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  1. #1
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    Lightbulb british rocketplane by 2020?

    Looks like the skylon project is going forward and might have a working spaceplane flying around 2020
    BBC news article
    Skylon concept page

    highlights from the info i have found so far:
    the craft will be a ssto vehicle
    it will use the air breathing sabre engine for it's ascent to orbit.
    projected cargo capacity is about 12 tons to LEO
    it is designed to be fully reusable up to 200 flights between refurb and rebuild.
    also. it will use active cooling during reentry.

    somehow i get the feeling that 200 flights between engine refurb might be a tad optimistic. but i could be wrong.
    what i do wonder the most about tho is the use of active cooling during reentry. has that ever been done before?
    what do you guys think of the progress of these kinds of vehicles so far?

    oh yeah. i didnt want to open up the old dead thread about air breathing rocket engines. Esp. since it's in the wrong place imho. So
    linky instead

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    While I wish them well, this article points out why airbreathing space vehicles are difficult to achieve.

    For the engines, we can compare rockets to jets, and the rockets come out ahead. Rocket engines generally are simpler than jet engines, not more complicated. And although they have to lift more weight, they can do that: the best modern jet engines produce thrust equal to about 10 times their weight. But 40-year-old rocket engines can lift 100 times their own weight. For a given thrust, a rocket engine is much smaller and lighter than the corresponding jet engine.

    Why the difference? It's all about density. A litre of LOX contains as much oxygen as about 4500 litres of sea-level air. At high altitudes, the air is much thinner yet, and the LOX is unchanged. Jet engines are huge and heavy because they must handle such enormous volumes of air to gather each kilogram of oxygen. As the old sailor's adage goes: "the wind may be free, but the sails bloody well aren't".

    Yes, LOX weighs a lot, but the tanks to put it in, and the engines to carry it, are light - and it's the tanks and engines that you have to build and maintain. Moreover, in a single-stage spaceship, the liquids in the tanks burn off on the way to orbit, but the engines have to be carried all the way... so how much they weigh is actually rather important.

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    That´s an old idea. All you get on their website is the description of the concept. I´d like to see an actual explanation of how they´re going to make it work.

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    I´d like to see an actual explanation of how they´re going to make it work.

    More to the point, I'd like to see how they're going to pay for the R&D. It'll cost quite a bit more than the million pounds mentioned in the article.

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    some of the enabling technologies appear top have other commercial uses outside of rocketry. like the highly advanced heat exchanger they are developing for cooling down the air input, Before compressing it for use in the rocket stage of the engine.
    Also. going by the business model they portray. the company uses the developement of these technologies as a way to keep ahead and as a selling point for their real business: consultant services for technological development.
    call it "know how" farming if you like.

    back to rocket planes in general:
    One of the advantages of a rocket plane that is not readily aparent is that it needs not accelerate to high speed until it is pretty high up.
    you don't just use air to reduce on board oxidiser weight. you also use it to help you lift your ship up to altitude before starting the real burn to orbit.
    it would be interesting to see a comparison between this concept and the 2 stage spaceship one concept on mass savings on the ground. most of the fuel in a rocket is spent just getting the first couple of km off the ground after all. the third dataset in that comparison should be a normal rocket.
    i have assembled arguments both for and against the use of rocketplanes:

    points in favor of rocketplanes:
    1. reduced need for initial thrust due to aerodynamic lifting help from the wings. can be further enhanced by using a steam catapult to help the craft gain speed on the runway
    2. reduced need for oxidiser in the initial part of the ascent profile.
    3. using a runway for both lifting off and landing means reduced need for specialized launch pads with a slow turnaround. one runway multiple preparations bay's can enhance the turnaround rate by a magnitude or more.
    4. reusability puts hardware investement per kg cargo put in orbit lower. resulting in cheaper access to space

    points against rocketplanes:
    1. increased complexity due to complicated dual mode engines.
    2. wings makes re-entry harder on the craft. might demand new and experimental tech to accomplish safe return at all.
    3. can only lift small payloads in the 10 to 12t range. also severe volume restrictions apply. (this might change over time as the concept matures with use)
    4. initial capital investement is very high compared to normal launchers, and is dependant on having enough customers for all the launches the vehicle can do within a reasonable time frame.

    To me it seems that the biggest issue is to actually have enough cargo to carry to orbit so that you utilize your shiny launcher in a profitable way. a hangar queen ain't making money, and unless you have cargo to carry you aren't getting off the ground no matter what kind of launch system you employ.


    imho. there will always be a market for big boosters. large oversized cargoes like big station modules etc needs be launched only now and then during construction phases, and would be outside the normal market for fast turnaround rlv's.

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    I had some comments on air breathing hybrids back here in 2005:

    http://www.bautforum.com/astronomy/1...tml#post359434

    I see them as missing the point. The idea is to get the cost of operations down, and the air breathing schemes will add great complexity and mass. Anyway, in that post, I linked here, which had mentioned Skylon:

    http://www.aerospaceweb.org/question...ft/q0202.shtml

    From that page:

    Although Reaction Engines has been pursuing this concept [Skylon] for over 15 years, it does not appear to have attracted much interest. In 1992, the company estimated that it would take 10 years and $10 billion to develop the Skylon. To date, the group has only been able to raise a small fraction of that total, most of which has been invested into basic research on the Sabre engine. Given the number of unproven technologies incorporated into its design and the lack of capital to develop them, it seems unlikely that Skylon will be completed anytime soon. Additional information is available at Reaction Engines.
    - answer by Jeff Scott, 14 November 2004
    So, they've been pushing this idea for a long time, and a million euros isn't going to cut it.

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    Not a chance. Our government will toss money at failing banks, and when that fails they even start printing more money - but they won't invest in R&D.

    Skylon is a good project. It isn't just a bunch of random nutters, the men behind if have serious aerospace pedigree, and they deserve a shot at trying to make this work. They just simply won't get it.

    (I should also point out that the company has plans to use a variant of the same air-breathing rocket engine for a hypersonic antipodial passenger jet that can go from Europe to Australia in 4 hours or something daft. Having common parts with a commercial airliner helps the economy of a space plane a lot)

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    Quote Originally Posted by Van Rijn View Post
    I had some comments on air breathing hybrids back here in 2005:

    http://www.bautforum.com/astronomy/1...tml#post359434

    I see them as missing the point. The idea is to get the cost of operations down, and the air breathing schemes will add great complexity and mass. Anyway, in that post, I linked here, which had mentioned Skylon:

    http://www.aerospaceweb.org/question...ft/q0202.shtml

    From that page:

    So, they've been pushing this idea for a long time, and a million euros isn't going to cut it.
    The Skylon proposal for using hybrid engines may work but they still need to use rocket propulsion to make the final push to orbital velocity. Note also the jets they use lower in the atmosphere are heavy which will cut down on performance.
    This is because of the characteristic known as thrust-weight ratio, which has always been quite poor for jets compared to rockets. See here:

    Thrust-to-weight ratio.
    http://en.wikipedia.org/wiki/Thrust-..._ratio#Engines

    Henry Spencer also discusses the poor performance of jets compared to rockets on this one measure in this article:

    Rockets, not air-breathing planes, will be tomorrow's spaceships.
    March 4, 2009 5:33 PM
    "For the engines, we can compare rockets to jets, and the rockets come out ahead. Rocket engines generally are simpler than jet engines, not more complicated. And although they have to lift more weight, they can do that: the best modern jet engines produce thrust equal to about 10 times their weight. But 40-year-old rocket engines can lift 100 times their own weight. For a given thrust, a rocket engine is much smaller and lighter than the corresponding jet engine.
    "Why the difference? It's all about density. A litre of LOX contains as much oxygen as about 4500 litres of sea-level air. At high altitudes, the air is much thinner yet, and the LOX is unchanged. Jet engines are huge and heavy because they must handle such enormous volumes of air to gather each kilogram of oxygen."
    http://www.newscientist.com/blogs/sh...hing-plan.html

    I have an alternative method for producing a hybrid engine: circulate the incoming air around and around to allow sufficient time for combustion to occur even at the high hypersonic speeds required to reach orbit:

    From: Robert Clark <rgregorycl...@yahoo.com>
    Date: Sat, 8 Mar 2008 06:26:00 -0800 (PST)
    Newsgroups: sci.astro, sci.physics, sci.space.policy, sci.engr.mech
    Subject: Could we just circulate the air in scramjet propulsion?
    http://groups.google.com/group/sci.a...8d9a4b4f0a9be9

    In that post I suggested using regular jets or ramjets in the lower atmosphere but it occurs to me a key advantage here would actually be to use the same engine all the way from launch to orbital velocity, thus saving on the weight of traditional jet engines.
    Note that a key component for propulsive efficiency specifically for jets is the compression they generate of the air before they direct it into the combustion chamber: the more compression they can generate the more efficient the jet engine.
    This is why they use the huge turbine compressors in front of the engines. This contributes most to their heavy weight. Then the method of having the air just circulate around and around would dispense with this heavy weight. Note also that according the the formula for acceleration around a circle a = v^2/r, the pressure, and therefore the compression, would be greater for the higher velocities. Henry Spencer mentions in his article the problem of the huge amount of air that has to be gathered to get sufficient oxygen for combustion with jet engines, but note that at the high supersonic and hypersonic velocities, where the efficiency would be greatest, the high incoming velocity of the air automatically makes it so that you can acquire that large amount of air, and oxygen.


    Bob Clark

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    The Skylon proposal for using hybrid engines may work but they still need to use rocket propulsion to make the final push to orbital velocity. Note also the jets they use lower in the atmosphere are heavy which will cut down on performance.
    You seem to misunderstand what they are doing; they are attempting SSTO using just one kind of engine - that can either draw oxygen from the air at low altitudes or draw from an on board supply and function as a rocket engine at higher altitudes.

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    Hmmm...It seems to me, from the images and the descriptions, that the engine have one ramjet part, the area between the outer walls of the engine and the turbojet/rocket motor assembly(this is mounted in the center of the whole engine assembly).

    The precooler apperantly uses more hydrogen for cooling than the turbojet can burn, so the surplus is used to feed the ramjet part of the engine. Precooling allow the use of lighter parts than in a traditional supersonic jet engine, and you supposedly do not need to ramp back the power on this engine like the traditional supersonic jets. The numbers given estimates the thrust to weight to be about 14 in atmosphere, that isn't that bad for a jet engine, supersonic jet engines are rarely much above 5 or so.

    A point of the Skylon design I found interesting is that it doesn't need to use as heavy heat shielding as the shuttle, the maximum reentry temperature being estimated to less than 850 celcius, so no tiles to fall of or break.

    Though I am not really that enthusiastic to the probability of this project succeding, getting enough money to make the thing is probably a bigger challenge than the engineering itself... Still, It will be interesting to see if they get an engine up and running.

    I expect even if the Skylon fails, the data collected would be a help in the design of the other project, the A2 hypersonic liner, and its Scimitar engines, though if anyone wants such a thing is another issue...

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    i can see several non space launch uses for a hybrid engine like the one they are developing.
    passenger suborbital flights being just one of them.
    The engines could easily be used to make a mach 5+ long range strike aircraft. i am certain there would be some military uses for that.
    secondly. the engine runs on hydrogen. altho hydrogen is very bulky to move it's as clean a fuel you can get.
    so even kept in just air breathing mode as a engine for supersonic passenger craft it would have some nifty benefits. however that means solving some of the issues that kept Concorde ticket prices so high in the first place.

    as long as they develop the engine, witch btw is what they are doing. then there is potentially a pretty decent market for it.
    If the engine sells then cash for making skylon is liable to be available at some future date.
    btw. how much of that 10 bill figure is needed for the engines is hard to say. since it seems like a lot of the enabling technologies are already nearing the maturity levels to start half scale prototype building.
    however in my experience it's the last 5% of the development that really cost the big $$ as it is at that point you have to build working prototypes of the entire system and actually fly them. so let's hope they get it funded properly.

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    Quote Originally Posted by Antice View Post
    The engines could easily be used to make a mach 5+ long range strike aircraft. i am certain there would be some military uses for that.
    I think a military application for a long range aircraft is about the only likely one, assuming they weren't worried about complexity, maintenance, or cost, but were concerned about range and speed in atmosphere.

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    What is the cost of hydrogen compared to kerosene? I don't even know how they produce it. If they use an electrolysis then it has to be very expensive.
    The whole thing sounds like the areocar concept.

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    Quote Originally Posted by samkent View Post
    What is the cost of hydrogen compared to kerosene?
    I don't know, but for its use in an expensive application is probably negligable in the entire picture.
    Quote Originally Posted by samkent View Post
    I don't even know how they produce it. If they use an electrolysis then it has to be very expensive.
    That's one way. It's also produced by chemical reactions and extracted from hydrocarbon sources like natural gas.

    Electrolysis is expensive now, but if we can bring the cost of renewable electric sources low enough, it might not matter.
    I think I recall some country (Iceland?) uses geothermal to produce it for a comperable price to petroleum products.

  15. #15
    Quote Originally Posted by samkent View Post
    The whole thing sounds like the areocar concept.
    There is an areocar available in the US beginning in 2011.

    http://www.flightglobal.com/blogs/fl...a-reality.html

    However, I agree with the spirit of your comment. In my opinion, SSTO might be technically possible at some point in the distant future. At best it will always be a compromise just like the aerocar, which is really neither a very good car nor a very good aircraft.

    Thus far even partially re-usable launchers have failed to live up to cost expectations. STS is a prime example. The Orbital Sciences Pegasus XL system is better, but that's only because of the L-1011 part of the system. The Pegasus itself is more or less a conventional rocket - three stages at that. SpaceShipOne and SpaceShipTwo are not capable of orbit so in my opinion they don't really qualify as launchers. The fastest airbreather so far is the X-51A Waverider and it has only managed about Mach 6 for 200 seconds. That demonstrates the extreme difficulty of achieving sustained hypersonic velocities.

    The Wiki article (http://en.wikipedia.org/wiki/Reaction_Engines_SABRE) about the SABRE engine makes some amazing claims. To quote, "The engine gives good fuel efficiency peaking at about 2800 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds." Are they talking about specific impulse or specific thrust or what? Are they claiming efficiencies in the range of ion engines? Then there is the matter of the helium powered turbopumps re-cycling waste heat. Why not do the same using the hydrogen fuel? The helium system just seems like extra mass.

    My gut feeling is that this whole concept is fundamentally flawed. I find it hard to believe that they can achieve the engine performance claimed. I don't believe it is possible to design an airframe that can be sufficiently aerodynamically efficient across such a wide range of airspeeds. I don't believe they can build it light enough to carry sufficient fuel for SSTO and still handle the dynamic stresses and thermal issues of SSTO. And even IF they could do all that, I seriously doubt that the life cycle cost could be competitive with more conventional commercial launchers such as those being demostrated already by SpaceX.

    whoops - I forgot the X-43A 2nd and 3rd flights. The 2nd flight reached Mach 7 and the 3rd almost Mach 10. Both included very brief periods of scramjet (airbreathing) operation and there was I believe some acceleration during the 2nd flight.
    Last edited by KB73RR; 2010-Jul-07 at 05:12 PM. Reason: add x-43A information

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    Quote Originally Posted by KB73RR View Post
    There is an areocar available in the US beginning in 2011.

    http://www.flightglobal.com/blogs/fl...a-reality.html

    However, I agree with the spirit of your comment. In my opinion, SSTO might be technically possible at some point in the distant future. At best it will always be a compromise just like the aerocar, which is really neither a very good car nor a very good aircraft.

    Thus far even partially re-usable launchers have failed to live up to cost expectations. STS is a prime example. The Orbital Sciences Pegasus XL system is better, but that's only because of the L-1011 part of the system. The Pegasus itself is more or less a conventional rocket - three stages at that. SpaceShipOne and SpaceShipTwo are not capable of orbit so in my opinion they don't really qualify as launchers. The fastest airbreather so far is the X-51A Waverider and it has only managed about Mach 6 for 200 seconds. That demonstrates the extreme difficulty of achieving sustained hypersonic velocities.

    The Wiki article (http://en.wikipedia.org/wiki/Reaction_Engines_SABRE) about the SABRE engine makes some amazing claims. To quote, "The engine gives good fuel efficiency peaking at about 2800 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds." Are they talking about specific impulse or specific thrust or what? Are they claiming efficiencies in the range of ion engines? Then there is the matter of the helium powered turbopumps re-cycling waste heat. Why not do the same using the hydrogen fuel? The helium system just seems like extra mass.

    My gut feeling is that this whole concept is fundamentally flawed. I find it hard to believe that they can achieve the engine performance claimed. I don't believe it is possible to design an airframe that can be sufficiently aerodynamically efficient across such a wide range of airspeeds. I don't believe they can build it light enough to carry sufficient fuel for SSTO and still handle the dynamic stresses and thermal issues of SSTO. And even IF they could do all that, I seriously doubt that the life cycle cost could be competitive with more conventional commercial launchers such as those being demostrated already by SpaceX.
    Well luckily your 'belief' isn't relevant. Try expanding your research beyond Wiki, perhaps even to, for example the company actually building the SABRE and Skylon:

    Reaction Engines

    Seriously if you think this bears any relation to the aerocar you really have no clue about the Skylon project.

  17. #17
    Quote Originally Posted by Garrison View Post
    Well luckily your 'belief' isn't relevant. Try expanding your research beyond Wiki, perhaps even to, for example the company actually building the SABRE and Skylon:

    Reaction Engines

    Seriously if you think this bears any relation to the aerocar you really have no clue about the Skylon project.
    Thanks for the suggestion. I was intrigued by the OP and progress reports in the previous posts so I actually did look at their site, the previously cited wiki article, all readily available info about the principals, and I ran a D&B on the company too. The bottom line is they are just a handful of smart guys with a grand vision, an apparently politically-related ESA contract, and a slick web site. I wish they would succeed with their grand vision, but it's just a CGI spaceplane - and that's probably all it will ever be. "No Clue"? Perhaps, but for now my venture capital is going elsewhere.

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    Quote Originally Posted by KB73RR View Post
    Thanks for the suggestion. I was intrigued by the OP and progress reports in the previous posts so I actually did look at their site, the previously cited wiki article, all readily available info about the principals, and I ran a D&B on the company too. The bottom line is they are just a handful of smart guys with a grand vision, an apparently politically-related ESA contract, and a slick web site. I wish they would succeed with their grand vision, but it's just a CGI spaceplane - and that's probably all it will ever be. "No Clue"? Perhaps, but for now my venture capital is going elsewhere.
    Errant nonsense, and no doubt your money will going on a bridge to lurk under...

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    Quote Originally Posted by KB73RR View Post
    ...

    The Wiki article (http://en.wikipedia.org/wiki/Reaction_Engines_SABRE) about the SABRE engine makes some amazing claims. To quote, "The engine gives good fuel efficiency peaking at about 2800 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds." Are they talking about specific impulse or specific thrust or what? Are they claiming efficiencies in the range of ion engines? Then there is the matter of the helium powered turbopumps re-cycling waste heat. Why not do the same using the hydrogen fuel? The helium system just seems like extra mass.
    The Skylon user's manual gives the following projected data for the engines on the Skylon C2:
    Maximum air-breathing thrust 2 x 1350 kN
    Isp in air-breathing mode 35000 N s / kg
    Maximum thrust in rocket mode 2 x 1800 kN
    Isp in rocket mode 4500 N s / kg

    The helium system is in fact one of the improvements in this engine design over earlier precooled designs. These earlier precooler systems using hydrogen tended to become brittle and break as the hydrogen diffused into the metal(a process called hydrogen embrittlement). This would pose a problem to the reliability and rapid re-usability of a craft using SABRE engines.

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    hydrogen can be produced from natural gas with the help of steam reforming of natural gas. this method invalidates the no co2 release of the hydrogen cycle tho. it just moves the co2 release to the hydrogen manufacturing plant. where it potentially could be sequestered.
    alternate methods involve high temp electrolysis in steam cycle nuclear reactors. but none of those reactors are built yet. and might never get built unless we have a big political shift about nuclear power in general.
    generally hydrogen is a more expensive fuel than kerosene, since the cheapest manufacturing methods rely on natural gas to make hydrogen.
    Ironically. in some places. like my own country hydrogen is cheaper than kerosene due to high amounts of taxes on kerosene/diesel used in vehicles.

    how viable it is to convert the rocket part of the sabre engine in order to use the cheaper fuels is unknown. it could be a fairly easy thing to do since as far as i can see from the design specs on the sabre engines the rocket part is distinct from the air breather/pre-cooler part. with helium used as coolant in the pre-cooler.
    I guess that sort of development would be market driven, as a means to lower cost per flight. it depends on hydrogen costs at the time the engine nears completion of development.

    even as a layman with only hobbyist interest in these matters i am aware of some of the fallacies regarding the production and use of hydrogen as a fuel.
    methane/ethanol and other waste to energy cycling technologies might very well provide far cheaper and more sustainable fuels than hydrogen.

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    I look at the concept and I can't help but think: "Thunderbirds are GO!"

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    One of the biggest drawbacks of an airbreathing booster is that it must spend a lot of time flying at high speed within the atmosphere where the heating loads are extreme. This drives up the weight of the thermal protection system and structure. A rocket can climb vertically until it is above most of the atmosphere before beginning its horizontal acceleration to orbital velocity. This greatly reduces the thermal and aerodynamic loads on the rocket leading to lower weight. The article I linked to above claims that a Goddard Space Flight Center study showed a pure rocket powered spaceplane (if that's the approach you'd want to take) outperforms an air-breathing spaceplane by a significant margin. Also, you'd save the R&D costs of developing the air-breathing engine and required thermal protection system.

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    Quote Originally Posted by Larry Jacks View Post
    One of the biggest drawbacks of an airbreathing booster is that it must spend a lot of time flying at high speed within the atmosphere where the heating loads are extreme.
    Does that matter with an SSTO? It needs the protection for re-entry anyway.

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    Quote Originally Posted by NEOWatcher View Post
    Does that matter with an SSTO? It needs the protection for re-entry anyway.
    Yes, in this case there would be a long sustained thermal load, not a relatively short period for just reentry.

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    is it not so that the greatest thermal and aerodynamic loads are encountered upon re-entry?
    any rocket plane that is fully reusable would need some form of active cooling instead of a tile based shuttle derived or expendable Apollo derived system.
    I can imagine quite a lot of development being required to pull that off. afaik that is as of today a rather immature concept that might not be doable currently.

    pure rocket propulsion might very well outperform the air breathing tech altogether. time will be the judge of that. i think it depends on wither extra engine weight is higher or lower than the oxidiser saved by using them. again we can only wait and see how the end performance of this concept will end up. at least we know that there is a lot of margin for improvement from the lace experimental engines from earlier times, and what will come out of the engineering labs in 10 years or so.
    However a lifting body do save on fuel for lifting the launcher up to altitude. a 2 stage system might be more sensible for this tho. again. time will tell.

    One thing i have noticed, is that most manned launch architectures needs a depressed trajectory for reducing g loads on abort. this means that a lot of the savings for going straight up first are not really there any more. effectively negating one of the key advantages to using a rocket in the first place.

    IMHO rocket planes dont really make sense for bulk cargo. they make more sense for flying people, and experiments to a station and back down to earth.
    however i do see the need for verifying such radically different designs in a long series of unmanned flights first.

    one thing that a lot of ppl seem to overlook tho. it's not always about efficiency of fuel and lift that makes sense. but also how many launches you can get done on the same amount of capital investment, and amounts of recurring costs associated with one launch. ground crew and infrastructure maintenance is the biggest factor here. a craft that uses a runway can share crew with other non space based craft thereby utilizing the infrastructure and ground crew's capacity more fully.
    If done right, rocket planes can have a lot lower cost per launch than rockets by simply being able to do a lot of runs in a fairly short time frame. however this demands that the craft is 100% reusable, and can perform multiple launches between servicing rounds.
    I don't think that we are at that stage in development for quite a while yet. i might get to see it happen in my lifetime tho.

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    Quote Originally Posted by Antice View Post
    one thing that a lot of ppl seem to overlook tho. it's not always about efficiency of fuel and lift that makes sense. but also how many launches you can get done on the same amount of capital investment, and amounts of recurring costs associated with one launch. ground crew and infrastructure maintenance is the biggest factor here. a craft that uses a runway can share crew with other non space based craft thereby utilizing the infrastructure and ground crew's capacity more fully.
    Yes, that was a point I made earlier in thread, and in more detail in my earlier post that I previously linked to. That's one of the arguments against hybrid designs, as they add a great deal of complexity and are likely to inhenerently require more maintenance than a much simpler pure rocket design.

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    Quote Originally Posted by Van Rijn View Post
    Yes, that was a point I made earlier in thread, and in more detail in my earlier post that I previously linked to. That's one of the arguments against hybrid designs, as they add a great deal of complexity and are likely to inhenerently require more maintenance than a much simpler pure rocket design.
    While I think they will likely get it working, you have a good point.

    Thats why I like Spaceship One's approach. Use a huge carrier wing craft to get the bulk of the ship as high and fast as possible with minimal fuel. Then use a small rocket to get it into orbit.

    Technically, you could opperate a heavy lift opperation with Spaceship One's approach (as long as you were not bringing the weight back in reentry) on about 1/10th of the fuel of conventional heavy lifters.

  28. #28
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    Quote Originally Posted by dgavin View Post
    Thats why I like Spaceship One's approach. Use a huge carrier wing craft to get the bulk of the ship as high and fast as possible with minimal fuel. Then use a small rocket to get it into orbit.

    Technically, you could opperate a heavy lift opperation with Spaceship One's approach (as long as you were not bringing the weight back in reentry) on about 1/10th of the fuel of conventional heavy lifters.
    First, fuel's cheap. It's the vehicles that are expensive, and the launch operations and infrastructure.

    Second...no, you can't. You need a rocket nearly as big as you do for a launch from the ground, and an enormous carrier aircraft:

    A launch from 15.2 km altitude at 4200 km/h (what WhiteKnight2 is expected to do with a much smaller suborbital payload) only gets you about 2% of the way to a circular 350 km altitude orbit, in terms of energy. You've gained almost nothing with the altitude, and only dropped required delta-v from about 8.1 km/s to about 7 km/s, and the rocket still needs 70-78% of the fuel it needs for a ground launch using plain old LOX/RP-1.

    Proportion of energy saved =
    (15 km*9.8 m/s^2 + 0.5*(4200 km/h)^2)/(350 km*9.8 m/s^2 + 0.5*(7.69 km/s)^2) = 0.025

    Delta-v required for ground start (rough approximation):
    sqrt(2*(350 km*9.8 m/s^2 + 0.5*(7.69 km/s)^2)) = 8.12 km/s

    Delta-v required for 15.2 km altitude, 4200 km/h start (again, a rough approximation):
    sqrt(2*((350-15.2) km*9.8 m/s^2 + 0.5*(7.69 km/s - 4200 km/h)^2)) = 7.01 km/s

    Tsiolkovsky's rocket equation: Ve*ln(m0/m1) = deltaV
    deltaV/Ve = ln(m0) - ln(m1)
    e^(deltaV/Ve + ln(m1)) = m0
    Setting m1 to 1 so the result is total vehicle mass in units of dry mass, ln(1) = 0, so:
    e^(deltaV/Ve) = m0

    For LOX/RP-1 engines, Ve is around 3240 m/s.
    e^((7.01 km/s)/(3240 m/s))/e^((8.12 km/s)/(3240 m/s)) = 0.7

    For LOX/LH2 engines (lighter, but bulkier, meaning more aerodynamic drag on the resulting vehicle), Ve is around 4420 m/s.
    e^((7.01 km/s)/(4420 m/s))/e^((8.12 km/s)/(4420 m/s)) = 0.78

    These are very approximate calculations, but they get the point across. There's tank mass for the ~25% larger fuel tanks for the ground launch version, but that only makes for slightly heavier tanks. There's also further subtleties with staging on both alternatives, structure to allow the air-launched vehicle to survive being carried and dropped by an aircraft after prolonged supersonic flight, the difference in aerodynamic losses, etc.

    Notice that air launch is very complicated. Wings aren't magic, it takes the same amount of energy to lift a payload to a certain altitude whether you do it vertically or up an inclined plane...though the inclined path will often require more energy overall due to friction and gravity losses, even if it can be done with less power over a longer interval of time. On top of this, just altitude gets you very little. Reaching orbit requires about an order of magnitude more energy to accelerate the payload up to orbital velocity than it does to lift the payload up to orbital altitude. And as airspeed goes up, aircraft become less and less efficient and more complicated. You need a complex and enormous supersonic carrier craft, and a booster that can handle stresses and heating conditions that are actually worse than reentry. All this for saving a bit of fuel that's practically free?

  29. #29
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    I see there is a Wikipedia Article on the sabre engine.

    I am not so sure either of how feasible the concept is, it looks interesting. It doesnt' seem as complex as a traditional jet engine. At first I wondered if it was some sort of pulsejet, but I see that it actually do have a fan based compressor...

    Hmmm... Ah well, if nobody invests in projects based on risky technologies, no new approaches will ever get proven, we will be stuck with the current approaches forever. We are bound to learn something, if they get it of the computer and into actual testing, at least.

    The space craft has a pretty nice design, kind of a classic sci-fi feel.

  30. #30
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    Quote Originally Posted by dgavin View Post
    Technically, you could opperate a heavy lift opperation with Spaceship One's approach .
    And technically, you could replace one supertanker or container ship with a million john boats too.

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