Exo-Atmospheric Suborbital Refueling

[john smith 19]

So you're conducting a mid air refueling having attained enough momentum. I suspect the dynamics of unreeling that pipe will be quite interesting. I also suspect that pipe system will be a bit more complex than a conventional air to air system.

On that count I'd suggest looking at stuff on eletromagnetic and momentum transfer tethers, specifically their unreeling. The Shuttle did a few experiments for power generation, lowering a conductive tether into the atmosphere.

The bottom line is - there's so few litterature available on suborbital refueling, there might be some big flaw hidden somewhere that could prevent that thing from working and that I'm not able to guess.
Perhaps the flaw is hidden somewhere in the rocket equation, for example the refueling could add more gravity losses.. The SSTO business is so hard, even with suborbital refueling the marges remain razor thin and the payload might be wiped out entirely within the blink of an eye.
That's where I stand as of today, and why I post on this forum. Engineering criticism is welcome.

Well winged lift cancels out a lot of those losses. The theory is HTOL widens the margins,

Skylon

I wish that machine work, because Alan Bond and his team are really worth it. The day it works, suborbital refueling will be much less useful for sure. Skylon payload to orbit will certainly be superior, closer from the shuttle.

You really need to look up the mass for the shuttle stack versus the payload (call it 57500lb). STS had the very dubious distinction of a multi stage vehicle with a payload mass fraction as bad as an SSTO.  The amazing thing about Shuttle was that it was built at all, given the ludicrous budget limits it was built under.  Again it's the economics.

What really fascinate me with suborbital refueling is that it represents a kind of fourth path toward a SSTO.
 I class SSTOs according to failed atempts: there is the VentureStar, the Orient Express, and HOTOL / Skylon. Each represent a different path: all-rocket, atmospheric air through scramjets, atmospheric air through liquefaction. 
But truth be told - none of them could have worked in the 70's, when NASA tried the shuttle. 
- HOTOL was unworkable, Skylon took 30 years to correct the flaws if it ever works someday.

Actually the theory was corrected by the early 90's, particularly by moving the engines to the wing ends.
The rest has been getting the funding and moving from theory to practice. 

- This has been discussed endlesssly, but whether Chrysler SERV or Lockheed VentureStar, I'm far from convinced the "all rocket SSTO" mass fraction issue can be solved

I'd suggest you look at the mass fractions of the Saturn V 1st stage, and the fact it was carrying another 100 tonnes and the 2nd stage on it's back. Now imagine it with RD180's instead of F1's. You might also look at the Titan II, which were explicitly designed to exclude the pressure stabilization of the original Atlas.
Mass fractions for VTOL are tough. They are not impossible and have been possible for decades.

- and the Orient Express was the worse of the lot, courtsey of Tony Dupont, who is a fraud. Today scramjet research has been re-directed toward hypersonic missiles.

I think that's what they call "libel," unless he's been convicted. But I can certainly understand anyone reading around the subject thinking exactly the same thing.

By contrast with the three SSTOs above I think suborbital refueling might have worked at any point is space history, from 1963 to, well, that day in the future when Skylon will make it obsolete. 

Imagine if, from 1960 onwards winged SSTOs had been build with turbofans and rockets and nothing else. No scramjets, no RBCC / TBCC, no air liquefaction, no impossible mass fractions.
Wings and rocket propellants and turbofans are certainly too heavy, so the thing will never reach orbit alone. So what ? as I said before, cheat with the damn rocket equation. Refuel on the way.

It was (and still is) a path not taken. And that, by itself, is fascinating.

There are various reasons for this but mostly it's financial. If you want to take this concept further that has to be overcome.   

[e of pi]

So, I've been fiddling a bit with the spreadsheet I made to look at this. The major addition was trying to add some calculation of the difference made by various air-assist. Currently, you can pick one of three modes: airdrop, air-breathing, and no air assist at all.

However, the results aren't very pretty. To get even 1% of one vehicle's gross mass as payload to LEO (and remember this needs two vehicles, so that's 0.5% of the total system mass) using a rather optimistic 325s ISp for the kerosene/peroxide mix, I needed a 9% structural mass. That's pretty low, and not particularly impressive payload-wise. The big problem is the losses of transferring fuel--you're transferring 44 tons of the stuff, and I can't see that being done in much less than a minute, during which gravity will take a big bite from the speed and altitude you'd built up pre-rendezvous.

If you were to switch to a biamese configuration with continuous crossfeed, you'd be able to get to more like 3.25% to LEO (1.65% of system mass) with the same fuel mix, but that's not so easy to fly off a runway. With a two-stage configuration, with each stage having the same 9% structural fraction, then you can get 2.06% or so of the system mass to LEO, the best yet.

With a more high-performance fuel mix it's a better performance for the exo-atmospheric refuel, but the competition does better as well. Thus, the issue I keep having is that while it can work, in the cases where it works more conventional configurations work much, much better. I dig the vision of "space 737s," but the math doesn't seem to work.

[john smith 19]

With a more high-performance fuel mix it's a better performance for the exo-atmospheric refuel, but the competition does better as well. Thus, the issue I keep having is that while it can work, in the cases where it works more conventional configurations work much, much better. I dig the vision of "space 737s," but the math doesn't seem to work.

That's the problem.  I think you can do better than 325 with something like a stage combustion cycle IE a Russian engine. Likewise going with something like a large expansion nozzle, SSME ran with 1:77 (or 2 dual bell or bell in bell or plug nozzle)
But they all increase risk and cost.

That ability to do ELV payload fraction in an SSTO (unheard of) is why Skylon is being funded. 

It's not that it can't work, but other solutions work cheaper or better or cheaper and better. 

[EDIT This is where where the air breathing part comes in. SABRE gives Skylon an Isp of 3300 secs during it's 0-M5.5 period).

I doubt you'd get that from any other engine but you might like to look at what happens when you can use the atmosphere for reaction mass or Oxygen, or both. From papers on air launch historically high speed aircraft have managed mass fractions of something like 26% and the Vigin global Challenger that Steve Fossett was flying managed IIRC 15% but was subsonic.

so if you split the trajectory into an "air breathing" and a "rocket" part, possibly on both vehicles or possibly on the "booster" only (so it's a big tanker for the rocket only "upper stage," using air breathing jets to need less fuel and leave more room for propellant to transfer).

The actual question becomes what's the lowest  air breathing Isp and/or mach range that gives major assistance? Historically turbojets or low BPR turbofans like those on the B1 or F111 have been looked at up to M4+. Water injection has been used on jet engines for thrust increase (into the combustor mainly , but sometimes into the inlet) and LOX injection has some experimental work (but AFAIK no deployment).

The other issue is what this does to the cost of the overall system  ]

[Bob Shaw]

Why transfer fuel, when you can simply add a fully-fuelled boost stage? I'm thinking of a lightly built, prop-heavy upper stage launched at the same time as an orbiter with third-stage style prop levels suited to RTLS/downrange emergency recovery. Like two engines on a mountain railway...

[Archibald]

Ok folks, that's the technical expertise I've been waiting for, and I'm not dishonest. I won't try to challenge it. 
It was an excellent discussion. You grasped my vision, I accepted the engineering criticism.
My opinion is that it works well enough for a sci-fi novel  - and I'm more a dreamer than an engineer.

[clongton]

Ok folks, that's the technical expertise I've been waiting for, and I'm not dishonest. I won't try to challenge it. 
My opinion is that it works well enough for a sci-fi novel  - and I'm more a dreamer than an engineer.

I think you would enjoy "Perigee" by Patrick Chilies.
It's a scifi novel along similar lines to what is being discussed here.
I completely enjoyed the story.

[Archibald]

I'd suggest you look at the mass fractions of the Saturn V 1st stage, and the fact it was carrying another 100 tonnes and the 2nd stage on it's back. Now imagine it with RD180's instead of F1's. You might also look at the Titan II, which were explicitly designed to exclude the pressure stabilization of the original Atlas.
Mass fractions for VTOL are tough. They are not impossible and have been possible for decades.

Those are expendable SSTOs. Try adding the recovery gear and things get harder.

[john smith 19]

Those are expendable SSTOs. Try adding the recovery gear and things get harder.

True, the question of how much payload is lost with the addition of recovery hardware has long been debated, but it does show that that sort of mass fraction is possible,  which was my point.

[Archibald]

That ability to do ELV payload fraction in an SSTO (unheard of) is why Skylon is being funded.

Rebounding the thread about this. I have to admit suborbital refueling will never match Skylon or ELVs payload.
Surely, the comsat market is perhaps the only viable moneymaker as of today.
Reading that novel again however I wonder if even a small payload RLV couldn't help bringing new markets.
http://www.hobbyspace.com/AAdmin/archive/SpecialTopics/RocketCom/titlePage.html
Surely 5000 pounds or less is not much, but (once again) you can try and cheat. Propellant depots, OTV, OMV, space tugs would help.
There is a whole bag of tricks that might improve the payload to orbit. Steve Pietrobon NSTO is one of them.

The basic machine consist of a N2O or H2O2 + JP-5, single suborbital refueling space plane with a turbofan powerful enough for an horizontal liftoff without subsonic refueling.

E of pi is right, payload to orbit would be modest (although, reading Clapp Black Horse litterature again, I think the american military would easily trade payload for flexibility and fast turnaround - essentially a space F-16 burning kerosene out of an ordinary air base)
So how could payload to orbit be improved ?
- drop the large turbofan - either tow, air-drop, or refuel subsonically with smaller jets
That should improve mass fraction a bit, at the expense of operational flexibility
- change the propellant mixture
a) switch to keroLOX and buy a NK-33.
Once the H2O2 / N2O space plane in regular service, try LOX transfer between them, using a pair of LOX tanks in the payload bay. It's a low risk approach akin to X-planes.
b) add a Thrust Augmented Nozzle to the NK-33
c) or drop kerosene in favor of propargyl alcohol (thanks RanulfC and Bruce Dunn for that) H2O2 + propargyl results in a whopping 40% + payload to orbit
- add more machines and more refuelings into the dance - three or four space planes, two or three suborbital refuelings. Not more otherwise you might fall into FLOC fantasy land.
- drop the turbofan in favor of "faster" airbreathing engines - for example, RanulfC beloved SERJ. Oh, for the record, Marquardt prototype SERJ used H2O2 for its ejector rocket. Marquardt found that easier than cryogens, and the US military, too.

[john smith 19]

Rebounding the thread about this. I have to admit suborbital refueling will never match Skylon or ELVs payload.
Surely, the comsat market is perhaps the only viable moneymaker as of today.
Reading that novel again however I wonder if even a small payload RLV couldn't help bringing new markets.
http://www.hobbyspace.com/AAdmin/archive/SpecialTopics/RocketCom/titlePage.html
Surely 5000 pounds or less is not much, but (once again) you can try and cheat. Propellant depots, OTV, OMV, space tugs would help.
There is a whole bag of tricks that might improve the payload to orbit. Steve Pietrobon NSTO is one of them.

True.
Now do yo have about 7 or so billionaires to fund your plan?   

If you don't, you're faced with the problem of raising that money.

And you're back to the original problem.   

[clongton]

Reading that novel again however I wonder if even a small payload RLV couldn't help bringing new markets.
http://www.hobbyspace.com/AAdmin/archive/SpecialTopics/RocketCom/titlePage.html

That was an awesome read. I've read it thru twice and highly recommend it!

[Archibald]

Rebounding the thread about this. I have to admit suborbital refueling will never match Skylon or ELVs payload.
Surely, the comsat market is perhaps the only viable moneymaker as of today.
Reading that novel again however I wonder if even a small payload RLV couldn't help bringing new markets.
http://www.hobbyspace.com/AAdmin/archive/SpecialTopics/RocketCom/titlePage.html
Surely 5000 pounds or less is not much, but (once again) you can try and cheat. Propellant depots, OTV, OMV, space tugs would help.
There is a whole bag of tricks that might improve the payload to orbit. Steve Pietrobon NSTO is one of them.

True.
Now do yo have about 7 or so billionaires to fund your plan?   

If you don't, you're faced with the problem of raising that money.

And you're back to the original problem.   

so what ? do you have millions to invest in Skylon ? what's your point ?

[john smith 19]

so what ? do you have millions to invest in Skylon ? what's your point ?

I'm not proposing a design, you are.

And if you want to see it built you've got 2 choices. 1) Find some "angel" investors who will back your idea regardless of what the cost models say, unless like Elon Musk you are your own angel. 2)Revise your concept till it works out. 

There are lots of clever ideas for RLV's.

But they don't give you ELV mass fractions. So when the money men perform due diligence and run their mass through the aerospace cost models they will come up with a price which is much more than the equivalent capacity ELV.  And the world does not need another ELV.

mass fraction < ELV --> bigger vehicle for same target payload --> bigger development budget --> Proposal rejected.

Unless you can find a way to deliver that payload (about 3% of GTOW if you look at current ELV's) 

Otherwise it will join the list of "clever ideas for an RLV" in the bottom drawer. 
OTOH neither does F9R, but the difference Musk has an income stream from expendable F9 operations already.