

[Madidus_Scientia]

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 (OP)]

Matt gave a clear and concise answer but I just can't get my head around why lighting it makes a difference.

[Geezer]

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]

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

[chris]

I chanced upon this interesting discussion from a little while back. The "thrust" of the query was why should igniting the gas stream from a rocket generate more force? The very good answers above missed out one simple but fundamental point.

Put simply: burning produces heat; heat makes gases expand; if the gas expands then the pressure in the combustion chamber is higher, so the gases are forced out of the back of the rocket at much higher velocity.

Given:

Thrust = mass per second * velocity of exhaust
[ Newtons = Kg/s * m/s = Kg.m.s-2 ]

...the higher velocity means that the thrust is greater.

And while it looks like the combustion is occurring outside the rocket, the mixing of the fuel and oxidiser in happening up inside, but some burn continues outside the rocket and the carbon and other partially burned material in the gas stream glows orange.

[dutch]

We certainly can launch a bottle rocket by putting the initial contents under pressure. However, this initial pressure quickly drops off without a source to replenish it. 

With chemical reactions we can produce the very high pressures needed to accelerate the gas out at high speeds and we can continuously replenish this pressure by burning the fuel over a time span of seconds to minutes (or more).

Trying to put ALL the propellant under that much pressure at the start would blow the rocket to pieces. This over pressurization that often leads to an explosion has occurred in the countless failed launches that have occurred over the decades.

We can more efficiently store all energy we need to convert into kinetic energy in the form of chemical potential than we can as a static pressure. To release the energy stored as chemical potential we need to get the reactants in contact and generally we need to ignite it by adding some energy to start the reaction.

[evan_au]

I think that the discussion as a whole is missing the central point that you need a fuel pump.

The fuel (eg kerosene, liquid oxygen, liquid hydrogen) is often stored at a moderate pressure inside the rocket.
However, the pressure inside the combustion chamber is extremely high (10-200 atmospheres pressure; high pressure = high thrust). So you need a powerful pump to take the fuel (at low pressure) and inject it into the combustion chamber (at high pressure).

As I understand it, the typical sequence you see at liftoff of a liquid-fueled rocket is:
- Venting of excess gas (to keep pressure and temperature under control within the cryogenic tanks)
- Water injections starts (200dB noise can damage the payload)
- Sprays of sparks start (to ignite any pockets of flammable gases that might form once the valves open, but the rocket hasn't lit)
- The turbo-pump firing up to pump the fuel into the combustion chamber (this pump is itself a rocket motor, with exhaust below the rocket)
- The main rocket engines fire, and the flame takes a second or so to build up to maximum thrust
- Solid boosters fire
- Clamps release and classic "liftoff"

If you didn't light the fuel, you would rely on the turbopump to spit fuel out the back of the rocket, and the turbopump doesn't have enough force to actually lift the rocket off the ground.

It also leads to a recursive question: Why light the fuel in the turbopump?
It's because burning the fuel releases a lot of energy.

A rocket which doesn't burn the fuel is about as effective as a car that doesn't turn on the ignition.

See: https://en.wikipedia.org/wiki/Turbopump

[syhprum]

One of the things that impressed allied engineers when they got to examine V2 rockets was the power to weight ratio of the fuel pump motors.

[kamel2500]

It was to be sure a clever response, which previously clarified why it carries on various?

The gas moves out quicker when it touched off. The quicker it moves, the more active vitality it has. The more motor vitality it has, the more dynamic vitality the van gains from the equivalent and inverse response.

[Paul25]

I wonder why they don't use a spring to assist the take off

[Kryptid]

I wonder why they don't use a spring to assist the take off

You resurrected a thread that hasn't been replied to in over a year just to say that?

[Bored chemist]

I wonder why they don't use a spring to assist the take off

You resurrected a thread that hasn't been replied to in over a year just to say that?

It's hardly the worst example of necromancy (even within this thread).

The simple answer is that springs are uncontrollable and the force they supply falls off rapidly as they expand.

[Just thinking]

The pumps that push the fuel out of the exhaust cone do not provide any thrust to the rocket of any significance. When the fuel is ignited the heat generates expansion and the blast that you can see has a huge opposite force within the cavity of the exhaust cone. So the blast that is going down is free to escape but the pressure at the top can only push the rocket up.

[Petrochemicals]

I wonder why they don't use a spring to assist the take off

Because the rocket would never cope with any worthwhile force being applied to it. If you where to accelerate a one ton object to half orbit velocity which is about 5000ms, the force on the rocket would crush it and the fuel should spill out, much like a pop can being trodden upon. You could reinforce the rocket but that tends to make it much heavier meaning you would still require at least a similar amount of fuel as with a conventional take off.

[Just thinking]

For the same reason that we need to light the fuel to run a car's motor as a car cannot be propelled just by its fuel pump.