MoreAmphibians [none/use name]

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Joined 4 years ago
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Cake day: February 6th, 2021

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  • The Delta-V (change in velocity) to reach the ISS (400km) in Low Earth Orbit (LEO) is 9.4km/s. This is the absolute minimum so it doesn’t include things like drag or other inefficiencies. Getting to space takes only about 1.4km/s, just about all the rest of that delta-v is used to accelerate the spaceship to 7.6km/s. This is about the minimum speed, altitude, and delta-v to get into orbit and stay there. Going much lower than the ISS will have your orbit decay too much to stay in orbit for more than weeks or months. I think it would take about 8000km/s to orbit right at 100km but your orbit would decay really quickly at that altitude, you would only get a few hours or orbiting.

    I think I answered you below about why you can’t to orbit using only a railgun.

    Edit: Going from earth orbit to other places doesn’t take as much delta-v as you might think. Going from earth orbit to lunar orbit only takes about 3km/s and going from earth orbit to mars orbit takes 5.7km/s.


  • The problem is that you still need an additional acceleration in order to get into an real orbit. In any theoretical orbit (without an acceleration) you’ll always go around in a circular motion until you reach your starting point, if your starting point is in the atmosphere then you’ll go around your orbit until you hit your starting point in the atmosphere again. This will obviously cause your orbit to decay. There are a few solutions.

    1. Just use a rocket engine. This is the simplest and most practical solution. Vacuum engines are very efficient and you don’t need that much fuel.
    2. Use a skyhook. This is a giant spinning thing in orbit. Your spaceship “grabs on” to an end of a line and takes momentum from the already orbiting skyhook station in order to get in orbit yourself.
    3. You might be able to do something weird with the Moon’s gravity in order to change your orbit. This would take a while and you would need a huge velocity from the railgun and your orbit would be very high and probably no circular, but there’s no physics reasons it wouldn’t work (I think).