Say you had a practically sized hollow glass ball with a hole in it, through which you placed the thruster of a small rocket; say you then extracted all the air from said ball to create a vacuum, and then fired the thruster up?
What would happen? Wouldn’t the jet engine just die like a candle trying to light up in a vacuum? Or would the rocket take off, ball and all?
It wouldn’t take off. The rocket would feel a thrust up-wards when it first fired, but as the gas coming out of the rocket hit the opposite side of the glass ball, there would be an equal thrust downwards, and it wouldn’t go anywhere.
If the ball is air-tight, presumably pressure would build up in it from the expanding hot gases expelled by the rocket, and eventually the whole thing would explode.
Also, jet engines are not the same things as rockets.
Typically, rocket engines contain fuel and oxidizer and will operate once they are ignited until either the fuel or the oxidizer runs out. Jet engines take air in at one end and heat it with fuel and let it shoot out the other.
Jet engines work by compressing intake air, mixing it with fuel, igniting the fuel, using some of the generated energy to drive the compressor, and leaving a hole in the back to create a thrust imbalance. Without air to compress, there is nothing to oxidize the fuel and drive the engine.
Rockets, OTOH, carry both fuel and oxidizer, so the presence of vacuum would not interfere with its operation.
Is the rocket sealed inside the ball, or is the ball attached to the rocket as payload (the rocket’s thrusters being outside the ball via the hole)?
Napier gives one above (basically, rockets are what they attach the Space Shuttle to, jet engines are the things on the wings of planes). Your experiment wouldn’t work with a jet engine since both ends of a jet need to be exposed to the air, and air needs to be able to move from one end to the other, and so you couldn’t pump just one end down to a vacuum.
So, a jet engine cannot work in a vacuum, but a rocket can, as evidenced by all the ones we have sent up. When the rocket leaves our atmosphere, can you still see the booster firing in space, or have they dropped off by then?
For the space shuttle, the boosters separate at approximately 45km altitude. Space is defined to begin at 100 km. The orbiter has multiple rockets as main engines, but they’re hydrogen fueled and don’t look that impressive firing in the atmosphere. The exhaust is essentially invisible in vacuum.
The rocket could move about within the ball and roll the ball via hamster-like forces, but the ball would not take off.
Yeah rockets are self contained hence their suitability for operating in space. The space shuttle also has rocket engines as part of the main hull, not just the boosters.
I would think that the rocket would move toward the edge of the bubble where it will then stop as the net thrust from the bubble to the outside world is zero.
Scale an Apollo rocket down by a 1,000; instead of the base being above a blast pit, it now has a large hollow steel ball attached, it has all the air sucked out and is sealed. The rocket is fired up. Now what would happen?
Could I have a cite for this, please? How often has something been sent up to observe the rocket from behind, as it leaves our atmosphere? Or do they track it with telescopes?
The non-business end of the rocket is secured to the top of the ball on the inside. In which case, the rocket fires and the ball doesn’t move an inch. It doesn’t matter how big the ball is. Every erg of upward force from the rocket is canceled when the rocket’s thrust hits the bottom end of the ball. Actually, there might be a slight delay in the thrust reaching the bottom if it was big enough, so maybe it would pop up for a second, but then it would go right back down. It’s like if your shoe is untied and you step on the shoelace with the other foot when you try to jump.
The rocket is not secured. It bounces around all over the place, and the ball random walks over the landscape.
There’s still a mass transfer from the rocket engine into the ball, right? Shouldn’t there be associated momentum with this? Granted, the velocity isn’t going to be very high…
You won’t see any effect until the rocket hits the side of the ball. The miniature pilot of the rocket could fly all around inside the ball and nothing would happen as long as he doesn’t crash into the side.
Whenever even the most crazy idea appears to have a remote chance of being used as a weapon, assume that the military thought of it already (though somewhat the other way around compared to what you’re asking about).
As for the OP, I assume that the rocket is fastened to the sphere in such a way that its nose points outward, and the engine into the ball? If so, there will be no total momentum transferred to the contraption as a whole.