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A jet engine on the other hand takes in air which is 'stationary' and pushes it out the back, so a jet plane can't go faster than it can push the gasses out of the back.

I'm not sure there is much difference between a rocket and a jet Dave. Don't they both develop thrust my accelerating the exhaust gas? After that, the thrust is simply a function of the mass of the exhaust times the acceleration.

This gets hot and expands, increasing the exit speed of the air. This is particularly useful as noone has got a practical jet engine where the air flows through supersonically.

So if a jet plane is going near to the speed the air is coming out of its jet, the amount it accelerates the air is less, hence less force.

One weird thing about rockets; if you calculate the power the rocket generates- it *increases* with time/speed, even though the thrust is the same (energy is force x distance, and distance per second is increasing), and the propellant flow rate is constant. [] []Puzzle: explain why this is!

The equation for net airbreathing jet thrust is: F = m_dot * (c-v)where v is the speed of the aircraft, c is the exhaust speed, and m_dot is the mass of air going through the engine per second. As you can see if you put c=v, you've get no net thrust. If v > c then you're slowing down (this is actually used that's what a lot of jet 'reverser' cups do- they throw the air sideways, which has the same effect as setting c to 0).For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.

UHB turbines were retrofitted to early jets as soon as they became viable and had enough life history to prove reliability. They are quieter than straight jet engines, and consume less fuel per pound of thrust than the equivalent jet engine. As the drive engine is smaller and lighter it has lower maintenance costs, as well as being cheaper to overhaul. The only drawback is that the engines are fatter and have less ground clearance on underwing pods. The main driver was lower fuel consumption and noise during takeoff, a good thing to all living near airports.

Quote from: wolfekeeper on 01/07/2010 14:07:17One weird thing about rockets; if you calculate the power the rocket generates- it *increases* with time/speed, even though the thrust is the same (energy is force x distance, and distance per second is increasing), and the propellant flow rate is constant. [] []Puzzle: explain why this is!Power is work done in time. Work is force times distance. The greater the distance in time, the greater the power.

Quote from: wolfekeeper on 01/07/2010 14:07:17For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.Is this really any different from Force = mass x acceleration?

For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.

Quote from: Geezer on 04/07/2010 01:27:13Quote from: wolfekeeper on 01/07/2010 14:07:17For rockets the equation is just F = m_dot * c, and it's independent of the rockets mass and speed.Is this really any different from Force = mass x acceleration?Well, technically it'sF = d/dt (mv)Where mv is the momentum of the exhaust.Which is slightly different. The f = ma only applies when m is constant.

At constant aircraft velocity I suppose the mass flow rate would be constant.

Hmmm...Rockets are a bit simpler. I wonder why they don't say "it's not jet engine science"?

Where does the extra power come from?

You know that bit in qi where it goes whoop-whoop, well... you fell into the trap- rockets have no top speed, and their generated power is simply proportional to speed!However much energy is being generated from combusting the fuel, or even in the fuel, at some speed, the rocket is generating more power than that!

Hint: there's a hidden source of energy, other than chemical energy of the fuel.

No, it doesn't tell you where the energy ends up. You need to look at the speeds to calculate the kinetic energy, F=ma only gives you the rate of change of speed.

Pretty much, but only if you know the speeds at some point (and if the accelerations are known to sufficient accuracy).The thing is that energy and forces are fundamentally different things, albeit related by numerous equations.

How do you differ between its kinetic energy and the chemical?

Doesn't the kinetic energy include all energy that leaves the exhaust?