Propulsion System Thrust

As long nothing else adds momentum to the system componets, what's got a very slight chance of happening in such a big system.

We can be pretty sure that the COG of the Universe still seats where the big bang singularity once was though ;)

Speaking of the Big Bang - does anybody else feel slightly uneasy about this theory? The entire universe the size of a pea (or smaller)? I have read the evidence for the BBT - surely the universe didn't start like this.
I once heard a well-known astronomer say privately that he was in the "uneasy" category but that he was not going to say so publicly.

If it is, we should be able to measure a resulting centrifugal force.

Actually I'd think you'd have more success with measuring Coriolis acceleration. Trouble is with the whole bent space thingo it all gets a bit tricky. Incidentally if you haven't, read Simon Singh's "Big Bang" where he talks about how we can work out some amazingly interesting stuff without leaving our little rock.

Edited by Greg Locock, 18 July 2013 - 01:05.

Speaking of the Big Bang - does anybody else feel slightly uneasy about this theory? The entire universe the size of a pea (or smaller)? I have read the evidence for the BBT - surely the universe didn't start like this.
I once heard a well-known astronomer say privately that he was in the "uneasy" category but that he was not going to say so publicly.

I don't feel any more uneasy about the Big Bang than any other proposal, at least most measurements point to this being 'the truth'. It doesn't mean that there weren't other Big Bangs after countless Big Crunches, or that this is the only Universe. I like the idea of 'branes, although I hate the contraction of 'membranes'.

Also, there is no 'centre' where the BB happened, as space is curved - or so I read recently. "Curiouser and curiouser", said Alice...

If it is, we should be able to measure a resulting centrifugal force.

Actually I'd think you'd have more success with measuring Coriolis acceleration.

I should have said "we should be able to infer a resulting centrifugal force by observing accelerations out there in the cosmos".

Still - your idea of measuring Coriolis accelerations would be even better.

If you specify a frame of reference like the rocket ship and its fuel only or the rocket, its fuel and the earth the COG for each FOR I suspect the COG is fixed no matter what complications or diversions you might consider.

Consider this example: When the Space Shuttle, traveling in a LEO, needed to return to the surface of the Earth, it did so by firing its Orbital Maneuvering System (OMS) engines in the direction of orbit to reduce its orbital velocity.

http://en.wikipedia....euvering_System

Consider this example: When the Space Shuttle, traveling in a LEO, needed to return to the surface of the Earth, it did so by firing its Orbital Maneuvering System (OMS) engines in the direction of orbit to reduce its orbital velocity.

http://en.wikipedia....euvering_System

Considered. And exactly what conclusion are we supposed to be drawing other than that Newton was pretty damn much on the money?

The crewman would definitely slow down a tiny bit, while the wrench would also speed up a tiny bit.

exactly, and that is why "space elevators" will never work..

Why?

because when you lift an object into space with a space elevator, you not only have to lift it up and as such increase it's potetial energy, but you also have to increase it's velocity, from some 465 m/s on the equator to some 3000 m/s at geosynchonous orbit... How you are to do that with a weight hanging from a string in orbit I do not really see, because that means that the counterweight in orbit will loose some of it's speed... My guess is that you will need some sort of a rocket to accelerate the counterweight again or accelerate the payload..

Ah, OK, it's just that you said '"space elevators" will never work..'! Not that it would be difficult to make it work.

ok, I meant "work practically.."

because when you lift an object into space with a space elevator, you not only have to lift it up and as such increase it's potetial energy, but you also have to increase it's velocity, from some 465 m/s on the equator to some 3000 m/s at geosynchonous orbit... How you are to do that with a weight hanging from a string in orbit I do not really see, because that means that the counterweight in orbit will loose some of it's speed... My guess is that you will need some sort of a rocket to accelerate the counterweight again or accelerate the payload..

If the counterweight is positioned higher than a geosynchronous orbit, its "tether" will have considerable tension due to centrifugal force and thus will always tend towards vertical.

Being tether the critical word here, how are you building this cable 35,000+Km long that will not desintegrate under it's own weight...

Being tether the critical word here, how are you building this cable 35,000+Km long that will not desintegrate under it's own weight...

Tapered.

Although of course I am not building or designing it myself.

Yup, there's a competition at the moment that is encouraging people to develop suitable fibres. It turns out a ribbon is better than a rod. We are a long way off finding a material that is strong enough per unit mass that can be manufatured in reasonable sizes.

...but it will come.

Considered. And exactly what conclusion are we supposed to be drawing other than that Newton was pretty damn much on the money?

Newton was correct with his theories about the motion of bodies, but he went a bit astray when it came to his views regarding alchemy and religion.

The conclusions you were supposed to draw from my response was that the space shuttle firing its OMS engines into the direction of travel to reduce orbital velocity (ie. the OMS engines are transferring momentum to a fuel mass and ejecting it forward from a nozzle) is basically the same principle as the crewman tossing the wrench.

I suspect that they would still share the same COG. Presumably a rocket could be treated in the same way by considering the unchanged initial COG of rocket plus fuel. To extend this idea - a rocket which reached a nearby star would still have its COG at its starting point on the launch pad. The rocket has gone to a star but its COG hasn't gone anywhere. This seems to be an odd concept but I suspect it is true in theory.

I think this model only works if it starts in space with no other friction, gravity, rotations, etc. I also don't know if it accounts for parts the spaceship might shed. Gases thrown at the launch pad don't go past it, but end up swirling around next to it in the same atmosphere the spaceship is being pushed through. Consider Apollo 10 100,000 miles out on its way to the moon. It's "coasting," burning no fuel, most of the Saturn V has been dropped into the ocean, and its COG is allegedly rotating around on the surface of the earth in Florida. A few days later, the command module is back to the same distance from the earth, falling to it, but a huge amount of fuel has been thrown out around the moon, the descent stage dropped onto the moon, and the ascent stage fired off into the solar system, and the COG is supposed to still be in Florida?

I think this model only works if it starts in space with no other friction, gravity, rotations, etc. I also don't know if it accounts for parts the spaceship might shed. Gases thrown at the launch pad don't go past it, but end up swirling around next to it in the same atmosphere the spaceship is being pushed through. Consider Apollo 10 100,000 miles out on its way to the moon. It's "coasting," burning no fuel, most of the Saturn V has been dropped into the ocean, and its COG is allegedly rotating around on the surface of the earth in Florida. A few days later, the command module is back to the same distance from the earth, falling to it, but a huge amount of fuel has been thrown out around the moon, the descent stage dropped onto the moon, and the ascent stage fired off into the solar system, and the COG is supposed to still be in Florida?

The model does only work if the space ship starts in space with no outside influences at all - or any outside influences during its journey. However the spaceship can shed any number of parts, made any number of course changes, fire guns or launch missiles in all directions etc. etc. and even after years of doing this and light years of travel its CoG will still be at its starting point.