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Name: Antti
Status: educator
Grade: 9-12
Country: Finland
Date: Fall 2013


Question:
When Rocket soars into the sky it has a huge flame behind. Doesn't big part of the fuel go in to waste because the burning happens outside the rocket nozzle. The burning happens in the open space?



Replies:
Hi Antii,

Thanks for the question. The phenomena you are describing is problematic to rockets and decreases their efficiency. At sea level, the flame is narrower due to the pressure of the atmosphere. At higher elevations, the flame fans out and spreads due to the lower pressure of the atmosphere. This phenomena is hard to correct for with solid fueled rockets--at least since I read about the problem in the mid-1990's. With liquid fueled rockets, one can alter the flaring of the nozzles while in flight to maximize the efficiency of the motors.

I hope this helps. Thanks Jeff Grell


Hi, Antti

I am an electrical engineer who can give you part of an answer.........

Some rockets may have combustion in the flame and you are right that this would reduce their efficiency. I think the flame is mostly due to the temperature of the gas, and possibly due to electrons changing energy state as in a fluorescent lamp.

However even without combustion in the exposed flame, rockets are very inefficient. Rockets develop thrust via a fundamental law of Newtonian physics; F=M*A or Force = Mass * Acceleration. In a rocket, we are discussing the mass and acceleration of the gas being expelled out the nozzle. At any moment, the mass of the gas is significantly less than the mass of the rocket.

Another applicable rule here is conservation of momentum. Momentum is mass * velocity and it has direction.

If your mass is 75 Kg, you are floating in space and throw a 75 gram ball away from you at 10 meters per second, the total momentum of (you + the ball) has not changed. You and the ball have each individually undergone a change in momentum that is equal and opposite. Because momentum = mass * velocity and your mass is 1000 times that of the ball, the velocity of the ball must change 1000 times as much as yours does. As a result, of that total velocity of 10 meters per second, the ball is now moving at 9.99 meters per second in one direction, and you are now moving at 0.01 meter per second in the other direction. In this context "0" was to your velocity before throwing the ball.

On the other hand, energy is (mass*(velocity squared))/2. Because of that squared term, after you throw the ball, the ball will now be travelling with 1000 times the energy that you will be travelling. When a heavier object discharges a lighter object, they get equal changes in momentum but the lighter object gets more energy according to the inverse ratio of the mass. (This is the main reason why a bullet causes much more damage than does the gun which shot it. Assuming that the bullet does not change, a very lightweight gun would receive more kinetic energy so may injure the person shooting it.)

Getting back to the rocket, the gas is lighter than the rocket so travels much faster and with much more energy than does the rocket. All of that energy in the motion of the gas is basically wasted. This makes a rocket very inefficient compared to most other vehicles.

I hope this long explanation is helpful and interesting. Best regards, Bob Zwicker


Hi Antti,

The "flame" you see is not a flame at all! It is simply the already- burned exhaust gases that are so hot from combustion that takes place inside the rocket motor's combustion chamber, that they are glowing almost white hot.

Almost all combustion of a rocket's fuel, takes place inside the rocket motor's combustion chamber, before the gases reach the engine's nozzle.

Regards, Bob Wilson



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