4 ms·

> But maybe it's zipping too fast there to be easily shot down? A satellite in LEO already has a speed of about 8 km/s. The highest speed of a satellite in a h

> But maybe it's zipping too fast there to be easily shot down? A satellite in LEO already has a speed of about 8 km/s. The highest speed of a satellite in a highly elliptical orbit is 11 km/s. It you can shoot a satellite at 8 km/s, you probably can also shoot it at 11 km/s. The problem is, of course, that the perigee of a satellite in a highly elliptical orbit can be over a different part of the planet than your SAM battery.

jjk166 2y ago
> It you can shoot a satellite at 8 km/s, you probably can also shoot it at 11 km/s. Assuming all else is equal (same altitude, same targeting capability, same flight characteristics), an interceptor for an 11 km/s target needs about 89% more kinetic energy than an interceptor for an 8 km/s target. Realistically, between the rocket equation and atmospheric effects, the faster interceptor would likely need to be far larger still. You're probably talking about a 2.5 to 3 times bigger missile. While a country that could develop one could likely develop the other if they so chose, it doesn't seem like a safe assumption that a country with one has the other.
- s1artibartfast 2y ago
  I dont think that it is correct to assume your interceptor needs to match velocity. Any speed will do if you can intercept it on approach instead of chase it down.
  - jjk166 2y ago
    I'm not assuming the interceptor needs to match velocity. There is some point you detect the target x1, and some range for your missile from your position x0 to an interception point x2. Your interceptor needs to get from x0 to x2 by the time the time the target gets from x1 to x2. If your target is travelling 37.5% faster, you have 27.2% less time to get to the interception point, which means that you need to fly 37.5% faster, which requires 89% more kinetic energy, regardless of what the baseline speed required was. Alternatively you need to detect things from further away or fire from a closer distance, both of which pose their own issues.
    - credit_guy 2y ago
      That's not quite correct, is it? Consider shooting at a bird. A falcon travels twice as fast as a pigeon, but you don't need a bullet that is twice as fast to shot the falcon. You simply adjust the lead distance when you shoot at the falcon. In your argument you assume that you need to cover some distance in the same amount of time, but you don't need to. You simply lead the target a bit more. As for detection time, well, it is basically a guarantee that whoever is considering shooting at a space plane, has the tracking capabilities to know its position at all times. If they don't, they probably can find some reddit where people do that out in the open.
      - s1artibartfast 2y ago
        Agreed, it all depends on the use case. If you are trying to hit an evading target, you have to be faster. If you are trying to deny the ship from a destination, you don't.
        jjk166 2y ago
        If the ship's current position is 200 miles from its destination and you are 400 miles from its destination, you need to go faster than it to deny the ship from its destination. Conversely, if you are 40 miles from its destination, you can go much slower, though if you go too slow you still won't make it. You have to get to a point ahead of it before it gets there, and the faster its moving, the less time you have available to get to any particular point. Dealing with evasion is a whole other matter. A stationary iceberg can hit a ship that tries to evade if it does so too late. Conversely simple error in your knowledge of a ship's position and velocity can cause it to be miles away from where you expect it later in the journey.
        s1artibartfast 2y ago
        Like the parent says, you already know where it is. I'm struggling to understand the case where you are further from the destination than the x37B, with an orbit 60,000km from earth.
- bagels 2y ago
  I don't think that is quite right. You can hit a satellite with a suborbital projectile. The kinetic energy of the satellite is enough on its own if you hit it.
  - jjk166 2y ago
    It's not about energy required to destroy it, it's about speed required to get to it.

jjk166 2y ago

> It you can shoot a satellite at 8 km/s, you probably can also shoot it at 11 km/s. Assuming all else is equal (same altitude, same targeting capability, same flight characteristics), an interceptor for an 11 km/s target needs about 89% more kinetic energy than an interceptor for an 8 km/s target. Realistically, between the rocket equation and atmospheric effects, the faster interceptor would likely need to be far larger still. You're probably talking about a 2.5 to 3 times bigger missile. While a country that could develop one could likely develop the other if they so chose, it doesn't seem like a safe assumption that a country with one has the other.

s1artibartfast 2y ago
I dont think that it is correct to assume your interceptor needs to match velocity. Any speed will do if you can intercept it on approach instead of chase it down.
- jjk166 2y ago
  I'm not assuming the interceptor needs to match velocity. There is some point you detect the target x1, and some range for your missile from your position x0 to an interception point x2. Your interceptor needs to get from x0 to x2 by the time the time the target gets from x1 to x2. If your target is travelling 37.5% faster, you have 27.2% less time to get to the interception point, which means that you need to fly 37.5% faster, which requires 89% more kinetic energy, regardless of what the baseline speed required was. Alternatively you need to detect things from further away or fire from a closer distance, both of which pose their own issues.
  - credit_guy 2y ago
    That's not quite correct, is it? Consider shooting at a bird. A falcon travels twice as fast as a pigeon, but you don't need a bullet that is twice as fast to shot the falcon. You simply adjust the lead distance when you shoot at the falcon. In your argument you assume that you need to cover some distance in the same amount of time, but you don't need to. You simply lead the target a bit more. As for detection time, well, it is basically a guarantee that whoever is considering shooting at a space plane, has the tracking capabilities to know its position at all times. If they don't, they probably can find some reddit where people do that out in the open.
    - s1artibartfast 2y ago
      Agreed, it all depends on the use case. If you are trying to hit an evading target, you have to be faster. If you are trying to deny the ship from a destination, you don't.
      - jjk166 2y ago
        If the ship's current position is 200 miles from its destination and you are 400 miles from its destination, you need to go faster than it to deny the ship from its destination. Conversely, if you are 40 miles from its destination, you can go much slower, though if you go too slow you still won't make it. You have to get to a point ahead of it before it gets there, and the faster its moving, the less time you have available to get to any particular point. Dealing with evasion is a whole other matter. A stationary iceberg can hit a ship that tries to evade if it does so too late. Conversely simple error in your knowledge of a ship's position and velocity can cause it to be miles away from where you expect it later in the journey.
        s1artibartfast 2y ago
        Like the parent says, you already know where it is. I'm struggling to understand the case where you are further from the destination than the x37B, with an orbit 60,000km from earth.
bagels 2y ago
I don't think that is quite right. You can hit a satellite with a suborbital projectile. The kinetic energy of the satellite is enough on its own if you hit it.
- jjk166 2y ago
  It's not about energy required to destroy it, it's about speed required to get to it.