Of course interstage structures need to be designed. The thing is though they have successfully been for decades, and the state of the art is still being advanced by systems like the Falcon Heavy.
So is the state-of-the-art for A2A refueling and doing so in a free-fall ballistic arc is in fact easier in a more benign environment.
I'd rather have some mostly passive (save for the separation points) connective structure than having to put up with an additional in flight maneuver that has a non-negligible risk of (potentially even catastrophic) failure, even though you handwavingly try to pass it off as "pretty damn easy". As an educational exercise, I'd recommend to go through an associated FMECA for your proposal.
Let's see, failure of the pyrotechnic system, vehicle and mission loss. Failure of upper stage engine to ignite, vehicle and mission loss. Fairing separation failure, vehicle and mission loss. I can go on.
SOR mission profile:
Vehicles ascend into space with a trajectory to bring them together at 1km or less separation at engine cut off. They flip so that their upper surfaces are towards each other. (This is where he sensor and booms are located) They then use active sensors, RCS, maneuver engines and a cooperative rendezvous system to approach to 15m or less of each other. They then use 'cold gas' thrusters to maneuver to less than 10m and activate the automatic refueling booms which are equipped with a beacon and sensor tuned to the same frequency which are used along with built in boom articulation to connect and seal the transfer pipe. A test 'slug' of propellant is sent through the system at high pressure and bleed off into vacuum through the purge valve to ensure connection, and check for leaks and contamination in the lines as well as to clear the for propellant transfer. Propellant is then transferred till completion at which point the vehicles separate and maneuver away from each other.
Sub-orbital vehicle approach risks:
1km to 50m: RCS or maneuver engine failure on one or both vehicles. Mission abort, vehicle and payload saved. Approach and docking systems failure, execture missed approach and abort mission, vehicle and payload saved.
50m to 15m: RCS failure on one vehicle, abort mission. If the vehicle with the failed RCS can still maneuver then mission is aborted and both vehicles (and payload) are saved. If the failure is total then the failed vehicle is unlikely to land safely so mission is aborted and the crew may EVA to the nearby operational vehicle or use an escape system to return to Earth. Non-failure vehicle aborts back to Earth. Possible loss of payload depending on vehicle that fails, possible loss of one vehicle.
15m to Rendezvous: Probes booms deploy at 5m to 10m and use active connection, (they are articulated to a degree and can extend/retract) systems to find and connect with each other. Once the booms are connected the receiver and feed nozzles connect, verify and propellant flow begins. As note the booms can adjust for slight variations and active RCS on the vehicles keep them stabilized within a +/- 1m distance during the process.
Risks at this stage are:
-Booms unable to connect or active connection systems failure. Mission is aborted and vehicles and payload recover to Earth. This includes only one boom failing or failure of the nozzles to connect or propellant to transfer.
-Propellant or other contamination cause detonation in propellant booms, nozzles or connections. As the systems are cleaned and inspected on Earth this should be a low probability but it has a high chance of significant danger so it is addressed. Depending on where the incident occurs in the system and what secondary damage there is determines the risk factor of this type of incident. In the main anything short of the main propellant lines in either the target or transfer vehicle is survivable with minimum risk since there is little power in this 'test' slug and since the system between the main propellant lines of the vehicles and the transfer system at this point is isolated. Failure points apply to both vehicles.
--"Failure" at the entry point to the transfer vehicle boom arm would damage the boom and likely joint assembly, Kevlar lining the boom bay should protect the vehicle from any damage and the boom will be able to be disconnected and jettisoned if needed.
--Failure is along the transfer vehicle boom length, in which case there may be some shrapnel damage or external damage to the upper side of the vehicle. Since the explosion would be of low order any shrapnel generated should have low velocity/energy so extensive damage is unlikely. As this is 'leeward' side of the vehicle during reentry danger should be minimum a even if there is significant surface damage. The boom would have to be jettisoned and the mission aborted but vehicle, payload and crew would be recoverable.
--Failure is at the boom-to-boom interface. Similar to the above but the more robust structure would absorb more of the blast energy. This would damage both booms and require they be jettisoned and the mission aborted. Both vehicles would abort and recovery intact to Earth.
-Failure of the propellant transfer system during operation would mean aborting the mission unless sufficient propellant had been transferred before system loss. Both vehicles return to Earth intact.
-The last 'failure' mode is that the booms do not disconnect after propellant transfer is completed. In this case the "orbital" vehicle with payload detaches its boom and moves away to fire its engines and proceed to orbit. Once clear the transfer vehicle detaches its boom and proceeds to return to Earth. If the automatic detach system fails then the booms joints are locked and RCS is used to provide force enough to 'shear' the base connector joint at designed points with vectors having the vehicles, (and any debris generated) moving away from both vehicles. If that does not work then the last ditch explosive decoupling system will be used to detach the booms. This risks some damage to the upper surface of the vehicles but as these are directed and controlled explosions the risk is low.
Finally there is some risk during separation of collision between the vehicles. This could happen because of momentary confusion of the pilots, automatic system failure or malfunctions of the guidance and control system of the vehicle. Collision damage will be low due to the distance and relative energy of the vehicles but it is possible that significant or catastrophic damage to one or both vehicles could occur. If such damage DID occur at any point while this would mean loss of the vehicle(s) and payload I would recommend the crew have the means to escape using a system similar to the B-58/B-70 "escape capsule" designs using a parashield (
https://spacecraft.ssl.umd.edu/publications/2010/SpaceOps2010ParaShieldx.pdf) reentry system so at least the crew is saved.
Please show *any* evidence or analysis to support your assertion that refueling equipment masses are less than for missile launch racks.
The standard LAU-128 (
http://marvineng.com/product/lau-128/) launch adapter rail weighs 87lbs and is a two man lift while the refueling receptacle (
https://www.vacco.com/images/uploads/pdfs/2710942_aerial_refuel_receptacle.pdf) weighs a bit over 28lbs and needs only one person to lift out and move. I have done both operations
(This is the point where you should probably go back and read what I wrote previously AND what you wrote in response before you reply. As you were being a bit more than a little condescending in trying to compare the relatively straight forward attachment of a missile to a guide rail/launcher to the integration and stacking of a Two-Stage-To-Orbit vehicle as a "strawman" for how 'easy' that is compared to how 'difficult' something like A2A refueling is I'm hoping you don't mind ME burning yours as much as you 'burned' mine
)
If you *truly* design both vehicles to be 100% identical, the refueling equipment may be 90% passive (however you measure that) during transfer on one vehicle, but 90% active on the other - one of them has got to do the lion's share of the actual propellant transfer work.
It's not that difficult really considering how A2A works, and how the workload is shared between Tanker and receiver aircraft. 90% "passive" on the receiver aircraft part is defined as they move into position and 90% of the work of hookup is by the Tanker boom operator, followed by 95% 'active' in pumping and flow regulation also by the Tanker boom operator with the 5% on the receiver pilots part being weight and balance management and keeping the his aircraft steady. The SOR concept would regulate many of the 'active' station keeping operations to systems on both vehicles and within the booms as much as possible with as few thruster events as possible. Meanwhile both vehicles would use pumps to expedite the propellant flow rate and they would both HAVE all the necessary systems already installed since either can function as a tanker as needed.
Since there is clearly no need to perform transatlantic flights with composite aircraft that have to be stacked and launched together, I'm sure you don't mind if I set your strawman on fire.
Well, yours is burning nicely so it could use the company I suppose... But may I make a pertinent point? Going back to your original assertion that "To tie in with your military background, how complicated and time consuming is it these days to mate a missile to a fighter (oooh, two separate yet mated air vehicles - mind blown!)?" You seem to be missing the fact that MY example is actually applicable even if the system is not used in current trans-Atlantic travel the principle and operation are more similar to actual operations and systems than your "strawman" example. While you can 'burn' the idea of it being applicable to Trans-Atlantic flight you have to face the very real and quite sold 'core' that integrating and stacking a composite aircraft IS directly applicable in comparison to integrating and stacking a Two or more Staged Orbital vehicle. Vastly more so than locking a rail to a hard point and then sliding an all up missile onto that rail.
At a launch mass of 717 klbs, Skylon has a projected maximum payload of 37 klbs, which is *notably* less than the "well over" 100 klbs as you claim above, and as a HTHL design, it seems highly doubtful, to put it mildly, that the concept could be scaled up to that performance.
And that is what I get for doing a 'quick-reference' (
http://www.astronautix.com/s/skylon.html) AND not only misplacing a comma but reading the wrong number/mass category (What's the biggie between 120,000 and 12,000 anyway ) You're mostly correct sir and I stand/sit corrected. However I'll point out that there are references to the sub-orbital capability of Skylon being about twice what it's 'given' orbital capacity was and I also recall that the 'orbital' mass included an orbital transfer vehicle for GEO payloads? Anyway you're right on that point
I never advocated for an RLV to deliver as much payload as possible per flight, but there are payloads that are non-divisible (people come to mind, for example, and once you fly even one of those pesky passengers, now you need an ECLSS with associated masses as well). I may have missed it, but I don't think it was ever explicitly stated that the original idea was exclusively for a space station light logistics vehicle that can deliver cargo or a few passengers very often, economically and reliably and to do so even on short or little notice missions, but even then other architectures are perfectly viable and in my view technically and operationally preferable.
While some payloads are not recommended to be divided, (like people ) it's really not as much of a worry as you'd think till you get DOWN below 500lbs to orbit. (And that's not fixed as NASA did an ISS support study where they proposed using a Scout {
https://www.nasa.gov/centers/langley/news/factsheets/Scout.html} that's 385lbs mind you, to launch one person to the ISS in a fully recoverable capsule design. And keep in mind that the "NASA standard astronaut" of 200lbs with space suit was most of that mass! NASA, you gotta love em ) Once you're up to around 1,000lbs of payload you can start thinking of people hauling as well. (Four 'standard astronauts' with minimum comfort but it was one basic idea for Blackhorse)
I don't think I said the concept was exclusively for space station logistics support but it was a clear suggestion from the beginning since it had a low initial payload mass. The "main" use, beings it was a suggestion initially for the military, was small-recon-sats and 'pop-up' intelligence gathering satellites as well as a possible global sub-orbital/orbital strike weapons platform. Once Zubrin got involved he was more vocal about the commercial small-sat market and less about tourism or space station support though I, like Archibald think he was generally aiming towards something that could be used to prove/support his hypersonic skyhook concept.
That there are other systems that could do the job is a given, whether they are technically and/or operationally preferable is part of what we're discussing
Randy
(Edited to fix spelling errors, grammer errors, and general artifacts in the post generated by doing my spellchecking in an email
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