4 ms·

I think railguns are better at sending dumb payload masses, there's a fairly high amount of electricity pumped into the rails and IIRC the military ones have a

by pkinsky 11y ago

I think railguns are better at sending dumb payload masses, there's a fairly high amount of electricity pumped into the rails and IIRC the military ones have a sort of plasma muzzle flash effect. Cubesats tend to be lightweight and small, and probably wouldn't survive the launch.

lifeisstillgood 11y ago
Aha - sounded too good to be true. I suppose it depends on how fast we accelerate them upto right speed So if we imagine a Nimrod style dish on top of an aircraft near space, and say orbital speed is 30Mach or about 21,000mph call it 30kph for ease, and minimum useful weight would be 500g we need to input ... 54MW? And control travel around a dish tighter than anything previously seen. Yeah - nice idea ...
- sandworm101 11y ago
  It's a little more complicated. A railgun launch would see the payload moving at above orbital velocity immediately upon leaving the gun, creating serious aerodynamic headaches (drag). Rockets don't accelerate to such speeds until well above most of the atmosphere. For a tiny/lightweight object like a cubesat, the drag climbing up from the surface will be monumental. It's a trap. Drag requires more speed, which creates more drag. So you go with a heavier, more aerodynamically efficient object, which requires a larger railgun. The minimum size for this railgun is probably so large as to make the entire endeavor futile in comparison to rocketry. Artillery gets around this by firing relatively high. This wouldn't be practical for launch to orbit where you want as much momentum as possible laterally rather than vertically.
  - logfromblammo 11y ago
    The engineering required for an orbital launch gun is beyond the scale of anything yet built. Let's say, for the sake of argument, that your space gun's acceleration track climbs the slopes of Chimborazo in Ecuador and releases the launch vehicle at the summit. From there, if you launch eastward, you get about 460 m/s for free from the rotation of the Earth near the equator. LEO requires about 7800 m/s. Earth escape requires about 11180 m/s from that altitude. So if you go to LEO, you need another 7340 m/s of velocity. If you accelerate at 3 g, your track needs to be 920 km long. That isn't quite the distance from the end of your track to the Galapagos Islands. The same length of track could hit escape velocity with constant acceleration at 6.5 g. For reference, the Large Hadron Collider is 27 km in circumference, and cost about $5 billion. A space gun would be 34 times longer, and portions of it would have to be evacuated and neutrally buoyant at some depth under the ocean. The entire length would need to be powered. If you built a circular track 920 km in circumference, 147 km radius, the centripetal acceleration required to keep something moving at 7340 m/s on that track would be more than 37 g. That only increases as you reduce the radius. So no getting away with a shorter track.
    - sandworm101 11y ago
      Why only 6.5g? This isn't launching hamsters. Numbers like 100 or even 1000g are an option when dealing with tiny sats. There is already a small industry surrounding building electronics capable of being fired from artillery guns.
      - logfromblammo 11y ago
        That's just the minimum required acceleration to hit Earth escape velocity (at the peak of that mountain) with a track of that length. If you're going to build a megastructure like this, you might as well make it capable of launching manned vehicles. If your typical payload can handle 1000 g, you wouldn't need anywhere near that length of track. You would only need 3 km or so, I think. A 3 km long coilgun is possible with today's technology.
    - rakoo 11y ago
      This may be a stupid question, but why not do 2 stages, like today ? The railgun replaces the first stage but there's a second stage, like today, that gives all the lateral velocity.
      - logfromblammo 11y ago
        Stage I of a Saturn V contributes a maximum of about 3600 m/s of delta-v. Replacing a first stage of that magnitude would require a 3g acceleration track 220 km long. It still takes a lot of launches before the track starts to be cheaper than just building a new first stage for each launch. It takes a lot more if the first stage rocket is reusable.
        rakoo 11y ago
        There has been documented tests of railguns launching raw material around those speeds (https://en.wikipedia.org/wiki/Railgun#Tests https://en.wikipedia.org/wiki/Railgun#Tests). As the sibling comment said, there's no reason to limit ourselves to 3g (yet), and indeed someone laid out a plan to launch material to space with a railgun, where the first "stage" would be a railgun and the second stage would be a traditional rocket (https://en.wikipedia.org/wiki/Railgun#Launch_or_launch_assist_of_spacecraft https://en.wikipedia.org/wiki/Railgun#Launch_or_launch_assis...). It says that acceleration can reach a little less than 2000g. Unfortunately I haven't found any realization of this.

by pkinsky 11y ago

lifeisstillgood 11y ago

Aha - sounded too good to be true. I suppose it depends on how fast we accelerate them upto right speed So if we imagine a Nimrod style dish on top of an aircraft near space, and say orbital speed is 30Mach or about 21,000mph call it 30kph for ease, and minimum useful weight would be 500g we need to input ... 54MW? And control travel around a dish tighter than anything previously seen. Yeah - nice idea ...

sandworm101 11y ago
It's a little more complicated. A railgun launch would see the payload moving at above orbital velocity immediately upon leaving the gun, creating serious aerodynamic headaches (drag). Rockets don't accelerate to such speeds until well above most of the atmosphere. For a tiny/lightweight object like a cubesat, the drag climbing up from the surface will be monumental. It's a trap. Drag requires more speed, which creates more drag. So you go with a heavier, more aerodynamically efficient object, which requires a larger railgun. The minimum size for this railgun is probably so large as to make the entire endeavor futile in comparison to rocketry. Artillery gets around this by firing relatively high. This wouldn't be practical for launch to orbit where you want as much momentum as possible laterally rather than vertically.
- logfromblammo 11y ago
  The engineering required for an orbital launch gun is beyond the scale of anything yet built. Let's say, for the sake of argument, that your space gun's acceleration track climbs the slopes of Chimborazo in Ecuador and releases the launch vehicle at the summit. From there, if you launch eastward, you get about 460 m/s for free from the rotation of the Earth near the equator. LEO requires about 7800 m/s. Earth escape requires about 11180 m/s from that altitude. So if you go to LEO, you need another 7340 m/s of velocity. If you accelerate at 3 g, your track needs to be 920 km long. That isn't quite the distance from the end of your track to the Galapagos Islands. The same length of track could hit escape velocity with constant acceleration at 6.5 g. For reference, the Large Hadron Collider is 27 km in circumference, and cost about $5 billion. A space gun would be 34 times longer, and portions of it would have to be evacuated and neutrally buoyant at some depth under the ocean. The entire length would need to be powered. If you built a circular track 920 km in circumference, 147 km radius, the centripetal acceleration required to keep something moving at 7340 m/s on that track would be more than 37 g. That only increases as you reduce the radius. So no getting away with a shorter track.
  - sandworm101 11y ago
    Why only 6.5g? This isn't launching hamsters. Numbers like 100 or even 1000g are an option when dealing with tiny sats. There is already a small industry surrounding building electronics capable of being fired from artillery guns.
    - logfromblammo 11y ago
      That's just the minimum required acceleration to hit Earth escape velocity (at the peak of that mountain) with a track of that length. If you're going to build a megastructure like this, you might as well make it capable of launching manned vehicles. If your typical payload can handle 1000 g, you wouldn't need anywhere near that length of track. You would only need 3 km or so, I think. A 3 km long coilgun is possible with today's technology.
  - rakoo 11y ago
    This may be a stupid question, but why not do 2 stages, like today ? The railgun replaces the first stage but there's a second stage, like today, that gives all the lateral velocity.
    - logfromblammo 11y ago
      Stage I of a Saturn V contributes a maximum of about 3600 m/s of delta-v. Replacing a first stage of that magnitude would require a 3g acceleration track 220 km long. It still takes a lot of launches before the track starts to be cheaper than just building a new first stage for each launch. It takes a lot more if the first stage rocket is reusable.
      - rakoo 11y ago
        There has been documented tests of railguns launching raw material around those speeds (https://en.wikipedia.org/wiki/Railgun#Tests https://en.wikipedia.org/wiki/Railgun#Tests). As the sibling comment said, there's no reason to limit ourselves to 3g (yet), and indeed someone laid out a plan to launch material to space with a railgun, where the first "stage" would be a railgun and the second stage would be a traditional rocket (https://en.wikipedia.org/wiki/Railgun#Launch_or_launch_assist_of_spacecraft https://en.wikipedia.org/wiki/Railgun#Launch_or_launch_assis...). It says that acceleration can reach a little less than 2000g. Unfortunately I haven't found any realization of this.