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>If we could build a robust fully-reusable space plane that can go Mach 15, and we could also find a way to rendezvous it with a rotating tether, we could free

>If we could build a robust fully-reusable space plane that can go Mach 15, and we could also find a way to rendezvous it with a rotating tether, we could free ourselves from the nastiness of the rocket equation and achieve inexpensive space access. If we could build a robust fully-reusable space plane that can go Mach 15 free from the nastiness of the rocket equation then we already practically achieved inexpensive space access :). If you look at the rocket equation - it is the first 15 Mach that require to burn almost all this mass of fuel through all these big engines pumped from all these big tanks. Once you're going 15 Mach - to get to the 30 Mach for the payload of N kg, you'd need to burn only ~N kg of LH2+LOX (4.5km/s nozzle speed) - so not much gain would come by replacing rocket propulsion with anything else on the second half of the acceleration. The first half is the problem.

stcredzero 15y ago
If we could build a robust fully-reusable space plane that can go Mach 15 free from the nastiness of the rocket equation then we already practically achieved inexpensive space access :). If you look at the rocket equation - it is the first 15 Mach that require to burn almost all this mass of fuel through all these big engines pumped from all these big tanks. By not requiring full orbital velocity, we are freeing ourselves from the particular nastiness of the rocket equation on the surface of our planet. Our gravity well is just deep enough, that SSTO is just beyond the edge of what we can achieve with chemical rockets. I proposed Mach 15 because it's significantly less than the Mach 20 result, but in this case it's clear that I still chose a number too high.
- onemoreact 15y ago
  The problem with rockets is the 0 part of H20. 0 = 15.9994, H = 1.00794 so by burning oxygen from the air you drop 15.9994/ (1.00794 * 2 + 15.9994) = 88.8% of the weight of your fuel which makes everything a lot simpler. Less weight takes less fuel to lift which reduces the size of the craft which saves even more fuel or let's you build a stronger craft or one better suited for reentry. An air breathing craft that can get to Mach 10+ would easily drop the cost of getting to space to 1/10th what it is today and let do things like have some reserve fuel in case you messed up the landing and want to come around for another pass. If you can hit Mach 20 then LEO starts to take less energy than flying around the world.
  - deleted 15y ago
    [deleted]
  - stcredzero 15y ago
    The HASTOL scheme was originally proposed with an air-breathing Mach 10 (12?) hypersonic plane.
  - VladRussian 15y ago
    >The problem with rockets is the 0 part of H20. 0 = 15.9994, H = 1.00794 so by burning oxygen from the air you drop 15.9994/ (1.00794 * 2 + 15.9994) = 88.8% of the weight of your fuel which makes everything a lot simpler. there are 2 issues with oxygen from the air: 1. the air is moving fast relatively to the aircraft frame of reference. To burn it, the air should be slowed down in the aircraft frame of reference. That means acceleration of that air in the Earth frame of reference. It isn't a noticeable energy loss below Mach 2, yet above the Mach 2-3 it starts to impact the efficiency of the air breathing scheme to the point that rocket engine carrying oxygen with it (ie. accelerating it in the Earth frame of reference and keeping it non-moving in the aircraft frame of reference) doesn't look that less efficient, and the higher the speed the less the efficiency gap. Scramjets (no slowing down of the air) while seemingly fixing that problem face the other side of the same issue - trying to impact even small amount of additional momentum on the already fast moving air (in the aircraft frame of reference) requires an unproportionally increasing (the same v square) amount of power. 2. due to composition of the Earth atmosphere, to burn 1kg of oxygen, the aircraft need to pump through engine - ie. accelerate as described in the point 1. above to the speed close to its own in the Earth's frame of reference - 5 kg of air. The jet propulsion itself doesn't care that mass m being thrown out of the nozzle with velocity v consists of - H, O2, H2O, steel balls or foam - doesn't make difference as long as it is of mass "m", and the aircraft needs to come up with that mass somehow - carry it with itself or gather from outside (and accelerate the mass or try to impact momentum on the already fast moving mass). Take the points above, add the simplicity and low weight of the rocket engine in comparison with air breathing engines and you will see why rocket engines are dominating the arena.
    - onemoreact 15y ago

stcredzero 15y ago

If we could build a robust fully-reusable space plane that can go Mach 15 free from the nastiness of the rocket equation then we already practically achieved inexpensive space access :). If you look at the rocket equation - it is the first 15 Mach that require to burn almost all this mass of fuel through all these big engines pumped from all these big tanks. By not requiring full orbital velocity, we are freeing ourselves from the particular nastiness of the rocket equation on the surface of our planet. Our gravity well is just deep enough, that SSTO is just beyond the edge of what we can achieve with chemical rockets. I proposed Mach 15 because it's significantly less than the Mach 20 result, but in this case it's clear that I still chose a number too high.

onemoreact 15y ago
The problem with rockets is the 0 part of H20. 0 = 15.9994, H = 1.00794 so by burning oxygen from the air you drop 15.9994/ (1.00794 * 2 + 15.9994) = 88.8% of the weight of your fuel which makes everything a lot simpler. Less weight takes less fuel to lift which reduces the size of the craft which saves even more fuel or let's you build a stronger craft or one better suited for reentry. An air breathing craft that can get to Mach 10+ would easily drop the cost of getting to space to 1/10th what it is today and let do things like have some reserve fuel in case you messed up the landing and want to come around for another pass. If you can hit Mach 20 then LEO starts to take less energy than flying around the world.
- deleted 15y ago
  [deleted]
- stcredzero 15y ago
  The HASTOL scheme was originally proposed with an air-breathing Mach 10 (12?) hypersonic plane.
- VladRussian 15y ago
  >The problem with rockets is the 0 part of H20. 0 = 15.9994, H = 1.00794 so by burning oxygen from the air you drop 15.9994/ (1.00794 * 2 + 15.9994) = 88.8% of the weight of your fuel which makes everything a lot simpler. there are 2 issues with oxygen from the air: 1. the air is moving fast relatively to the aircraft frame of reference. To burn it, the air should be slowed down in the aircraft frame of reference. That means acceleration of that air in the Earth frame of reference. It isn't a noticeable energy loss below Mach 2, yet above the Mach 2-3 it starts to impact the efficiency of the air breathing scheme to the point that rocket engine carrying oxygen with it (ie. accelerating it in the Earth frame of reference and keeping it non-moving in the aircraft frame of reference) doesn't look that less efficient, and the higher the speed the less the efficiency gap. Scramjets (no slowing down of the air) while seemingly fixing that problem face the other side of the same issue - trying to impact even small amount of additional momentum on the already fast moving air (in the aircraft frame of reference) requires an unproportionally increasing (the same v square) amount of power. 2. due to composition of the Earth atmosphere, to burn 1kg of oxygen, the aircraft need to pump through engine - ie. accelerate as described in the point 1. above to the speed close to its own in the Earth's frame of reference - 5 kg of air. The jet propulsion itself doesn't care that mass m being thrown out of the nozzle with velocity v consists of - H, O2, H2O, steel balls or foam - doesn't make difference as long as it is of mass "m", and the aircraft needs to come up with that mass somehow - carry it with itself or gather from outside (and accelerate the mass or try to impact momentum on the already fast moving mass). Take the points above, add the simplicity and low weight of the rocket engine in comparison with air breathing engines and you will see why rocket engines are dominating the arena.
  - onemoreact 15y ago