• nev posted an update 1 week, 5 days ago

    Why is it so hard to escape Earth’s gravity?
    This may be a stupid question to a rocket scientist, but i’d appreciate it if somebody could dumb down the science to help explain it to me.

    • Getting up there is relatively easy. But staying in orbit needs enormous sideways velocity; you need to be going so fast that rather than falling into the earth you miss it. There is an excellent way to think about this involving a really tall tower, and throwing a ball from it faster and faster – the ball lands further and further away, and eventually you throw it so hard that it falls all the way around the earth and hits you in the back of the head.

      ISS, for example, is only 400km above the earth, but to stay there it’s doing 7.5km/sec sideways.

      This is a pretty good primer:

      https://what-if.xkcd.com/58/

    • You need a certain force (the rocket) to lift whatever you want against gravity. But that also needs to lift itself so it needs to be bigger.

      Then add in the air resistance once it’s moving and you need a bit more rocket to overcome that resistance, and even more rocket as it moves faster as drag increases at the square of the increase in velocity so the rocket needs to be bigger to start with, oh and it needs to be that bit extra bigger to overcome the extra you added to overcome air resistance and so on up to a point where it’s big enough to overcome gravity affecting itself and the payload, max air resistance etc. (I think as I’m not a rocket scientist)

      Mark

    • Seems an odd question. You might as well ask why is it so hard to breathe underwater or why is Andromeda so far away.

      It’s not hard, or easy, it’s exactly as you would expect.

    • It isn’t as hard as people think – rockets are a very poor way of doing it. It comes down to the fact that you need rocket fuel and reaction mass to push the rocket, and you need more rocket fuel and reaction mass to push that rocket fuel etc etc.

      If there was a solid road to push against you wouldn’t need reaction mass, or reaction mass to lift the reaction mass etc. But the materials to build that solid road (space elevator) are currently beyond us.

      It’s still not trivial because whilst Earth’s gravity is weak (you are stronger than it!), it extends a long way from Earth so you have to do work against it for a long time.

      Something like a “space gun” needs almost incomparably less energy (fuel) to do the same thing as a rocket. Read about Gerald Bull to find out some more. No reason why one couldn’t have been built by now that launches lumps of metal, fuel or oxidiser Ito orbit, it’s just that there’s no need for one of the type we could built right now. We are a long way from the engineering needed to make a space gun that’s big enough to not squish meat bags and satellites on launch.

    • I think if you integrated the equation for potential energy to increase height above surface with respect to distance, g decreases, you would be surprised by the magnitude of energy required works surprise you, add to that the energy to burn to a velocity to keep orbit stable.

      Fuel is heavy.

      • Most of the energy is in forwards velocity, not height, for a stable orbit. Although the OP didn’t specifically refer to orbits.

        It’s not actually that much energy required. A quick estimate suggests the minimum energy required to put mass into geostationary orbit is about 60 MJ/kg or 16 kWH/kg, or about £2.50 worth of electricity at retail prices.

        It’s just that rockets are orders of magnitude worse than the theoretical minimum specific energy. Because, as you say, fuel (and reaction mass) is heavy.

    • It’s not gravity itself but the energy required to accelerate the payload and the initial but decreasing propellant mass up to the required velocity to achieve a more or less stable orbit, the point where instantaneous velocity and acceleration due to gravity reach a balance. remember the formula for kinetic energy includes V squared, so the energy input has to rise with the Square of the change in velocity required. (The same reason why stopping distance and killing power of a motor vehicle rise steeply as speed increases, they rise with the square of the velocity)

      • The energy required for a change in velocity also helps to understand why the Voyager missions were fly-by affairs rather than orbital insertion and long term study. At the speeds required to escape earth and, with the gravitational assistance of some of the planets, and at the eventual approach velocity there was never any question of something like Voyager having the necessary energy available to generate enough force to change its velocity enough to be captured in an orbit. That makes the Cassini-Huygens and similar missions very impressive.