For those who don’t know, SKYLON is a fully reusable spaceplane with Ramjets for speeds up to Mach 6. The current version is a bit beefier than the original design, now promising about 16 tons of payload delivered to LEO from a suitably low inclination launch site. Further north or south, or tilted to the equator, and the payload is less.
A notable feature is the suborbital delivery mode promises 30 tons at SKYLON’s release of the payload. To then circularize the orbit we can use the figures for the SKYLON Upper Stage (SUS) to get an estimate of payload that can be orbited. The SUS masses 0.95 tonnes empty and can carry 7 tonnes of propellant (LH2/LOX) with an Isp of 4562 N.s/kg – to circularize the payload in LEO needs another 900 m/s delta-vee, thus meaning a mass-ratio of 1.21 and after we subtract the SUS that leaves 23.7 tonnes of payload. Expanded slightly that would mean an SUS based space-tug with ~23 tons of propellant and about 1.5 tonnes dry mass. Reaction Engines Ltd is designing just such a space tug it dubs Fluyt.
Fluyt has a larger propellant load (~47 tons) and higher payload, but a simpler interim space-tug might prove useful even before an orbit tanker facility is available to support larger FLuyt operations. Back in the early 1970s the European Launcher Development Organization (ELDO) designed an even smaller space-tug system massing just 12.5 tons fully tanked, which in a tandem configuration could deliver 7 tons to Lunar Orbit. Thus a “small” SKYLON tug system could deliver ~12 tons to Lunar Orbit, potentially enough to provide support for a Lunar base. The ELDO tug is featured on Adrian Mann’s “This is Rocket Science” art gallery …
If Adrian’s work looks very familiar, that’s because he’s the chief graphic visualiser for Reaction Engines Ltd. Bringing SKYLON to virtual life is something we can thank him for!
Hi Adam, an idea that I’ve thought that the potential of has been unrecognised is that of APT use for spaceplanes. I first came across the idea reading an article in the June 1995 issue of: Analog; Black Horse:One Stop To Orbit by Robert Zubrin and Mitchell Burnside Clapp.
Earlier today I found a revision of the idea from Burnside Clapp.
http://www.islandone.org/Launch/BlackHorse-PropTransfer.html
Cutting down to table 5, I found a variation that I find scintillating, if the weight of the H2/O2 spacecraft in the final column were doubled you’d have something capable of taking a 15 ton payload to orbit that’s fully reusable and with a dry mass of only ~50 tons. Only one of the biggest planes now flying would be adequate as the tanker, the AN225 or an A380, and even then some of the planes own wing fuel tanks would need to be replaced with LOX tanks to supply the orbiter.
Why I find the whole idea so attractive is there’s very little new technology required and with the re-usability, easy of handling, use of present infrastructure and quick turn around having the potential to make even the SpaceX systems look bloody expensive.
Have you come across these studies yourself? What do you think of their potential?
Thanks,
Andrew.
I am undecided. I read the Black Horse essay years ago and it seemed dubious, but Zubrin is a great persuader so I gave it some credence, but it does seem excessively complex. Getting oxidizer en route to orbit, even in part, from the air direct seems a better option than tanking on the fly. But thanks for the link – I’ll have a read.
I look at the weight allowances for Skylon, with the engines having t/w ratios of 14, landing gear to support a nominal take-off mass of 345 tonnes, an overall length of 83 metres but with an unladen mass of just 53 tonnes, Isp in rocket mode of 460s, and well, I’m not an engineer, but it does look like they’re pushing everything to the absolute limit. in comparison the numbers for an enlarged Black Horse with a similar payload to Skylon look to be pretty conservative.
It’s getting to be a bit frustrating trying to compare the two, on the Reaction Engines website they now have an empty mass of 41 tons, 220 tons, propellant and 12 ton payload launching east, and Mach 5 rather than 5.5 as fuel switchover. But as far as I’ve found, the thrust and t/w of the SABRE’s hasn’t changed, and their mass goin, on wiki thrust and t/w figures, is about 9.5 tons each, which would leave a ridiculously low mass for the rest of the vehicle.
Then there’s the $12 billion dollar development price, I just don’t see a competing APT system using existing aircraft (modified for the tanker role) and existing rocket engines costing nearly that much.
Had another look at the site, the SABRE’s that they’re talking about might be only 7 tons each if the t/w of 14 for air breathing holds. Still not much weight left for an 82M fuselage though.
Slowly working it out, users manual shows the later C2 version, do you suppose I can label the changes as “growth problems”?
OK, here’s an APT based on the Skylon figures:
Fueled mass at separation from tanker aircraft 345 tonnes
Of which:
Engines 2XSSME 7 tonnes
propellant 290 tonnes
empty mass (exc SSME) 33 tonnes
Payload 15 tonnes
MR at separation 345/55=6.27
Delta V at ISP 460 = 8.3 km/s
Which from the numbers in the Black Horse study is enough to get to an Eastward orbit from 10,000m at Mach 0.85.
I’ve subtracted the 19 tonnes of the SABRE engines, and added 7 tonnes for SSME’s.
The APT would need bigger wing area for rendezvous with the tanker, but as that would mean lower take-off speed (can you believe Skylon’s is Mach 0.5?), and also because of the much lower take-off weight ~90 tonnes vs 345 tonnes, I assume mass added to wings can be taken directly from mass subtracted from landing gear.
In the Black Horse only the LOX was transfered, but I dont see any insurmountable problems transferring H2 as well, I’d also look at a refuelling boom designed and positioned to take a towing load, the boom would latch on, and only after the link was secure would the LOX and H2 couplings swivel into position, as long as the LOX and H2 coupling separated sequentially so that there was no possibility of any spillage getting together I see no safety issues.
I’ve also been conservative in assuming the same mass in structure and tankage of the orbiter when far more of the Skylon’s propellant is H2.
Another advantage APT would have over the direct ascent from the runway is that, if the tanker can tow the still to be fueled orbiter for a while before fueling commences, there is the ability to position the ascent for rendezvous with an orbital destination. Compare that to the direct ascent which can only achieve a quick rendezvous with the destination on the occasions when the destinations orbit passes over the space port.
Hi Andrew
Start throwing LH2 around and you spoil any advantage that APT had. The main point was simplicity because no cryogens were being handled. I am also unaware of any vehicle which can carry that much propellant and transfer it. Way outside the original design envelope. The original had just 1,000 lbs of payload, the next iteration just 10,000 lb. SKYLON is designed to orbit to LEO directly 16,000 kg (35,300 lb) payload, but orbit ~30 tonnes as a sub-orbital booster.
Not saying it can’t be done Andrew. My points are a response to your claim it has to be easier/cheaper than SKYLON, but your modifications are deleting any advantage the original concept might’ve had over SKYLON in cost effectiveness terms. Bigger tankers able to handle hard cryogens would require as much development – if not more – than REL’s system. Existing tankers wouldn’t be able to handle it IMO, but go ahead and prove me wrong – that’s how new ideas are born. You might even be able to convince the guys at REL.
“Bigger tankers able to handle hard cryogens would require as much development – if not more – than REL’s system.”
Well I can’t prove which would be the cheaper option, I can point out that with the ATP there are a range of possible approaches, for example the A380 freighter can carry 150 tonnes of cargo in the fuselage, if we just go for only a 130 tonne top up at altitude I get these figures:
take-off mass from the runway:
structure (incl engines): 56 T
LOX: 111 T to get to the tanker, 119 T for ascent.
LH2: 18 T to get to the tanker, 41 T for ascent.
Total runway take-off mass: 345 T
Required tank volume: 220 (after refueling) LOX + 890 LH2 = 1110 cubic M
That’s allowing for a delta V of 1.6 km/s to get to the tanker, and no towing load (but doesn’t allow for refueling time propulsion).
So my tank volume is still only 78% Skylon’s, and runway mass is the same as Skylons.
And that’s the using worst assumptions, with an existing aircraft with the only modification being the addition of LOX tanks in the cargo hold and a cryogenic LOX boom, the An225 can carry 225 tonnes in the fuselage, which would allow more LOX at refueling and therefore less runway mass.
“You might even be able to convince the guys at REL.”
Are you serious? They’ll be married to their idea, and switching to something else would mean too much loss of face.
No new idea would ever be developed if it wasn’t taken serious enough to be pushed to its limits, to either convince the doubters or fall into pieces. Your idea is worth that sort of testing to destruction. And the REL guys want Space Access to succeed regardless of how.
Mitchell Burnside Clapp did try to get the APT idea off the ground with Pioneer Rocketplanes but that went under.
I just did another exercise on the Skylon numbers, interestingly if you use the propellant volume of Skylon (1440 m^3) and replace the Sabres with SSME’s and then change the mixture to the 6:1 ratio LOX:LH2 of the SSME’s, the higher LOX fraction means the runway mass increases to 525 tonnes. Keeping the structural weight the same but adjusting for the lighter SSME’s and Using an Isp of 350s to 10,000m and 450s thereafter, in theory you still get to orbit!