I just couldn't help thinking about the possibility of the tow vehicle using atmospheric oxygen to resupply itself so here goes:-The vehicle grabbed by the tow vehicle would contain payload and a tank of LH2.-Upon connection the LH2 would begin to feed the tow vehicle engines using the tow vehicle's LOX-Upon achieving orbit the payload would be separated but the LH2 tank retained-The tow vehicle would use extra LH2 in the tank to boost itself to a highly elliptical orbit -From this orbit the tow vehicle would use a ram scoop to gather air by dropping its perigee into the upper atmosphere-The compressed air would be separated by expansion to vacuum of enough air to begin liquefaction after which the difference in boiling points of O2 and N2 would allow separation. Alternately a filter could be used I suppose. After that initial step, N2 would be exhausted to vacuum to provide cooling to liquefy O2 which would be stored in the tow vehicle's tank. -Subsequent passes into the upper atmosphere would gather more O2 until the tank was full and the apogee lowered leaving the tow vehicle in a circularish low orbit-The tow vehicle would re-enter to rendezvous with another payload and LH2 tank to begin the process againLN2 could be retained and used to cool the ram scoop inlet and ejected as it was heated - this might lessen the slow down due to drag somewhat if it was ejected through a nozzle in the proper direction perhaps Assuming the tow vehicle had similar dry mass as the payload vehicle the amount of LH2 would be about 3 -4 times what would be needed for a second stage to raise the payload to orbit(I think). Compared with a second stage which would have to carry LOX, and assuming a 6:1 LOX to LH2 ratio, the vehicle could have extra payload equivalent to about half the mass of LOX a second stage would have to carry. If the tow vehicle used a different stochiametric ratio say 10:1 the amount of LH2 needed would be less I suppose. If payload represented 25% of second stage mass normally, in this case the payload would be 3 times what it would otherwise be.OK - I realize this is imprecise and simplistic but its just me taking a stab at it. It looks like it may triple the payload and between that and the re-usability of the tow vehicle vs. a throw away second stage perhaps reducing costs itself, Skyhitch may significantly reduce cost to orbit a given payload even without a space based propellant supply infrastructure.
This is probably correct but the "reason" behind the tethers was your actual "window" is going to be about half or less of that and "slamming" a couple of vehicles together at high speed with little time is not seen as a "feature"
QuoteI think maybe a tether might make sense if you designed around rendezvous in mid-air. That way, the LV could stay comfortably outside the shock cones of the skyhitch, and drag could keep the tether taut and trailing. However, I propose using pure rocket propulsion, so neither vehicle is airbreathing. I think it would be better to get out of the atmosphere.Actually the reverse, you can't use tethers while in the atmosphere because they couldn't stand up to being hit by the shock waves. (They would in fact "lay" on the shock wave and melt away if you tried)
I think maybe a tether might make sense if you designed around rendezvous in mid-air. That way, the LV could stay comfortably outside the shock cones of the skyhitch, and drag could keep the tether taut and trailing. However, I propose using pure rocket propulsion, so neither vehicle is airbreathing. I think it would be better to get out of the atmosphere.
Outside the atmosphere they don't have to worry about drag or aerodynamics. The "reasoning" behind the idea is you could have one vehicle with a "target" and have the other fire a tether and grapple that would attach to the target and allow the vehicles to be hauled together similar to mid-air-refueling. Once attached the "tow" vehicle reels in the tether until the two vehicles "latch" together rather than having to rely on (and be subject too) close use of active RCS.
The trajectories seem too different for this.They could reach same position and same speed, but with different velocity vector
anyone remember the buck rogers hawk ship ?
[...]But then that starts to make things sound unnecessarily complicated perhaps. Going back to the hard dock rendezvous, perhaps something akin to the Phoenix/Dove spacecraft pair from the movie "Journey to the far side of the sun"? Have essentially a wide hollow cone skyhitch, with potentially an airbag rendezvous receiver cone area in the rear center to guide docking?
Quote from: RanulfC on 01/29/2013 08:52 pmThis is probably correct but the "reason" behind the tethers was your actual "window" is going to be about half or less of that and "slamming" a couple of vehicles together at high speed with little time is not seen as a "feature" Thing is...I don't see tethers as necessarily helping this situation, compared to directly docking.
Quote from: IsaacKuo on 01/31/2013 07:31 pmQuote from: RanulfC on 01/29/2013 08:52 pmThis is probably correct but the "reason" behind the tethers was your actual "window" is going to be about half or less of that and "slamming" a couple of vehicles together at high speed with little time is not seen as a "feature" Thing is...I don't see tethers as necessarily helping this situation, compared to directly docking.See I'm not actually advocating tethers for this concept... Just trying to explain why they came up A whole lot depends on the vehicle dynamics involved. "MY" personal take is that you'd be looking at an advanced version of the "sticky-boom" followed by a vehicle-to-vehicle hard-dock. The booms allow all the control of an RCS cluster without the vehicle impingment issues. (And they should be about as fast acting)The problem with the "nose-to-tail" docking that I see is that the "tug" nose is going to need to be pretty robust for atmospheric manuvering which is probably going to require some active cooling and the last thing you want is an impact to damage that capability. (I'm seeing the Skyhitch-Tug as something akin to the Spacecruiser vehicle: http://www.up-ship.com/apr/extras/scruiser1.htm)Randy
The vehicles must approach each other at a high rate and slow down quickly in order to prevent damage don't they? The way it seems to me the tow vehicle is slowing down on the last skip and the towed vehicle is accelerating to a path that takes it above the atmosphere. I question whether they can reliably get to the exact same position and velocity without some maneuvering which requires a robust RCS on both vehicles and more importantly, time.
One thing to point out is how will the Skyhitch be propelled after capture?
Conventional chemical propulsion requires both oxidizer and fuel. PROFAC type systems generally only provide oxygen and nitrogen in large quantities.
One thought that springs to mind is something similar to Jordan Kare's laser heat exchanger rockets. A suitable shape could have a large area absorber plate, which might double as an actively cooled heatshield during the skip. Would a hot nitrogen rocket (fueled by PROFAC liquid nitrogen) have appropriate thrust and exhaust velocity? The laser power requirements might be lower than Kare's laser SSTO, but range/LoS issues become more dominant, as well as laser beaming station location.
You could use oxygen acquired by atmospheric scooping combined with hydrogen carried on the launch vehicle.
With an atomic mass of 14, N2 gas molecules have 14 times the mass of H2 gas molecules. So, while the specific impulse of laser thermal heat exchange for hydrogen propellant may be around 800s, using nitrogen propellant reduces that potential to around 800s/sqrt(14) = 214s. This has much the same problem as using nitrous oxide monopropellant--the specific impulse is too low for the desired 5km/s delta-v. With 214s specific impulse, 5km/s requires a mass ratio of 10.8.