Author Topic: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing  (Read 15617 times)

Offline IsaacKuo

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Everyone knows ISRU propellant can't help you launch from Earth to LEO.  Or can it?

The idea behind "skyhitch" is for a suborbital launch vehicle (LV) to hitch a ride with an orbital tow vehicle (skyhitch).  The skyhitch meets the LV halfway by aerobraking, and then it carries the LV back to orbit.  The sequence of events is:

1) skyhitch de-orbits from LEO
2) LV launches into a 3km/s hypersonic hop (costs 5km/s)
3) skyhitch S-turns down to 3km/s, aeromaneuvering to match LV
4) skyhitch and LV ascend from atmosphere in parallel
5) skyhitch uses thrusters to rendezvous with LV
6) skyhitch latches onto LV
7) skyhitch boosts back to orbital speed (costs 5km/s)

With a specific impulse of around 450s, the mass ratio required for 5km/s is only about 3:1.  Multiple stages are not required.

The skyhitch and LV may share a common airframe, but the LV engine is optimized for low altitudes while the skyhitch engine is optimized for vacuum.  Also, the LV engine might use denser fuels so it has more room for cargo, while the skyhitch uses its entire volume for fuel.

This hypersonic skyhitch system shares some features with the hypersonic skyhook concept.  But a hypersonic skyhook requires development of tether technology and presents problems with other satellites.

Hypersonic skyhitch uses existing technology, and it plays nice with other spacecraft.  In particular, the skyhitch may be based on the ISS.  It can undock, pick up an LV, and return all while keeping the same ground track as the ISS.  This isn't an option for a hypersonic skyhook.

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #1 on: 01/24/2013 10:24 pm »
 Great minds think alike - good to see you posting here - seems like a while since you were describing your idea for using collision to transfer momentum from a spacecraft perihelion thrust to an orbiting vehicle.
 I actually submitted a similar idea to NASA in an RFI solicitation and discussed it here: http://forum.nasaspaceflight.com/index.php?topic=21530.90.
 As I developed the idea I proposed using small rugged vehicles based on kinetic kill type ABM vehicles carrying tethers from each vehicle that could home in on each other, slow down and latch to each other. The tow vehicle would then slowly take up the slack and tow the payload or entire launch vehicle into orbit.
 
Steve Mickler

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #2 on: 01/24/2013 10:37 pm »
 Here's an excerpt from the RFI:

The obvious question is: how can space based activities aid launching payloads to orbit? Production of propellants  and space based spacecraft servicing servicing  are nothing new, at least conceptually, but what is new in this proposal is the means to have a space based vehicle capture a suborbital payload launched from the ground and accelerate it to orbital velocity. This vehicle, call it the boost vehicle, will remain in LEO and be serviced in an inflated hangar, perhaps attached to or near the ISS. This supports the large inflatable space structures part of the new technology development plans and turns the ISS into a true space station and a vital link in the space transportation infrastructure.
   The proposal also hinges on solar powered and propelled vehicles to mine asteroid regolith and act as OTV’s to resupply the boost vehicle’s propellant which supports the FTD-1 and beyond parts of the new plan, particularly SEP and AR&D. The need for large amounts of propellant to be produced from NEO regolith creates a situation quite unlike that imagined for proposed propellant depots. These depots  would be so infrequently used that keeping the propellants cold and stationkeeping become major problems. In contrast, this proposal imagines telerobotic based ISRU systems in GEO where the speed of light delay between operator command and robot response is constant unlike LEO. Large amounts of NEO regolith could be returned by the robotic mining vehicles for refining at GEO provided recently discovered asteroids with very low dV prove suitable. Demand would be so high that long term propellant storage would not be a factor and surplus would be available for stationkeeping. 
   The space based space transportation concept  also leverages the burgeoning suborbital launch market to provide launch services.
Advantages : To understand the advantages of a space based space transportation system, it is first necessary io explain how the space based space transpotation system might work.
The launch sequence would be as follows:
- The boost vehicle(BV) uses retro fire to de-orbit from LEO
- BV uses a combination of lift and drag to “skip” onto a suborbital trajectory
- BV repeats the process untill its velocity and trajectory match the launch vehicle’s
-The launch vehicle(LV) would have acheived a suborbital trajectory such that it remains in space  for several minutes at a minimum.
-  LV and BV rendezvous in space and use a capture mechanism to exchange payload-
capture system could take many forms but one possibility is that each vehicle would send out small, rugged tethered capture vehicles that would quickly home in on each other and mechanically connect their attached tether cables. The payload module (or even the entire LV) could be then towed to orbit or winched into a payload bay in the BV
- LV seperates and re- enters for landing
- BV LH2-O2 main engine(s) ignitie and boost BV to LEO
- BV rendezvous and docking at ISS/hangar for servicing and “refueling” and payload drop-off. Alternately, the BV could be met by a solar OTV carrying propellent to keep flight rate higher that would then transfer the payload module to the ISS
- Robotic Solar OTV’s support mining, refining and transportation of NEO derived hydrogen and oxygen propellant
    Obviusly the challenging part is payload exchange on a suborbital trajectory. This will require new speacialized guidance and capture technology that at first examination may seem unacheivable in the near term, given that on-orbit rendezvous and docking requires hours after days of manuvers today. There is however the possibility of repeated flight tests with unmanned subscalevehicles and an example from aviation’s past that give reason to be optimistic. In-flight refueling was once considered a daredevil stunt and yet today it is routine and essential. Indeed, in-flight re-fueling is the inspiration for the previously noted possible capture mechanism similar in some ways to the probe and drouge type in-flight refueling used by the Navy.
   A positive feedback establishes as more launches are required first to build up the required infrastructure and then maintain it. With higher flight rates come lower costs which in turn brings explorers, tourists, and perhaps even GEO based telerobotic industrial parks. High flight rate will bring rapid amortization of the initial investment and as robotic mining of  NEO’s develops to satisfy the large propellant demands; the cost of raw materials in space will plummet leading to increased industrial production in GEO raising the possibility that after a decade or so it will become less expensive to telerobotically produce consumer goods there than in any factory on Earth. It’s the 21st century, shouldn’t our space technology reflect that fact?



Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #3 on: 01/24/2013 11:12 pm »
 I even made a crude pic of the idea:



Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #4 on: 01/24/2013 11:14 pm »
Great minds think alike - good to see you posting here - seems like a while since you were describing your idea for using collision to transfer momentum from a spacecraft perihelion thrust to an orbiting vehicle.
 I actually submitted a similar idea to NASA in an RFI solicitation and discussed it here: http://forum.nasaspaceflight.com/index.php?topic=21530.90.
Interesting!  The idea as you originally describe it sounds essentially identical to what I was thinking.  Obviously there are a few details which are different, such as the suggested ISRU fuels, but it's essentially the same.

The tone of the ensuing discussion is a bit depressing and unfortunately typical, I think.  There is a lack of comprehension and vision.

As I see it, the main advantages are reducing operational costs.  The launch vehicle only needs 5km/s, so it can avoid LH2 ground infrastructure and multistage integration costs.  The reduced mass ratio means it can have one small engine rather than ten (like Falcon 9), reducing costs.  The reduced mass ratio also eliminates the really tall first stage.  This further simplifies integration and reduces time/costs.

LV return might involve gliding landing like the Space Shuttle, or it could use a parachute.  The former costs more mass, but can speed up throughput.  The end result can be aircraft-like operations.
Quote
As I developed the idea I proposed using small rugged vehicles based on kinetic kill type ABM vehicles carrying tethers from each vehicle that could home in on each other, slow down and latch to each other. The tow vehicle would then slowly take up the slack and tow the payload or entire launch vehicle into orbit.
This seems complex and not necessary, I think.  What's wrong with using clamps?

Offline Asteroza

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #5 on: 01/25/2013 01:39 am »
If you want two tethers/booms working together to hook up, maybe the Sticky Boom guys (Altius Space Machines) might be good.

I have this strange urge to call this a reverse bomber or an unbomber.

The payload hookup/transfer, or wholesale grabbing of a suborbital RLV (sRLV) seems like a solvable problem considering automated UAV refueling work and extending back to the Harrier Skyhook concepts for the Royal Navy.

Speaking of skyhooks, what are the defining advantages of skyhitch over a rotovator hypersonic skyhook? It seems like they cover many of the same areas.

1. exoatmospheric rendezvous at "hypersonic" suborbital speeds
2. orbital energy of the upper half is ISRU restored (PROFAC fuel for skyhitch, solar electric something for a skyhook rotovator MX station)
3. Lower gross weight of sRLV compared to SSTO/TSTO, making takeoff from conventional airports more reasonable. Cuts down on ground infrastructure/ops requirements. Think stuff like XCOR Lynx.

Both suffer from the problem of where the active portion of the hookup is designated, and how dumb the receiver side design of the connection is.

Skyhitch may have some interesting points in it's favor though;

1. Since the skyhitch is essentially a skip glide vehicle, it is also capable of aeroassist plane changes in theory. This allows redeployment into other orbits, compared to the orbital energy investment you make in a skyhook due to the much larger mass of the rotovator MX station.
2. Arguably better spatial/temporal flexibility for the rendezvous. A rotovator skyhook has much tighter docking requirements in terms of the docking window. The dispatchable timing aspect for skyhitch is on demand for the window as well, compared to the fixed schedule of skyhook. A bit of a truck vs train issue here?
3. Easier to do a heavy launch campaign quickly, provided there is sufficient available ISRU fuel (at the ISRU propellant depot). A skyhook would likely be electric in the nature of it's orbital energy recovery, thus a recharge downtime is imposed once it has reached it's momentum exchange limit (or risk deorbiting), though the downtime can be shortened with external power receivers of some kind (or a heavy nuke as a MX counterweight, but we know how people will react to that...)

Skyhitch also has some bad aspects;

1. If the skyhitch has a propulsive problem during the lift, you ditch (no landing gear) or abort to ground, along with a heavyweight landing of the sRLV after disconnecting, which means your system is down until a replacement skyhitch can be launched. Skyhook focuses on a tether break during lift, but even then the skyhook facility should be still okay, and a tether replacement would make it operational again in short order.
2. Skyhitch consumes PROFAC ISRU fuel, which has potentially higher market value as fuel for exclusively orbital assets. Rotovators recovering orbital energy via electric thrusters and/or electrodynamic tether thrust (likely via solar) arguably do better in this regard.


Total system cost, and specific orbital energy spot price value ($ / Joules per kilogram / inclination) might be interesting parameters to play with. With ISRU, you are essentially banking solar energy (if solar powered PROFAC or solar electric skyhook). What is the market valuation of that orbital energy, if it can be provided as a service from above rather than hauled from below?

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #6 on: 01/25/2013 04:35 pm »
 @IssacKuo: I just want to add that the S-turn idea to slow the tow vehicle down you propose offers advantages over the purely Sanger "skip bomber" inspired idea I proposed and I like the name "Skyhitch" better than "Space Based Space Transportation System".
 The tether grappling vehicle idea may indeed be too complex but abort might be easier and the risk of the two main vehicles colliding might be reduced.
@Asteroza What a great evaluation of the ideas especially the suggestion of "sticky boom's" releveance and the orbit plane change capability that dipping into the atmosphere for lift affords. I hadn't thought of thaat.

Offline Archer

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #7 on: 01/25/2013 06:27 pm »
You have stolen my idea) :)

Though I was thinking that "skyhitch" should be just an external fuel tank with TPS and some guiding.
« Last Edit: 01/25/2013 06:30 pm by Archer »
The future is better than the past. Despite the crepehangers, romanticists, and anti-intellectuals, the world steadily grows better because the human mind, applying itself to environment, makes it better. With hands...with tools...with horse sense and science and engineering. (c) R. A. Heinlein

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #8 on: 01/25/2013 07:14 pm »
Woah!  I hadn't really thought about the significance of plane changes.  I described using "S" turns, but if you just keep turning in the same direction, you can do a "C" turn and maybe even a "U" turn.

Slowing down from 7.8km/s down to the 3km/s rendezvous speed bleeds away 85% of your kinetic energy.  This gives you the ability to turn a lot even with a mediocre lift/drag ratio.

This means that skyhitch can be used to launch into just about any inclination.  That is huge.  You can use it for everything from equatorial to sun synch to retrograde to everything in between.

Of course, this depends on the details of how the skyhitch is refueled.  Assuming you want the skyhitch tow vehicle to be reusable, you'd rather have a resupply depot waiting in the destination inclination.

@Archer - an knat-pord is an interesting idea, but I see two problems.  The first is that you need to hook up cross-feed lines, which adds to the technological challenge and risk.  The second is that the LV engine needs to compromise between atmospheric and vacuum performance.  It also needs to be restartable, of course.
« Last Edit: 01/25/2013 07:48 pm by IsaacKuo »

Offline RanulfC

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #9 on: 01/25/2013 09:10 pm »
IsaacKuo:
Why not just exchange a cargo pod rather than tow the whole sRLV into orbit? That will save a "ton" of mass and allow the sRLV to have a faster turn around won't it?

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #10 on: 01/25/2013 11:48 pm »
IsaacKuo:
Why not just exchange a cargo pod rather than tow the whole sRLV into orbit? That will save a "ton" of mass and allow the sRLV to have a faster turn around won't it?

Well, if I have to chose one or the other, I prefer the LV to go all the way to orbit, because:

1) If you launch from KSC, the LV's path goes out into the Atlantic Ocean...so landing the LV after a suborbital hop would be a problem.

and

2) My hope is airplane-like operations for space tourism.  This means the people need to get back down somehow...makes sense to keep everything single stage to orbit and back rather than try to cram a return capsule in a cargo module.

That said, one size need not fit all.  I like the idea of nose-to-tail latching.  The LV has twin rockets to the sides so the central region can have a conical docking port in the rear.  The skyhitch has a conical nose tip which latches to the docking ring.

With this configuration, you can also have cargo LVs which eject the cargo module rearward so the LV can return by itself.  The LV also acts as an aeroshell for the cargo module.

Offline KelvinZero

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #11 on: 01/26/2013 01:18 am »
I think this concept was the basis of a long argument between Jim and a Paul Klinkman on space.com when it still had a forum.

Im a bit unclear because I remember zany catches on tethers being included, but im sure the case of the orbital element also decelerating part way was also discussed.

At the time I was more interested in the concept of using an orbital atmosphere collector to get the propellant in orbit in the first place.
http://forum.nasaspaceflight.com/index.php?topic=30104.0
« Last Edit: 01/26/2013 01:20 am by KelvinZero »

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #12 on: 01/27/2013 12:59 am »
 I believe on an earlier discussion the idea of the tow vehicle collecting air as it decelerated was discussed. The tow vehicle could perhaps collect air as it made a series of skips off the upper atmosphere Sanger style and cool and liquefy it and perhaps separate the N and exhaust it, between skips. The extra drag collection requires is not a problem if you are trying to slow down anyway.
 If the ground launched (or air launched I suppose) launch vehicle was to use LH2 as fuel and this could be transferred to the tow vehicle after mating as needed, then for the price of a larger LH2 tank the single stage launch vehicle could drop its LOX tank/engine part for recovery while the payload and stretched LH2 tank was connected either directly or by a tether that contains a fuel transfer line perhaps. 
 The tow vehicle would never need other refueling and the launch vehicle, while having to carry enough LH2 to loft both itself (or at least the payload and LH2 tank part) and the tow vehicle to orbit, would not have to supply the LOX and as a result, should realize a significant payload mass fraction increase vs. having a conventional second stage.
 Regarding this, it might be desirable to design the tow vehicle engines to burn very oxygen rich since you might have a surplus of it to increase thrust to perhaps reduce the amount of LH2 the launch vehicle must carry and thereby increase its payload mass fraction.   

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #13 on: 01/27/2013 05:50 am »
I believe on an earlier discussion the idea of the tow vehicle collecting air as it decelerated was discussed. The tow vehicle could perhaps collect air as it made a series of skips off the upper atmosphere Sanger style and cool and liquefy it and perhaps separate the N and exhaust it, between skips. The extra drag collection requires is not a problem if you are trying to slow down anyway.

These skips won't last very long unless you have a really good lift/drag ratio.

My gut feeling is that the only way you could liquefy the oxygen quickly enough is if you were doing something like LACE, where you sacrifice LH2 to liquefy oxygen.  But where would you get all this hydrogen from?  Hmm...doesn't make sense.

On the other hand, maybe it could work with liquid nitrogen?  Atmospheric scooping could provide plenty of nitrogen.

I don't know...the entire idea seems doomed to failure due to compression heating.  It's one thing for LACE to operate at relatively low speeds, but this will be scooping up air at near orbital speeds.

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #14 on: 01/27/2013 07:00 am »
I think this concept was the basis of a long argument between Jim and a Paul Klinkman on space.com when it still had a forum.

Im a bit unclear because I remember zany catches on tethers being included, but im sure the case of the orbital element also decelerating part way was also discussed.

I think it's best to keep things simple and practical, rather than include a lot of fancy technologies.

Quote
At the time I was more interested in the concept of using an orbital atmosphere collector to get the propellant in orbit in the first place.
http://forum.nasaspaceflight.com/index.php?topic=30104.0

I have a lot of interest in atmospheric scooping.  I wish I had a better understanding of the technology and engineering required.  In particular, I wonder if it would be easier to scoop CO2 from Venus or Mars, since it wouldn't require such low temperatures.

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #15 on: 01/27/2013 11:19 pm »
I believe on an earlier discussion the idea of the tow vehicle collecting air as it decelerated was discussed. The tow vehicle could perhaps collect air as it made a series of skips off the upper atmosphere Sanger style and cool and liquefy it and perhaps separate the N and exhaust it, between skips. The extra drag collection requires is not a problem if you are trying to slow down anyway.

These skips won't last very long unless you have a really good lift/drag ratio.

My gut feeling is that the only way you could liquefy the oxygen quickly enough is if you were doing something like LACE, where you sacrifice LH2 to liquefy oxygen.  But where would you get all this hydrogen from?  Hmm...doesn't make sense.

On the other hand, maybe it could work with liquid nitrogen?  Atmospheric scooping could provide plenty of nitrogen.

I don't know...the entire idea seems doomed to failure due to compression heating.  It's one thing for LACE to operate at relatively low speeds, but this will be scooping up air at near orbital speeds.

 Now that I think about it, another problem is that the tow vehicle would be unable to abort to orbit as it would if all the propellant was loaded at the ISS or other facility in LEO.
 I started thinking about this in the first place because it occurred to me that given the relatively low Dv required to bring some NEO's back to Earth orbit or the EML points, a successful asteroid mining operation might have a huge amount of water once the metals were extracted and could benefit by having a market for it. There might be few opportunities to fuel BEO vehicles ay first and so I saw taking over the job now performed by upper stages as a potential market to help jump  start asteroid mining.
 BTW - As I remember now, the reason for the grappling vehicles was that an objection was raised in previous discussion that the window for docking was too short to safely bring the two vehicles together but that depends i guess on how you do it.

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #16 on: 01/28/2013 12:53 am »
I started thinking about this in the first place because it occurred to me that given the relatively low Dv required to bring some NEO's back to Earth orbit or the EML points, a successful asteroid mining operation might have a huge amount of water once the metals were extracted and could benefit by having a market for it. There might be few opportunities to fuel BEO vehicles ay first and so I saw taking over the job now performed by upper stages as a potential market to help jump  start asteroid mining.

I agree that the potential BEO market is not promising.  Doing something to reduce launch costs to LEO, on the other hand, could be a big deal.

Quote
BTW - As I remember now, the reason for the grappling vehicles was that an objection was raised in previous discussion that the window for docking was too short to safely bring the two vehicles together but that depends i guess on how you do it.

I imagine the two vehicles leaving the atmosphere close to each other, on an ascending trajectory around 3km/s.  While in the atmosphere, the skyhitch can use aerodynamic maneuvers along with its plentiful spare speed to maneuver into a close tail chase.  So, the vehicles will be ascending at perhaps a 30 degree trajectory close to each other.

This implies an upward velocity component of 1500m/s, or about 5 minutes until gravity pulls them back down to the upper atmosphere.

I don't see how these tethers would help the situation.  It would take extra time to deploy the tethers.

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.

Offline RanulfC

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #17 on: 01/29/2013 08:52 pm »
BTW - As I remember now, the reason for the grappling vehicles was that an objection was raised in previous discussion that the window for docking was too short to safely bring the two vehicles together but that depends i guess on how you do it.

I imagine the two vehicles leaving the atmosphere close to each other, on an ascending trajectory around 3km/s.  While in the atmosphere, the skyhitch can use aerodynamic maneuvers along with its plentiful spare speed to maneuver into a close tail chase.  So, the vehicles will be ascending at perhaps a 30 degree trajectory close to each other.

This implies an upward velocity component of 1500m/s, or about 5 minutes until gravity pulls them back down to the upper atmosphere.
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" :)

Actually you can probably "extend" the timing by fireing the vehicle engines at the "top" of the arc to compensate for gravity (should be around 0.5 or so at that point due to velocity) but then you have engines firing in close proximity while you're trying to "latch-on" to each other.
Quote
I don't see how these tethers would help the situation.  It would take extra time to deploy the tethers.

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.
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)

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 key here is to get a fast, accurate rendevous between two seperate moving objects on similar but not the SAME trajectory in as short a time as possible.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline hkultala

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #18 on: 01/31/2013 05:38 am »
The trajectories seem too different for this.

They could reach same position and same speed, but with different velocity vector

Offline KelvinZero

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #19 on: 01/31/2013 10:07 am »
I think it goes without saying that they have to achieve the same trajectory for this to work...

...unless you can figure out a way to build your vehicle out of this stuff ;)

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #20 on: 01/31/2013 06:16 pm »
 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 again
LN2 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.
     

Offline Tass

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #21 on: 01/31/2013 06:33 pm »
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 again
LN2 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.
     

Even if the hydrogen were free you would not get enough impulse out of the oxygen to offset the impulse lost when collecting it.

This is easy to see from the fact that orbital speed is much higher than the exhaust speed from a hydrolox engine. Shyhitch may make sense if you are using propellant from space, but for atmospheric scoping you need electric or similar high Isp propulsion. Note also that the plans for skylon stop airbreathing after mach 5.5 because it is not worth it at higher speeds. Whether you collect during ascent or during a apogee drop like you said doesn't really change that.

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #22 on: 01/31/2013 07:31 pm »
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" :)

Thing is...I don't see tethers as necessarily helping this situation, compared to directly docking.

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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.

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 don't see why the tether would even touch the shock waves.  The tether would be entirely within the lead vehicle's shock cones and entirely outside the trailing vehicle's shock cones.  I'm assuming the trailing vehicle has a probe sticking out from the nose, and this engages a drogue cone on the tip of the tether.

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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.

Neither the ISS nor Apollo nor any other spacecraft have used this sort of "harpoon" technique to reel in a target.  The reason, which I thought was obvious, was that dealing with RCS thrusters was preferable to reeling in a tether with no atmospheric drag force to slow it down on approach.  At best, you'll reel straight in, and the target will slam into you hard when it arrives.  At worst, the target will have had a slight bit of transverse momentum and the thing will come whipping at you slamming you somewhere in the side or something.  Think of what happens when you retract a spring loaded tape measure.  If it's from a short length, the thing will slam home, but if it's from a long length it will probably whip off to the side and hit you.

Of course, you could use the target's RCS thrusters to slow itself down as you reel it in...but this begs the question, why not just use the RCS thrusters by themselves?

So no...the only way I see a tether making sense is if you can conduct the main burn without reeling in the tether.  This is a bit of a dynamics headache, and obviously you may have to worry about the tether surviving the lead vehicle's exhaust.  But the alternative of reeling it in seems no better than just using RCS thrusters alone.

The trajectories seem too different for this.

They could reach same position and same speed, but with different velocity vector

They can match velocity vectors while still inside the atmosphere.  The LV simply follows a ballistic suborbital course, but the skyhitch performs hard turns to slow down to 3km/s.  This gives it a lot of flexibility on deciding its final vector.  Hard turns bleed a lot of speed, but the skyhitch has a lot of speed it needs to bleed anyway.  So, the final hard turn is an upward pitch to match the speed and vector of the LV.

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #23 on: 01/31/2013 09:17 pm »
@Tass Great answer -thanks for making the explanation so straightforward.
 If I understand correctly, speeding the O2 up to the speed of the tow vehicle in scooping it means the exhaust velocity of the rocket has to be higher to get a net benefit.
 Of course a separate PROFAC type system could resupply the tow vehicle in LEO I suppose as long as it used SEP.
 In that case, do you think the benefit would be worth it?

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #24 on: 01/31/2013 09:38 pm »
@ Issac Kuo:  I had actually suggested reeling the tow line in, but actual towing was my preference because it seemed faster and simpler since only a tether/"kill vehicle" combo pod need be added onto the payload to skyhitch vehicle. Actual docking would require some mods to make the vehicles compatible and towing has issues.
 The issue you raise has a possible solution - if a shock absorbing mechanism could be extended from the back of the tow vehicle as the payload is reeled in it could be slowed down.
 Also if the payload had RCS it could damp any tether ocillation and the tow vehicle has a good deal of control and once taut under thrust the high acceleration is likely to keep it that way. I would imagine a carbon fiber tether properly oriented could take the heat and/or have a protective perhaps ablative coating in hot spots.
 I think faster is much better for the "hitch" part - you want to thrust while still on an upward track and avoid another skip which would slow you down.   

Offline Asteroza

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #25 on: 02/01/2013 03:25 am »
Hrm, an actual tether towing skyhitch, using a rear trailing tether and 2 or more main nozzles gimbled out a bit to keep from frying the tether and towed payload is kind of attractive from a structures standpoint. Though that makes some assumptions. The skyhitch after skip would need to do minimal delta V thrusting at least just before the rendezvous to maintain tension on the tether for a chasing sRLV. Regardless, the tether grapple would need to be an active target (local reel winch to change target distance, though that doesn't have to be capable of reeling in the full sRLV mass, just reel out the remainder of the adjustment segment of the tether via winch braking after capture)(local RCS for radial swinging on the end of the tether) it seems, but the sRLV also has to be active since it is a chaser.

Which makes it sound more like a system composed of a hypersonic shaped mass slug with a rocket, a small maneuvering grapple vehicle with it's tether latched onto the rear of said large mass, and your sRLV (or a payload with a grapple point being served up by a sRLV). The interesting problem is then at the end of the lift burn. The grapple would need to have enough RCS/thrust authority to keep the snatched payload from hitting the skyhitch before the circularization burn due to tether tension snap back, unless you have a nice slow tapering off of thrust from the skyhitch. Which makes the grapple vehicle on the same order as a small orbital tug in terms of performance. Then there would be the issue of the final hard docking of the grapple+payload to the back of the skyhitch prior to the circularization burn.


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? The skyhitch could structurally resemble a semirigid dirigible, with three or four propellant tank backbones connected by a ring covered by a inflatable heat shield shell. During the skip phase bleed PROFAC liquid nitrogen out of the center of the heat shield as a buffer gas for cooling. Skyhitch maintains minimal delta V to not fry the chaser during rendezvous, and by design the wide spacing and possible gimbling of the nozzles provides an entry vector into the receiver cone area for the snatch payload, and potentially allowing a faster/harder docking. Since we expect to have a short skip in the atmosphere, a simpler cone reentry shape with some CG shifting to improve L/D for aeroassist plane changes is feasible, as it is not like we are building a hypersonic cruiser. This does force the chaser to be very active, but that should be mass/energy-wise more attractive?

Doing things like that makes it sound more like a pelican or a python since it is swallowing the payload whole...

Offline KelvinZero

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #26 on: 02/01/2013 03:51 am »
anyone remember the buck rogers hawk ship ? :)

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #27 on: 02/01/2013 01:41 pm »
anyone remember the buck rogers hawk ship ? :)

Haha.  That was, in fact, my original inspiration.

For those who don't remember it, or never saw Buck Rogers, that hawk shaped fighter ship dispatched its victims by swooping in from above, and grabbing it with piercing metal talons, and hauling the victim away.  If I recall correctly, the victim spacefighters were crashed into the ground.

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #28 on: 02/01/2013 02:59 pm »
[...]
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?

The more I thought about various methods to capture a probe into a cone, the more I figured that smaller was better.

http://en.wikipedia.org/wiki/File:Gemini_Docking_Mechanism_diagram_view2.png

http://en.wikipedia.org/wiki/File:Soyuz_7K-OK_docking_system_drawing.png

http://en.wikipedia.org/wiki/File:Russian_probe_extended.jpg

http://en.wikipedia.org/wiki/File:Russian_drogue.jpg

For skyhitch, something like the Gemini docking mechanism could be used, but with an aerodynamic nose cone:

http://hydrogen-peroxide.us/history/bell/X-15-Lakebed_Engle_Nose_RCS-1965.JPG

This mates with a conical drogue on struts; maybe the struts could be telescoping and/or somewhat shock absorbing.

The docking mechanism doesn't need to be huge, and I don't see a compelling reason to capture one vehicle entirely within the other, but maybe the LV could be a bit short and stubby to mitigate the "balancing pencil" effect...

I keep coming back to this idea of having a long thick spike on the nose, but this would insert into the centerline of the LV, causing a lot of design headaches.  But really...it could work.  The LV's fuel tanks could be behind the cargo region, providing a decent length for the spike to work with.  The tanks could have a structural tube running through them for the spike to enter; this cuts into tank volume, or there could be a cluster of tanks around the centerline.  Either way, the LV would use 2 or more engines, so the centerline is free for the skyhitch spike to enter.

Offline RanulfC

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #29 on: 02/04/2013 06:23 pm »
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" :)

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
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #30 on: 02/05/2013 03:10 am »
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" :)

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

 If I understand correctly, the requirement to actually crash the vehicles together dictates a particular and very constrained design for the towed vehicle and requires the addition of a sticky boom and some kind of receiver to the tow vehicle or towed vehicle to connect them.
 I say "crash" quite deliberately because the two vehicles must come together within a couple of minutes at most in order to keep from being on the down side of the skip - you'd like to have your velocity vector in the upward direction right? You certainly don't want to descend into the atmosphere again either.
 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.
 In contrast, the small rugged capture vehicles can maneuver quickly since they are much less massive than the two vehicles and they can dock violently if need be by design. it seems to me that tethers give much more margin in both safety and design.
 In flight refueling never has or will involve docking the two vehicles. Towing sail planes is common by contrast. Docking in space is a slow delicate practice today.
 
 

Offline Asteroza

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #31 on: 02/08/2013 06:34 am »
One thing to point out is how will the Skyhitch be propelled after capture? One would expect a propulsion system that provides a fair amount of thrust for the primary lift burn, less so for circularization burn.
 
Conventional chemical propulsion requires both oxidizer and fuel. PROFAC type systems generally only provide oxygen and nitrogen in large quantities. If Skyhitch is using conventional propulsion, where will the fuel/propellant come from? Some other system supplying an orbital fuel depot could potentially fill the gap, if the assumption is Skyhitch is carrying people/fragile cargo and the other system does bulk cargo. Though that adds other issues (for example gun launch favors high density cargo like water, which makes LH2 delivery difficult, but post delivery electrolysis and cooling is feasible).

Does this mean unconventional propulsion options might be necessary for Skyhitch due to propellant sourcing issues?

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.

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #32 on: 02/08/2013 01:45 pm »
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.

As I envision it, the LV boosts on its ascending path--transmiting current and projected GPS coordinates along the way--and the skyhitch vehicle reacts to it using aeromaneuvers while still in the atmosphere and braking.

The skyhitch vehicle could easily pull many gees, sustained, because it's bleeding away about 5km/s worth of speed along the way.  Near the merge, the skyhitch is gliding horizontally on the same ground track as the LV, but far above and behind.  It's maybe still going at 3.5km/s when it pulls up hard for the merge.

So, the LV is coasting upward at a 30 degree angle (it has spent its fuel), while the skyhitch is pulling hard in a roughly circular arc from horizontal to the upward 30 degree angle.  It's a bit like someone on a curved on ramp merging with straight highway traffic.

This isn't a maneuver which would be appropriate for RCS thrusters.  Without the atmosphere, going from horizontal to upward at 30 degrees at 3km/s would require 1.5km/s delta-v.  But with aeromaneuvering it doesn't cost any fuel--it only costs speed (which the skyhitch needs to bleed away anyway).

So, when the LV and skyhitch leave the atmosphere, they're already very close to each other.

Offline IsaacKuo

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #33 on: 02/08/2013 02:03 pm »
One thing to point out is how will the Skyhitch be propelled after capture?

I have been assuming a LOX/LH2 rocket engine, with fuel provided by ISRU--perhaps from lunar ice, but there are also other possible sources.

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Conventional chemical propulsion requires both oxidizer and fuel. PROFAC type systems generally only provide oxygen and nitrogen in large quantities.

One possibility with atmospheric scooping might be nitrous oxide monopropellant, but this is less well developed than LOX/LH2 and has a far lower specific impulse.  Assuming a specific impulse of 180s, a delta-v of 5km/s would require a mass ratio of 17!

So, if nitrous oxide monopropellant were used, you'd want the LV to reach a higher rendezvous velocity and the skyhitch to brake by less.  This reduces the potential benefit.

Quote
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.

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.

Offline Tass

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #34 on: 02/08/2013 02:26 pm »
You could use oxygen acquired by atmospheric scooping combined with hydrogen carried on the launch vehicle.

Offline Solman

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #35 on: 02/08/2013 05:30 pm »
You could use oxygen acquired by atmospheric scooping combined with hydrogen carried on the launch vehicle.

 This is what I was talking about above. In my simplistic way I was comparing a two stage launch vehicle to a LV using Skyhitch.
 The first stage gets the payload/propellant vehicle up to adequate speed for the hitch so the comparison would be to a normal second stage. The payload/propellant vehicle does not have to carry LOX, but must carry enough LH2 to get itself and the hitch vehicle to orbit. The hitch vehicle would carry only LOX obtained on orbit perhaps from atmospheric scooping vehicle, so its size would be reduced vs. a hitch vehicle fueled on-orbit with LH2 and LOX. It may be advantageous for the hitch vehicle engines to run oxygen rich.
 Compared to a normal second stage the payload propellant vehicle would have a larger mass fraction, I think, since even at a 6:1 ratio the amount of extra hydrogen it would have to carry would be much less than the mass of the oxygen the normal second stage would, and also it doesn't have engines although it would have extra mass for the docking or towing sys. and the larger LH2 tank.
 I would really appreciate feedback on whether this assumption of much greater mass fraction is correct.
 

Offline Asteroza

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Re: "Hypersonic Skyhitch" - Partial Reentry Suborbital Towing
« Reply #36 on: 02/13/2013 09:29 am »

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.

10.8 is rough, but it still uses a totally orbitally sourced propellant. Taking up LH2 from the ground seems to have the issue of being directly compared to TSTO systems, as the LH2 can potentially be the bulkiest part of the upward bound package.

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