# Why does a rocket need fire in its tail

accord. to newton's laws, action <=> reaction, like how a balloon moves up if u blow air in it and then leave it at once...the air comes out with a force(action) and the balloon moves up(reaction)...the same principle in rocket....but with a difference...the gas actually burns...so the question is why it shud burn...y not just eject with the same speed unburnt..will it not fly then...

russ_watters
Mentor
Sure, but then you'd need big pumps and something to power them (for example, a fuel-fired engine...). Instead of pumps, burning the fuel causes the pressure needed to push the gases through the nozzle. It is a very efficient way to use the chemical energy.

When you combine the parts of the rocket fuel they spontaneously combust, this increase in temperature has to be balanced, the volume of the combustion chamber is fixed to the pressure of the hot gases produced has to go up. this pressure increase squirts out the back producing thrust.

The same process happens in a car engine

You can make a rocket without fire by blowing up a balloon, and attaching an outlet pipe to the neck. When you let the air out the reaction pushes the balloon the other way. If you sat in a rowing boat and threw bricks over the stern, you'd move forward. etc...

DaveC426913
Gold Member
accord. to newton's laws, action <=> reaction, like how a balloon moves up if u blow air in it and then leave it at once...the air comes out with a force(action) and the balloon moves up(reaction)...the same principle in rocket....but with a difference...the gas actually burns...so the question is why it shud burn...y not just eject with the same speed unburnt..will it not fly then...
As other have said, perhaps I can add one more viewpoint:

The unburned fuel can be stored as a liquid or solid, contained in a very small space. If you simply let it out, it will not do much for your rocket.

The beauty of burning it is that the end products are
1] gasses (which take up a LOT more room than the liquids/solids), and
2] heat (which causes whatver is being heated - in this case the gas - to expand even more). It is this huge expansion that serves as the force.

Much like dymanite. You can look at dynamite as if it is simply "exploding", or you can look at it as a chemical reaction that goes from "compact solid" to "volumious gas" VERY rapidly.

D H
Staff Emeritus
A few more words on this topic:

A rocket of course does not need a "fire in its tail". Some space vehicles do not use chemical combustion rocket engines. Two extreme examples are cold gas thrusters and Hall effect thrusters.

Cold gas thrusters are basically balloons with valves. Gas stored under high pressure is released from the thrusters to generate force. These thrusters don't produce much force and use a lot of gas to produce the force. The advantages are simplicity (very low tech), reliability, and little plume contamination. Plume contamination often results when one vehicle needs to dock with another. The active vehicle must fire its thrusters directly at the target vehicle at times. Doing so with combustion-based thrusters can fry the target vehicle or spray it with some very nasty chemicals.

In contrast, Hall effect thrusters are quite high-tech. The exhaust is generated by accelerating ionized gas with a magnetic field. The exhaust leaves the vehicle at a very high speed compared to typical chemical combustion thrusters. A high exhaust speed means less fuel is needed to generate thrust. The problem with Hall effect thrusters is that they can only expel a very small amount of fuel at a time. The force is too small to enable their use on large vehicles.

Rocket designers use several measures of thruster performance. The most obvious is the force produced by the thruster. Another is "specific impulse" (Isp), essentially the force per unit of fuel. Think of the horsepower and miles per gallon used to compare automobiles.

Chemical thrusters:
Isp: 250 (solid rocket) to 450 (liquid bipropellant)
Force: Moderate to very high (depends on size of thruster)
Complexity: Moderate to high (depends on type of thruster and fuel)

Cold gas thrusters:
Isp: <100 (50-70 is more realistic)
Force: Low
Complexity: Low

Ion thrusters:
Isp: 3000
Force: Very low
Complexity: High

if u burn the fuel, then the air below the rocket is at very low pressure and the air above the rocket is at high pressure(relatively)..so will the rocket not face any knock by the air over its head...

D H
Staff Emeritus
if u burn the fuel, then the air below the rocket is at very low pressure and the air above the rocket is at high pressure(relatively)

What makes you think this? The fuel contains oxidizer (this is what distinguishes rockets from jets). The pressure will be higher in front of the rocket, but that is a function of the rocket moving through the atmosphere. It has nothing to do with whether the rocket burns fuel or uses some noncombustive form of propulsion.

..so will the rocket not face any knock by the air over its head...

What do you mean by this, in particular, what do you mean by "knock"?

Proper use of English is appreciated on this forum. It fosters understanding.

when u burn something it heats the air near its surroundings...when heat increases in an area, a partial vaccuum is created in that place causing the surrounding air to occupy this vaccuum...i.e. air moves from high pressure to low pressure area...
knock - its a sudden application of force...the ordinary knock that u know of..and its a knock because the rocket blasts off...this blast must cause a sudden pressure differential resulting in a knock...
Now back to the original question...does burning fuel not cause a detrimental effect because of the pressure differential...

russ_watters
Mentor
when u burn something it heats the air near its surroundings...when heat increases in an area, a partial vaccuum is created in that place causing the surrounding air to occupy this vaccuum....
That isn't anywhere close to what happens when you burn/heat a gas.

The simplest rocket fuel is hydrogen/oxygen. The combustion process goes like this:

2 H2+O2 --> 2 H2O + Heat

As you can see, you start with 3 molecules and end with two. By the ideal gas law, you would have 2/3 the volume you started with if the temperature stayed constant. But since the temperature is much, much higher, the volume/pressure is much, much higher:

N1P1V1/T1=N2P2V2/T2

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DaveC426913
Gold Member
when u burn something it heats the air near its surroundings...when heat increases in an area, a partial vaccuum is created in that place causing the surrounding air to occupy this vaccuum...i.e. air moves from high pressure to low pressure area...
knock - its a sudden application of force...the ordinary knock that u know of..and its a knock because the rocket blasts off...this blast must cause a sudden pressure differential resulting in a knock...
Now back to the original question...does burning fuel not cause a detrimental effect because of the pressure differential...
You are confusing a number of different principles, some have nothing to do with rocket engines and some are downright wrong. Back up and let's take this slowly.