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Thread: How far into the future? An aircraft that launches from a runway and enters low

  1. #151
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    Quote Originally Posted by danscope View Post
    Hi Neo, I am talking wing loading, not propulsion. There is nothing in the literature about wing loading or the wing area on this paper airplane. 275 tons is a considerable weight indeed.
    An SR -71 Blackbird has an empty weight of 67,500 lbs and only takes on as much fuel as it needs to get to a waiting tanker for a very much needed re-fuel in flight. And that aircraft has a wing of 1795 sq.feet plus a lifting chine.
    Well.... we will see. The aircraft is so far in the future, it's shape is going to be revised anyway.

    Best regards,
    Dan
    Though I can't recall at the moment if the numbers on wing area is given anywhere, REL did say something about the wings in their proposal to ESA under the discussion about improvements since the HOTOL design. One of the points they mention is that since the trim/stability issues is solved by moving the engines to the wings, "The wing area can be optimized for maximum ascent performance".

    The choice of avoiding fancy lifting or blended body or leading edge extensions designs is also a clear design choice: "SKYLON differs from other spaceplane configurations in several key respects. Apart from the unique propulsion system, the main difference is an aerodynamic configuration that comprises a definite wing plus body. This was selected because it proved to be more optimum in terms of weight, lift and volume than the more fashionable blended bodies frequently portrayed for spaceplanes."

    So it seems implied that the choice of wing design has been done intentionally and with deliberation. From the drawings, though, I guesstimate the wing area of the main wings to be around 150 meters square, though I suppose some design changes may possibly occur later.

  2. #152
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    It's got a smaller wing than SR-71 ,which weighs 34 tons. Skylon is projected to weigh 275 tons.
    This provokes my skepticism. So.... we'll just see.

  3. #153
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    Quote Originally Posted by danscope View Post
    It's got a smaller wing than SR-71 ,which weighs 34 tons. Skylon is projected to weigh 275 tons.
    This provokes my skepticism. So.... we'll just see.

    Apples and oranges. The SR-71 had a very different purpose with very different requirements. It's also significantly slower than the Skylon is intended to be.

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    The folks here think a rocket sled is a good idea
    http://www.g2mil.com/others.htm

    The Rockwell X-33 concept seemed the most do-able
    http://www.up-ship.com/apr/volume1.htm
    http://up-ship.com/blog/?s=Rockwell+X-33

    In the news
    http://www.popsci.com/technology/art...y-space-planes
    http://up-ship.com/blog/?p=12737

    Old School
    http://up-ship.com/blog/?p=14420
    http://up-ship.com/blog/?p=13541
    http://up-ship.com/blog/?p=13574
    http://up-ship.com/blog/?p=14201
    http://up-ship.com/blog/?p=14049
    http://up-ship.com/blog/?s=shuttle
    http://up-ship.com/blog/?p=13687

    Shades of stratolaunch http://up-ship.com/blog/?p=14616

    Rather odd tech...
    http://up-ship.com/blog/?p=13732 Square hole drilled
    http://up-ship.com/blog/?p=13633 Image Enhancement


    On Page 50 of The May 7 2012 AV WEEK & Space we see Alcoa 50,000 ton presses that can make parts normally needed 200,000-300,000 ton forges (China has an 80,000 tonner--but I want to see a million tonner) Composite isogrids, frictionstir and titanium machining wonders, plus an interview with DARPA brain Leo Christodoulou who has moved to the (Rick Perry/Ron Paul threatened) DoE to study materials.

    Those of you who have studied Korolyov know that he had a background in strength of materials at Russian public schools, which were even more formidable than his biographer James J. Harford,author of "Korolev," who had the best public schools available back decades ago.

    To make a long story short, I think SSTO spaceplanes really need another series of material breakthroughs.
    Last edited by publiusr; 2012-May-11 at 09:42 PM.

  5. #155
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    Quote Originally Posted by publiusr View Post
    Those of you who have studied Korolyov know that he had a background in strength of materials at Russian public schools, which were even more formidable than his biographer James J. Harford,author of "Korolev," who had the best public schools available back decades ago.

    To make a long story short, I think SSTO spaceplanes really need another series of material breakthroughs.
    And the folk at ESA clearly don't since they found no 'showstoppers' after studying the details of the Skylon design, and given that none of your links appear to have anything to do with Skylon or any other current project I'm not sure how you drew your conclusion.

  6. #156
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    They didn't find any showstoppers when they built the Brabazon either. But they spent a lot of money and they spread it around , so it didn't do any harm,..... from a certain point of view.
    Last edited by danscope; 2012-May-13 at 12:21 AM.

  7. #157
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    Quote Originally Posted by danscope View Post
    Theu didn't find any showstoppers when they built the Brabazon either. But they spent a lot of money and they spread it around , so it didn't do any harm,..... from a certain point of view.
    Yes from the point of view of someone who couldn't even be bothered to do even the least bit of Googling on the Brabazon. Had you done so you would have been aware that yes it worked fine from the technical point of view and while it failed commercially the infrastructure and technology developed during the program contributed greatly to later successes. A little research goes a long way.

  8. #158
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    Quote Originally Posted by publiusr
    To make a long story short, I think SSTO spaceplanes really need another series of material breakthroughs.
    Quote Originally Posted by Garrison View Post
    And the folk at ESA clearly don't since they found no 'showstoppers' after studying the details of the Skylon design, and given that none of your links appear to have anything to do with Skylon or any other current project I'm not sure how you drew your conclusion.
    I'm a bit confused here. Back when I worked at ESA R&D, Skylon's predecessor was "called" Lapcat M5 (well, the research program was Lapcat, the vehicle was called the A2 and it was the subject of the Lapcat M5 part of the program. The A2 looks like a Skylon with 4 engines and without the rocket cycle) and was to be a mach 5 passenger plane. Not an SSTO. Is ESA involved with Skylon as an SSTO? I left ESA before the second phase of Lapcat and I didn't follow the project after phase 1, so I'm not familiar with the more recent details. And I worked on the M8 program, with little interaction with the M5 folks. Perhaps the Lapcat phase 1 only looked into the scramjet cycle of the Scimitar variant of Skylon's Sabre engines and not into a rocket mode, so not its orbital potential? Maybe they did that in phase 2, though I doubt it as LAPCAT really was about hypersonic atmospheric passenger craft and not about SSTO.

    Edit: I found that ESA researched the Skylon/SABRE concept outside of the LAPCAT project. That explains a lot of things. Indeed they did not identify showstoppers.

    As the Skylon uses the same overall layout as the A2, I'm pretty sure they calculated the wings thoroughly. It has been many years since the atmospheric capabilities of the A2 were investigated. Someone would have noticed if it had been unable to lift off. Not lifting off tends to significantly increase your flight time.
    Last edited by Nicolas; 2012-May-12 at 05:46 PM.

  9. #159
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    Ironically there is a blurb on the Brabazon in the current Air & Space:
    http://www.airspacemag.com/history-o...Committee.html
    It "climbs out at a rate so flat that one fears it’s about to sink back onto the pastures of Gloucestershire."

    Jets were on their way, as we saw from B-36 which had an even larger wingspan and marked a transition. There was nothing wrong with Bristols plane except for being underpowered, having complex gearboxes, etc.

    My point earlier was that spaceplanes would benefit from newer materials. The spindle shape for the hydrogen tank in Skylon concerns some. The Air Force would rather have hydrocarbons and that is what they are spending money on.

    Perhaps a kerolox Skylon design would allow Bond and others to work with Lockheed on this:
    http://up-ship.com/blog/?p=12737

  10. #160
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    Quote Originally Posted by Nicolas View Post
    I'm a bit confused here. Back when I worked at ESA R&D, Skylon was called Lapcat M5 and was to be a mach 5 passenger plane. Not an SSTO. Is ESA involved with Skylon as an SSTO? I left ESA before the second phase of Lapcat and I didn't follow the project after phase 1, so I'm not familiar with the more recent details. And I worked on the M8 program, with little interaction with the M5 folks. Perhaps the Lapcat phase 1 only looked into the scramjet cycle of the Skylon engine and not its rocket mode, so not its orbital potential? Maybe they did that in phase 2 though.
    Skylon is the evolution of the HOTOL SSTO from the 1980's. the Passenger plane was a spinoff from Skylon, not the other way round.

  11. #161
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    Quote Originally Posted by publiusr View Post
    Ironically there is a blurb on the Brabazon in the current Air & Space:
    http://www.airspacemag.com/history-o...Committee.html
    It "climbs out at a rate so flat that one fears it’s about to sink back onto the pastures of Gloucestershire."

    Jets were on their way, as we saw from B-36 which had an even larger wingspan and marked a transition. There was nothing wrong with Bristols plane except for being underpowered, having complex gearboxes, etc.

    My point earlier was that spaceplanes would benefit from newer materials. The spindle shape for the hydrogen tank in Skylon concerns some. The Air Force would rather have hydrocarbons and that is what they are spending money on.

    Perhaps a kerolox Skylon design would allow Bond and others to work with Lockheed on this:
    http://up-ship.com/blog/?p=12737
    It's been explained before why Kerosene is non-starter for SSTO. When Sabre is a working technology then Lockheed can license it if they want.

  12. #162
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    Quote Originally Posted by Garrison View Post
    Skylon is the evolution of the HOTOL SSTO from the 1980's. the Passenger plane was a spinoff from Skylon, not the other way round.
    Yes, it's all falling into place now. See also the edit in my previous post (which coincided with your post): as A2 and Skylon look very similar, this means people have already quite thoroughly calculated the takeoff capability of such design.

  13. #163
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    Quote Originally Posted by Nicolas View Post
    Yes, it's all falling into place now. See also the edit in my previous post (which coincided with your post): as A2 and Skylon look very similar, this means people have already quite thoroughly calculated the takeoff capability of such design.
    Thanks for that, hope I didn't come across as short tempered with that response but I wanted to make sure the proper history was established.

  14. #164
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    Hey, I'm a sailor. Short tempered is throwing the guy through the window without excusing yourself to the bartender first.

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    Quote Originally Posted by Garrison View Post
    It's been explained before why Kerosene is non-starter for SSTO. When Sabre is a working technology then Lockheed can license it if they want.
    My only point was that it might be nice if the Skylon folks don't go it alone. As for kerosene being a non-starter for SSTO, the R-7 core tumbling after its lightweight sputnik payload debunks that. Winged kerolox is harder, yes. That doesn't mean it is necessarily impossible. New materials might make all the difference.

  16. #166
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    Quote Originally Posted by publiusr View Post
    My only point was that it might be nice if the Skylon folks don't go it alone. As for kerosene being a non-starter for SSTO, the R-7 core tumbling after its lightweight sputnik payload debunks that. Winged kerolox is harder, yes. That doesn't mean it is necessarily impossible. New materials might make all the difference.
    Skylon is baselined with current material technology. they did not want to add to the technological risks by speculating with stuff that is not current. the case closes without any breaktroughs in this field. If a breaktrough that can help skylon does come around, then great. I am sure they will happily include it if someone else develops it.

    now onto the fuel thing. you can't do the preecooler trick with a non cryogenic propellant. if you cannot do any cooling of the intake air, then you cannot feed enough of it to drive the rocket engine efficiently. the SABRE cycle is a rocket engine with an added air cycle mode. not the other way around.
    if you absolutely have to design something that runs on RP1 or something similar then i think the vehicle you want is a seriously upscaled XCOR Lynx

    I kinda like the Lynx btw. It too avoids silly stuff like mixing horizontal and vertical modes in one vehicle. it's a single stage type of deal, and it do sport that sweet rapid turnaround capability that is oh so incredibly useful when one is trying to close a business case. it's a suborbital joyride toy, but i'l not hold that against it. there IS a market for toys too out there. and somebody got to provide for it
    Is the lynx upscaleable to something that could sport a payload of some use? probably. but it would be a massive vehicle. much more massive than the shuttle stack I'm sure. the runway for such a beast would indeed be a challenge to build.

  17. #167
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    Quote Originally Posted by publiusr View Post
    Ironically there is a blurb on the Brabazon in the current Air & Space:
    http://www.airspacemag.com/history-o...Committee.html
    It "climbs out at a rate so flat that one fears it’s about to sink back onto the pastures of Gloucestershire."

    Jets were on their way, as we saw from B-36 which had an even larger wingspan and marked a transition. There was nothing wrong with Bristols plane except for being underpowered, having complex gearboxes, etc.

    My point earlier was that spaceplanes would benefit from newer materials. The spindle shape for the hydrogen tank in Skylon concerns some. The Air Force would rather have hydrocarbons and that is what they are spending money on.

    Perhaps a kerolox Skylon design would allow Bond and others to work with Lockheed on this:
    http://up-ship.com/blog/?p=12737
    As the amount of hydrogen needed as coolant for the pre-cooler helium loop is enough to fuel the rockets and the bypass burners, it seems rather unnecessary to add hydrocarbon fuel to the mix. The cooling system is the critical technology for getting the SABRE engines to work, and they are designed to use cryogenic hydrogen as coolant, so it would probably need a severe redesign of the entire craft if it was to use hydrocarbon fuels, if it is even feasible to use hydrocarbons fuel as coolant replacement for hydrogen, I have my doubts.

    I do not really see what it matters to the Air Force what fuel the Skylon would use, as it doesn't seem to me that REL has designed the concept in such a way that it is essential or even necessary to sell the craft to the Air Force. Of course, if the craft gets of the ground, I suspect that several Air forces may consider how the technologies may fit into a military use.

    In the Skylon Owners manual there is a chapter on the use of the Skylon for suborbital deployment of payloads that would then proceed to orbit by the use of an attached engine module(What REL has called the Skylon Upper Stage module), so the concept of using the Skylon as a booster has been looked at.

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    The Air Force just hates hydrogen. From what I've read over the years, they really want an all hydrocarbon vehicle. They don't really want all that fancy cryogenic cooling. Now I'm sure they would investigate it, and there might be some fans of hydrogen in the USAF, but it is hard to beat RP for energy density.

    Here folks want water for cooling:
    http://www.flightglobal.com/news/art...ys-dlr-333941/
    http://www.flightglobal.com/news/art...travel-207854/

    Other all rocket concepts
    http://www.popsci.com/military-aviat...ays-start-bang
    http://www.msnbc.msn.com/id/19211706...ourist-market/
    http://www.up-ship.com/eAPR/images/a.../designs.shtml

    More from Scott Lowther:
    http://up-ship.com/blog/?p=336
    http://up-ship.com/blog/?p=344
    http://up-ship.com/blog/?p=350
    http://up-ship.com/eAPR/ev1n1.htm
    http://www.aerospaceprojectsreview.com/blog/?p=250
    http://www.pmview.com/spaceodysseytw...lvs/sld001.htm
    http://www.pmview.com/spaceodysseytw...lvs/sld041.htm
    http://en.wikipedia.org/wiki/Spaceplane

    Funny http://en.wikipedia.org/wiki/File:A_...ion_System.jpg


    In the news
    http://www.digitaltrends.com/cool-te...ero-emissions/
    http://news.discovery.com/tech/new-r...yo-116020.html

    Big PDF
    http://www.andrews-space.com/images/...esentation.pdf

    http://www.flightglobal.com/blogs/hy...ack-first.html
    http://www.flightglobal.com/news/art...essure-197977/
    http://www.flightglobal.com/news/art...engine-371683/
    http://www.esa.int/export/esaCP/SEMH...Germany_0.html
    http://www.la.dlr.de/ra/sart/projects/dsl/dsl.php.en
    http://www.flightglobal.com/news/art...travel-207854/

    Not as cool as what we see in fiction http://space1970.blogspot.com/ but give it time...

    Lots of folks are working this
    http://www.hobbyspace.com/Links/RLV/RLVCountdown2.html
    Last edited by publiusr; 2012-May-14 at 09:58 PM.

  19. #169
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    Quote Originally Posted by publiusr View Post
    The Air Force just hates hydrogen. From what I've read over the years, they really want an all hydrocarbon vehicle. They don't really want all that fancy cryogenic cooling. Now I'm sure they would investigate it, and there might be some fans of hydrogen in the USAF, but it is hard to beat RP for energy density.
    publiusr,

    That is too much. 25 links with hardly an explanation of any of them. And no, I'm not going to check each one to see if it has anything to do with the thread. In the future, way fewer links, with an explanation of each.
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  20. #170
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    Quote Originally Posted by publiusr View Post
    The Air Force just hates hydrogen. From what I've read over the years, they really want an all hydrocarbon vehicle. They don't really want all that fancy cryogenic cooling. Now I'm sure they would investigate it, and there might be some fans of hydrogen in the USAF, but it is hard to beat RP for energy density.
    And it's been explained why as far as a spaceplane like Skylon goes it won't work. Now maybe someone will come up with an SSTO that uses Kerosene at some point and if they do I'll be keen to see it work but at the moment REL and Skyon are the only game in town as far as the scenario described by the OP goes.

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    Spindle shapes also have their own problems, but I do get your point. Hey, at least I'm not calling for a return to the Venture Star. I was always more of a fan of X-34 myself than anything that looked like this:
    http://up-ship.com/blog/?p=14737

    I put a lot of links there--especially about the all rocket systems--because there was some mention about heating problems with regards to kerosene burners. I understand that it can be a good thing to have a very wide airframe to disperse heating loads--but that is exactly what skylon doesn't have with its slim design. A kerosene burner would look a lot like that but be shorter. No, you don't have the cryogenic to use as a working fluid, but the DLR link in the all rocket section of my links was to use water as a working fluid. I don't know how well that would work for a true SSTO, but it was going to be used for Dyna-Soar, and that wasn't a whole lot more than a warhead.

    I do seem to remember some talk about problems with certain airbreathers in terms of the icing up of inlets, which can cause havoc even with pitot tubes. This is where having an interest in seemingly disparate interests pays off. Some of you may remember a recent science magazine aricle on clouds:
    http://blogs.discovermagazine.com/lo...n-aerobiology/
    http://www.bautforum.com/showthread....Microorganisms

    Doesn't seem to have much to do with aviation right? Maybe not so much. On page 8 of the May 14 2012 issue of Av Week we learn that bacteria have "proteins that are able to align water molecules in a crystaline structure that mimics the crystaline structure of ice." This would aid in the accumulation of ice. What is the relevance of this? In the letter below we learn that ice crystals are larger near convection than those specified for icing tests. Convection like that near Canaveral, say?

    "Above a certain size, the ice 'stopped behaving aerodynamically and crossed the streamlines ballistically..effectively doubling the amount of ice ingested." What do we learn from this? Well, if you are a biologist, you might see the footprint left behind by abiogenesis thru eutectic ice.

    If you are an aerospace engineer--it might make you want to quit messing with airbreathers and go for an all-rocket approach and be done with it. That having been said, on page 15 of the above cited magazine, (across from news of a succesful MDA intercept) we do see the blurb "Scramjet Success Hailed."

    HIFire accelerated from Mach 6.5 to Mach 8 for 12 seconds instead of the allotted 8 seconds. But that's the brute force approach. Still, it is nice to see some good news on the airbreathing front. Challenges remain, however: http://www.bautforum.com/showthread....ersonic-Flight

    Sadly, scramjets dictate that the whole airframe look like a chisel nose inlet. Fine at altitude, but perhaps even more of a brick than shuttle lower down. A cruciform shape is best there and works best with a compact propellant like kerosene, but that leaves it out in the heat with no working fluid. But if you want a hydrogen burning that is going to dictate a lot of volume--that can work for you in spreading the heating load--maybe it is best to ditch the whole shape of a plane and go for this:

    http://www.astronautix.com/fam/lenicles.htm
    "At hypersonic re-entry speeds it would undergo lower heating and require less shielding. At the same time it was more maneuverable at subsonic speeds than a winged design, and could land at sea or on land without undercarriage."

    But a "saucer-shaped vehicle was inherently aerodynamically unstable and would require aerodynamic surfaces to allow controlled gliding flight."

    It has been said that the Bono Saucer was just a drawing to show just how large winged re-usable systems would be in comparison with simple rocket shapes:
    http://www.astronautix.com/craft/bonaucer.htm

    But it would be nice if re-usable hydrogen burning RLVs with something other than cruciform plans were investigated.

    That having been said, I always thought this design was interesting:
    http://en.wikipedia.org/wiki/Bartini_A-57

    Instead of a float, a heat shield, perhaps.
    Last edited by publiusr; 2012-May-20 at 08:50 PM.

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    For all the airbreathing fans out there--there is this new concept:

    http://up-ship.com/blog/?p=14769
    http://hobbyspace.com/nucleus/?itemid=37961
    http://www.aviationweek.com/Article....458597.xml&p=1

    http://www.aviationweek.com/Blogs.as...0-5258308c847e

    More about it on page 25 of the May 21 2012 issue of Av Week, especially the 'sugar scoop' inlet of the wave-rider.
    Last edited by publiusr; 2012-May-25 at 10:31 PM.

  23. #173
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    Quote Originally Posted by publiusr View Post
    The Air Force just hates hydrogen. From what I've read over the years, they really want an all hydrocarbon vehicle.
    Someone should tell them that the Delta IV uses LH2 in its first and second stages - that's launched 14 military payloads in its 19 launch history.

    Plus the centaur upper stage of the Atlas V - which has launched 14 military payloads in its 30 launch history.

    Clearly having used it as part or all of 28 launches in the last decade, they don't hate hydrogen THAT much, do they?

    These are both LV's they bankrolled the development of as part of the EELV program to the tune of billions of dollars.

    Perhaps they should switch all their launches to Falcon 9's - which use RP1/LOX for both stages so they can avoid using the Hydrogen they 'hate'.

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    http://en.wikipedia.org/wiki/Prompt_Global_Strike

    USAF does not need a spaceplane to launch comsats into GEO. USAF needs something which can fly on a suborbital trajectory and deliver a team of marines to a location on the other side of the world on short notice. The "short notice" part makes hydrogen a complete non-starter.

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    Quote Originally Posted by kamaz View Post
    http://en.wikipedia.org/wiki/Prompt_Global_Strike

    USAF does not need a spaceplane to launch comsats into GEO. USAF needs something which can fly on a suborbital trajectory and deliver a team of marines to a location on the other side of the world on short notice. The "short notice" part makes hydrogen a complete non-starter.
    And if the USAF want that and can get the funding that's their affair, has nothing to do with Skylon or any other spaceplane proposal which is the topic of this thread.

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    Yes it does. Hydrogen is fine for standard launches. The USAF wants hydrocarbon **for spaceplanes** since it is easier to deal with on short notice. That is very relevant to the topic at hand.

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    A couple of points re wingloading. The 747 has a wing loading of about 162 lb/ft^2 (take off speed is about 190 knots), which is higher than that of the notoriously tiny-winged TSR. The SR-72, with a take off weight of about 172,000 lb (78 000 kg) has a wing loading of about 95 lb/ft^2.

    It looks like Skylon has about 1700 ft^2 of wing area (I'm eyeballing the aspect ratio and getting the wing area from the eyeballed aspect ratio and the span given in their specs) and a TOW of about 602,000 lb, which gives a wing loading of about 355 lb/ft^2. This is really, really high. If its wing is as good as a 747's, which is doubtful, it's takeoff speed would be about 280 knots.

    I suspect that the tire life is going to be rather short....
    Information about American English usage here and here. Floating point issues? Please read this before posting.

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    Quote Originally Posted by publiusr View Post
    Yes it does. Hydrogen is fine for standard launches. The USAF wants hydrocarbon **for spaceplanes** since it is easier to deal with on short notice. That is very relevant to the topic at hand.
    It's shown that the only current design for an SSTO spaceplane has to use Hydrogen, it's been pointed out that the USAF uses and helped develop EELV's that use Hydrogen, and yet still you make this claim.

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    Quote Originally Posted by publiusr View Post
    The USAF wants hydrocarbon **for spaceplanes**
    Cite.

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    Quote Originally Posted by publiusr View Post
    Yes it does. Hydrogen is fine for standard launches. The USAF wants hydrocarbon **for spaceplanes** since it is easier to deal with on short notice. That is very relevant to the topic at hand.
    What the USAF "want's" is irrelevant for the reality of the physics involved in making a real life working spaceplane.
    It's been shown that a dual purpose cryogenic coolant/fuel is required for closing the air breathing rocket case.
    without the air breathing rocket cycle then we get no spaceplane. it's not practical to make one without. the payload ratios you get just aren't feasible in any economic sense.

    spaceplanes are not suitable for prompt global strike imho. they are too slow on final approach, and are vulnerable to anti aircraft fire. if you want to insert troops fast and hard then a drop capsule approach is much better. altho nobody wants to do that because it's impossible to differentiate between a troop drop and a nuclear first strike.

    The argument that you cannot have a skylon ready in time to transport troops is however, false. troops aren't ready to drop in 10 minutes unless already pre prepped before the alarm goes (this only happens if they expect to be called upon on minimal notice). troops on normal peacetime short notice call don't sit in the ready room waiting for an alarm. 1 hour to kit up is minimum. you can have a skylon that is prepped to ready status fueled up and cleared for launch in the same time-frame if you are willing to do boarding while the tanks are filled up. for military operations this is an acceptable risk. the skylon in question would be kept in a ready state already connected to a specially fitted fueling apron with external power keeping the systems ready for launch.

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