• Question: To launch a spacecraft, you would need alot of fuel, but with all that fuel, to launch it, would'nt you need more fuel, to be able to get it off the ground, so you need more fuel, which makes the problem worse? (And so on)

    Asked by dawnofwarfan to Adam, Catherine, Karen, Leila, Nazim on 19 Mar 2012.
    • Photo: Karen Masters

      Karen Masters answered on 19 Mar 2012:

      Yes this is a problem. It’s something you can study once you know some calculus – we recently went through it in undergraduate Physics class here at Portsmouth – you can work out the optimal ratio of fuel mass to rocket mass to get the fastest speed.

    • Photo: Adam Stevens

      Adam Stevens answered on 19 Mar 2012:

      Your logic is impeccable, but for one thing. If you drew a graph of mass vs fuel requirement, it wouldn’t be linear. When you build a spacecraft you have to account for the mass of the fuel, which is actually most of the total mass.

      But if you add a little bit of extra fuel (and therefore mass) the amount of fuel you have to add to move that extra mass is a bit less. Therefore at some point you’ll have more fuel than you need.

      There’s actually an equation to work this out, it’s called the rocket equation (or Tsiolkovsky equation, after the Russian guy that developed it).

      The wikipedia article about it is quite good http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

      In the end you have to do a trade off analysis between mass and fuel (most of engineering is just trade off analyses)

    • Photo: Nazim Bharmal

      Nazim Bharmal answered on 19 Mar 2012:

      You only need enough fuel to get off the ground, but once you start to climb, because you are using fuel up, the spacecraft (or rocket) gets lighter so the fuel you do have left can make the rocket/spacecraft go faster and faster till you have used up all your fuel and have got to the maximum speed. What you do need is to have reached escape velocity otherwise you will fall back to Earth, which you might want to do if, for example, you launch another smaller rocket that will go all the way into space.

      The problem is one of tiny changes in tiny steps in time, which leads to calculus and was one of the big inventions/discoveries of Newton.

    • Photo: Leila Battison

      Leila Battison answered on 19 Mar 2012:

      I had no idea about this, so thanks for asking it and thanks to my fellow scientists for answering it!

      Because I have nothing to add, here is a video of the northern lights from space! (I’m such a bad space scientist!)
      http://www.guardian.co.uk/science/video/2012/mar/18/northern-lights-from-space-video

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