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How can this possibly work without at least some maneuvering rocket fuel? If the catapult is on the ground, any ballistic trajectory it can achieve intersects t

by dTal 2y ago

How can this possibly work without at least some maneuvering rocket fuel? If the catapult is on the ground, any ballistic trajectory it can achieve intersects the ground. You can blast stuff into space, sure, but orbit? And the closer to a circular trajectory you get, the shallower the launch angle, and the more velocity-sapping atmosphere you have to plough through before you get to space.

bunderbunder 2y ago
I mentioned this elsewhere, but the aggressively simple answer is that it still works pretty much the same as current launch technology: the big huge rocket gets the payload onto an appropriately high suborbital trajectory, and then the payload use its own (much smaller) rocket to circularize the orbit. They're replacing the launch vehicle with a centrifuge, but otherwise the mission architecture remains broadly similar. The launch vehicle only getting things onto a suborbital trajectory is a key reason why SpaceX recovering Falcon 9s is possible. They are not making a round trip around the planet before coming back, which would be necessary if it were actually doing orbital insertion too since the circularization burn happens a decent chunk of the way around the planet from the launch site. It would also be insanely wasteful; the fuel needed to get that whole giant chunk of steel up to orbital velocity would be vastly greater than the cost of just putting a small orbital maneuvering engine on the orbital vehicle so that it can finish the job for itself. And as an added bonus the spacecraft also gets to use that engine for general-purpose orbital maneuvering. The Space Shuttle did something similar. Those huge main engines only got the orbiter onto a sub-orbital trajectory, too. They'd then jettison the external fuel tank while still on a sub-orbital trajectory (helpful for reducing how much space junk you produce), and use the OMS for orbital insertion. You really don't need such a big engine for this part of it, anyway. The SSME produced about 150x as much vacuum thrust as the OMS.
- mannykannot 2y ago
  If you are on a sub-orbital trajectory, you have to do something more than “circularize” to get into any sort of orbit. How much more? Well, it depends on what your velocity is at the point where you fire the rocket. If your sub-orbital trajectory just makes it to about the von Kármán line with no more tangential speed than you had at ground level, you only have a fraction (~10%?) of the kinetic energy needed for an orbit. On the other hand, you cannot have anything approaching the minimum orbital speed while still in the atmosphere, as demonstrated by every satellite burning up on re-entry, but any unassisted catapult-launched object must have a higher speed at ground level than it will at altitude. The article’s claim, on a point that is fundamental to the concept, is misleading.

by dTal 2y ago

bunderbunder 2y ago

I mentioned this elsewhere, but the aggressively simple answer is that it still works pretty much the same as current launch technology: the big huge rocket gets the payload onto an appropriately high suborbital trajectory, and then the payload use its own (much smaller) rocket to circularize the orbit. They're replacing the launch vehicle with a centrifuge, but otherwise the mission architecture remains broadly similar. The launch vehicle only getting things onto a suborbital trajectory is a key reason why SpaceX recovering Falcon 9s is possible. They are not making a round trip around the planet before coming back, which would be necessary if it were actually doing orbital insertion too since the circularization burn happens a decent chunk of the way around the planet from the launch site. It would also be insanely wasteful; the fuel needed to get that whole giant chunk of steel up to orbital velocity would be vastly greater than the cost of just putting a small orbital maneuvering engine on the orbital vehicle so that it can finish the job for itself. And as an added bonus the spacecraft also gets to use that engine for general-purpose orbital maneuvering. The Space Shuttle did something similar. Those huge main engines only got the orbiter onto a sub-orbital trajectory, too. They'd then jettison the external fuel tank while still on a sub-orbital trajectory (helpful for reducing how much space junk you produce), and use the OMS for orbital insertion. You really don't need such a big engine for this part of it, anyway. The SSME produced about 150x as much vacuum thrust as the OMS.

mannykannot 2y ago
If you are on a sub-orbital trajectory, you have to do something more than “circularize” to get into any sort of orbit. How much more? Well, it depends on what your velocity is at the point where you fire the rocket. If your sub-orbital trajectory just makes it to about the von Kármán line with no more tangential speed than you had at ground level, you only have a fraction (~10%?) of the kinetic energy needed for an orbit. On the other hand, you cannot have anything approaching the minimum orbital speed while still in the atmosphere, as demonstrated by every satellite burning up on re-entry, but any unassisted catapult-launched object must have a higher speed at ground level than it will at altitude. The article’s claim, on a point that is fundamental to the concept, is misleading.