Science Questions

Could we launch rockets from rockets?

Mon, 16th May 2016

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Question

Loyiso asked:

Can a rocket or some sort of spacecraft be mounted on another - say travelling at 1000 km/h and then the mounted one fired at 1000 km/k to travel at 2000 km/h and another mounted on the second rocket to be fired and double the speed? If this is feasible, why is it not done to speed up the probes for the journey to Mars?

Answer

Gerry Gilmore blasted off with this theory...rocket launch

Gerry - Yes, is the answer. In fact, that’s the way we do it. It’s not the combined speed, it’s the combined acceleration that matters but, eventually, you get to the combined speed. So, for example, when you’re launching a big rocket you strap four or five small ones on. They’re called boosters, and then you toss that up, and then you have a second stage, third stage, and so on. The cumulative speed is built up by adding rocket engines at various stages through the flight.

Once you run out of rockets there’s another cute thing you can do which is bounce off a planet or the moon. You don’t actually, physically, hit it but if you go really close then you can go down close to a planet and then zoom away.

Chris - This is what they call a “slingshot” isn’t it?

Gerry - It’s a “slingshot” yes. “Gravitational slingshot.”

Chris - How does it work?

Gerry - Well, if you time your motion of the spacecraft appropriately, you can take a little bit of the energy out of the orbit of the planets. You slow the planet down slightly and speed yourself up so total energy is conserved but you transfer energy from the planet into the spacecraft.

Chris - But surely, if you’re being attracted by something. If you’re zooming towards a big planet, its gravity is accelerating you towards it, when you’ve gone round the other side of it and you want to get away again, isn’t it just going to pull you back as hard as it pulled you in in the first place, so there’s no net gain?

Gerry - It’s all a question of relative motion and the fact that things are spinning as well helps to confuse things a lot as well. If you just imagined a stationary planet and you went down and came back up again then, yes, you’d gain nothing. You would, in fact, go in what is called an orbit around that planet round, and round, and round, as the moon goes round the earth, nothing would ever change. But, if you go very close to the planet and remembering the planet is moving compared to where you want to be so, in a different reference frame, then you can pick up some of the energy. So that’s how you get to an outer planet like Jupiter or Saturn. You do it by heading into Venus and then you zoom around Venus at high speed and you get flicked out and that’s the only way…

Chris - So you let Venus pull you in but, by the time you get to Venus, Venus has moved off somewhere else so you’ve just got the gravitational acceleration as Venus was temporarily there, and then you’re on your way to somewhere else. And you do that a few times with a few planets and you can get a lot more speed than you would otherwise have with a rocket?

Gerry - That’s absolutely correct, yes. It’s just a question of precision timing and…

Chris - And it takes a long time, I presume, as well because you’ve got to keep zipping backwards and forwards on the solar system doing journeys of billions of kilometers instead of just going A to B?

Gerry - Yes, a typical mission to the outer planets will, actually, bounce off Venus and the Earth and Mars two or three times before it heads on out. So it’s bouncing around the inner solar system picking up speed, and picking up speed until, eventually, it gets flicked out in the sling shot.

Chris - And it’s still faster to do that, in terms of time spent, than just to have a really, really, really big rocket and accelerate yourself  through a really, really, really high speed?

Gerry - No. In terms of time spent, if you were rich and you had a really big rocket, that’s the way to do it but it’s just a waste of money to do that when Isaac Newton can do it for you.

Chris - And, also, is there not more inherent risk because if you’ve got a really, really, really big rocket it’s going to be a really, really, really big explosion if anything goes wrong and, also, you might as well take the lower risk pathway if you bounce off a planet like you're saying?

Gerry - Yes, it works. People have been doing it for a long time so we know how to do it, so it is cost effective. It does require some pretty cute calculations because you’ve got to know where everything is going to be at exactly the right time and the right place. But, that’s what space scientists do for a living!

 

 

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Why have rockets on rockets? That achieves nothing more than if you just made the first rocket bigger and faster. The mass that you are accelerating is the main determinant. More mass means more fuel is required. chris, Thu, 14th Apr 2016


That is exactly the concept of a multi-stage rocket, and it is used for space probes to Mars (as well as most rockets trying to reach space from ground level).

Most of the mass at launch is the mass of the fuel, and the tanks to hold the fuel. Once you have burnt all the fuel, the tank is just dead weight, which slows down the rocket unnecessarily. So the empty fuel tank is dropped, and another, smaller rocket starts firing.

This allows rockets with far greater range and payload than a single-stage rocket.

But there is a limit to how far you can take this concept - eventually adding another stage can double the mass (and cost, and risk), to give you a 1% increase in payload at the destination.

This is why concepts like a Space Elevator appear much more attractive than very large rockets to reduce the cost of launching payloads into space. evan_au, Thu, 14th Apr 2016

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