What obstacles exist towards building a vehicle which is capable taking off horizontally, entering low Earth orbit, and returning to a horizontal landing on Earth, and which looks exactly the same taking off as it does landing? By this I mean a space plane—a vehicle which requires no discarded external fuel tanks or aircraft piggybacking. Are the obstacles to achieving this primarily technological or political? If the obstacles are technological, is it because hydrogen/oxygen fuels are simply not powerful enough to permit this, regardless of how efficient an engine we can design? Is it likely that we will see such a vehicle before the year 2050?
This can’t be answered factually with a simple yes or know. A spaceplan can probably be built with sufficient funding (the political hurdle) but there are many technological hurdles to overcome in both materials and powerplants. The problems are well known since the X-15 but creative solutions will have to be found to overcome them in ways suitable to a reusable spaceplane. You’ll go a long way toward answering this by doing a google search and going to the NASA website.
The problem with a space plane is that it’s a technical challenge, and also that such a plane has to haul a lot of mass into orbit that has no business being there - wings, engines, landing gear, etc. The shuttle shares some of these problems, but not all. For instance, the landing gear of the shuttle only has to be strong enough to support its weight empty of fuel, or nearly so. Imagine how strong it would have to be if it had to support the weight of the external fuel tank. Also, the shuttle’s engines are optimized for high-thrust, high acceleration operation in which the shuttle is out of the atmosphere very quickly.
Taking off on wheels means the gear has to be very strong (and therefore heavy), and that engines operate efficiently through many very different regimes - low-speed takeoff, high-speed cruise, and airless rocket power.
The ultimate problem is that you lose a lot of efficiency when you carry all that structure into orbit - so much so that it may never be cost-effective to operate this way. If you must take off on wheels, a much better approach would be to have a gigantic mothership that lifts the spacecraft to 40,000 feet or so and then drops it. Now your spacecraft can run on rocket power, and it doesn’t need to carry heavy landing gear or other ground systems. Also, having it already above most of the atmosphere, 40,000 ft up, and already going several hundred miles per hour reduces fuel requirements dramatically.
It depends on what technology you’re referring to.
If you mean an air-breathing aerospace vehicle, then you’re talking about a scramjet: a vehicle capable of hypersonic flight at the fringes of the atmosphere using engines that accomodate high Mach flow. The biggest problem with this is that the aerodynamics of hypersonic air flow through an engine is fiendishly complex and difficult. Only now that very advanced supercomputers are available for the calculations has it been possible to design and build such an engine. The concept has been around for decades but it was only a few months ago that a brief test flight of an unmanned drone with a scramjet engine was successful.
Or you might be thinking of a rocket-powered aerospace plane. (Which is really more of an airplane-shaped rocket). The biggest problem with this configuration is one that is common to all SSTO (Single Stage To Orbit) designs: It has essentially zero leeway for redesign if anything goes wrong in the development process. If any part of the craft needs to be heavier than originally planned, if the engines don’t perform as well as expected, your payload percentage of gross takeoff weight becomes a negative number. In other words, it can’t even reach orbit much less carry a payload. And unlike an expendable rocket where if worst comes to worst you can make a propellent tank a few meters longer, a SSTO rocketplane has it’s fuel tanks integral with it’s airframe, which means tossing out the entire design and starting over.
The Space Shuttle was originally intended to be a compromise rocketplane, sacrificing total reusability for a less developmentally risky design. An external main tank, and soild rocket boosters instead of a reusable winged booster stage. But even then the Shuttle has had crippling problems. The balky, leaky main engines can fly once, then need major refurbishment, then can fly ONE more time- and then have to be replaced! After every flight the heat shielding tiles have to be laboriously replaced. The Shuttle requires so much nursemaiding that it never met the promise touted for it of being the “DC3 of space”. The Shuttle flies so infrequently, is so semi-expendable, and has such a low ratio of it’s takeoff weight to payload that it’s more expensive than expendable boosters, not less. A SSTO design would have most of the Shuttle’s problems and be even more likely to fail as a reliable space launcher. NASA has started and cancelled several X-vehicle projects that were supposed to develop spaceplanes, but never produced a working prototype.
Wot? We Dopers? Of course we can!
If ya still want more info, Howstuffworks.com has two relevant articles:
How Space Planes Will Work
How Air-breathing Rockets Will Work
Vehicles to orbit that do what you’re interested in are called Single-Stage-To-Orbit (SSTO) vehicles. On paper we might just about be able to build them with what we now know, if they have little or no payload capacity. We have lighter alloys, stronger fibres, better composites, and better computers for design purposes than the current aging fleet of launchers had when they were designed. (I’ve read a claim somewhere that the 2nd stage of Apollo was close to a SSTO all on its own, no idea whether that’s true. Not that it would be re-usable or come back again, but still…) Nobody big seems remotely keen in trying to develop them, although quite a few little firms have tried to get far enough to attract decent investment.
One technology that hasn’t been developed very far is variable geometry nozzles for rockets. The optimum shapes of those bells at the business end of any rocket are different for rockets at ground level, rockets high in the air and rockets in vacuum. With a staged rocket you optimise the nozzle of the first stage for ground level, the last stage for vacuum and compromise with the ones in between. SSTO fans like either nozzles that get shorter as you get higher, or something called “aerospike” that should be self-adjusting with altitude. Who knows, maybe if the 2nd stage of Apollo was lightened a little using kevlar and aluminium-lithium alloys, and fitted with variable geometry nozzles, it would be a true SSTO.
SSTO enthusiasts tend to be interested in two body designs - blunt cones that take off vertically and re-enter arse first, and winged designs. The wing gives you some steering leeway without using fuel on reentry, and it also allows you to fly up part of the way. Getting to say 40,000, 60,000 or even 100,000 feet on conventional jets before you start the rockets has advantages. You can fly up above the weather before kicking your rockets in, you’re clear of much of the drag-inducing atmosphere (half the atmosphere is in the first 18000 ft), and you can use rocket nozzles with vacuum-optimised geometry. Whether that is enough to offset the weight penalty of wings and jets is debatable, and depends pretty much on who does the math and what assumptions they make.
Beyond this are the air-breathing rockets - using supersonic ramjets or pulse-detonation engines, you fly up there, building up so much speed that by the time you’ve run out of atmosphere (and air for your engines), you’re going fast enough that it doesn’t matter. But the engines to do this are in their infancy, and we don’t really know how well they’ll work.
The real obstacle to the space plane is money and risk. Nobody wants to be the first person to develop one, with all the investment, risk and massive amount of new technology that has to be developed - the current market for satellite launches doesn’t warrant it. Arguably if someone did develop such a launcher then the market itself would change (some think it would explode), but that’s just it - you want to be the second person to develop a launcher, where you can see the strength of the market and learn from all the mistakes the first person made.
Unless I’m mistaken, the shuttle engines have gotten much more reusable since the beginning of the shuttle programs. While they still need tear-down and inspection after each flight, the current generation of shuttle engines can be used up to a dozen times before needing replacement. And the shuttle tiles only need replacing if damaged by debris impact, the reentry itself normally causes no damage to the tiles.
One big problem I foresee would be drink service. How would that be accomplished on a space plane when it is such a nightmare on a regular plane?
Who’s rocket do you need to stage to get a dry martini around here?
This actually is a problem for any commercial service. Back when I was on the fringe of this field, there was a lot of talk about a hypersonic transport that could fly LA to Tokyo in a couple of hours. It was the focus of scramjet development for awhile. The project was eventually deemed to be completely impractical for several non-technical reasons, one of which was that anyone paying a premium for this kind of flight would expect a first class meal, and there simply wasn’t time to serve it on any of the planned routes. The marketing people decided they would be better off with a slower vehicle.
Eh, if your flight time is only two hours to anywhere, the solution to that one is easy. In every airport equipped to handle them, you put a full five-star restaraunt near the gate. Purchase of a first-class ticket entitles you to a meal at the restaraunt at the starting or ending airport, your choice (or, of course, you could skip it if you were in a rush). The restaurant would be inside the secure area, so you could eat in that “Please arrive at the airport two hours before scheduled departure” window. And none of the infrastructure needs to fly.
Meanwhile, back on topic, I’m curious why the OP wants a space plane in particular. Eliminating piggybacking on other aircraft, for instance, makes the problem a lot harder, for no particular gain I can see. Cheap access to space is certainly a worthwhile goal, but the proper question is “Can we build a cheap space system”, with a space plane considered as only one of many open-ended possibilities, not to try to shoehorn a space plane (or any other single proposed solution) into the role.
The problem with a horizontal take-off spaceplane is this; you need a certain amount of thrust–delta V–to achieve a particular orbit. It doesn’t matter in what direction you’re pointed (as long as your vector doesn’t intersect the ground :o) but you have to carry the fuel and engine to change your energy state, period.
A plane “works” by generating lift via Bernoulli’s principle, i.e. the differential of airflow over and under the wing creates a lower pressure above the wing than below it. The engines provide thrust forward but not up (excepting VTOL craft like the Harrier or the JSF), so all “up” force has to be converted to lift by the wings. Now, there’s a limitation to how much lift you can get for a given area and configuration of lifting surface, and that decreases with altitude. Going faster doesn’t generate more lift (in fact, you can end up getting less) and creates other problems with stability. In order to lift a payload the mass of what can be carried by a Saturn V rocket, for instance, would require a gigantic lifting area, which you’d then have to haul up into space, and protect from heating on reentry. If you’re going into orbit, of course, you are going to have to switch to a rocket instead of lift at some point, as which all that wing surface and support structure is dead weight.
As a heavy lift vehicle, a spaceplane is hopeless. Too much area and structural mass is dedicated to being a plane. It is far more practical and efficient to go with a conventional multistage booster. This will remain the case, I think, until and unless we get beyond using chemical propellents. However, as a personnel carrier, a scramjet/rocket vehicle is feasable. What is even smarter, though, is a two stage vehicle, the first stage of which is a dedicated aircraft and the second stage being a dedicated rocket, a la the Scaled Composites White Knight and Spaceship One combination. You get away from carrying all the extra flight surfaces and hardware into orbit, but still have reusability (at least of the first stage) and rapid turnaround times.
The Shuttle’s biggest problem, from a conceptual design standpoint, is that its requirements and mission functions became a hodgepodge of conflicting and dissimilar commandments based upon a collection of differing interests. The Air Force and the NRO wanted something to place and service spy satellites; commerical interests wanted a cheap booster for communications satellites; scientists wanted a research platform (SpaceLab); and NASA wanted a delivery truck to support the (then) Freedom Station. Reusability became a watchword, even as design compromises and limitations of material science resulted in a vehicle whose main propulsion systems have to be virtually rebuilt every couple of launches. NASA would have been much better off to maintain and refine the Saturn V booster and the expanded Apollo missions. (There were plans, at one point, for a larger Apollo-style capsule that could crew six people on a round-the-moon or Lagrange point mission.) However, for political reasons (i.e. Nixon wanted to kill the lunar exploration space program as a major budget item once we “won” the space race) Apollo was destroyed; literally, dies and tooling were demolished, drawings archived incomplete, and the last two boosters delivered in pieces and further disassembled, never to be used again.
We need a heavy lift booster (maybe the Delta IV, though it hasn’t had a very good reliability record so far) for hauling materials, and a cheaper, mostly reusable personnel shuttle, either an SSTO rocket or a spaceplane, or combination thereof. Whether it takes of horizontally, or whether it is completely reusable should be less of an issue than the cost to orbit, period.
Stranger
:dubious: What was Spaceship One?
Spaceship One piggybacked on the White Knight. That, at least, was fairly conventional, but not what the OP is talking about. (I believe that’s how the military got either the U2 or the SR-71 that high.) It would be interesting to have another X-Prize, demanding a space plane. The prize would have to be huge, though, to help offset the costs of R&D.
Both the U-2 and the SR-71 take off under their own power. The SR-71, though is only partially fueled, owing to its propensity to leak fuel while not underway, and is typically fueled immediately after gaining altitude. (In flight, thermal expansion causes the tank and skin materials to expand, closing the gaps.)
Stranger
One idea I remember being kicked around in the “go go” Sixties was for a space plane that used inflatable wings. The idea being that you could have all the benefits of winged craft, but none of the disadvantages of hauling all that extra crap around in vacuum where it’s useless. A similar idea is starting to gain ground.
I like your attitude! 
I just found an interesting new take on the spaceplane concept: the Skylon. The key to this concept is a dual-mode engine that can breath air on take off, then shut it’s intakes and shift into rocket mode. If it worked, it would turn many of the criticisms of spaceplanes on their heads. For example, the contention that wings are a useless dead weight on launch. Yes for a vertical ballistic ascent. But for a true spaceplane, they reduce how powerful (and heavy) the engines have to be. Their thrust only has to be a fraction of the vehicle weight, not multiples of it. And enclosing the fuel tanks within the airframe has an advantage: on reentry, the plane has such a large area for it’s weight that it can glide into the atmosphere more slowly and with less fierce heating: hopefully enough that a permanent durable external hull coating can be used instead of expendable tiles or shields.
Of course lots of concepts look good on paper, and the caveat of “IF this works” must be applied. But it looks very interesting.
That´s a misconception; the bulk of the lift generatd by a wing comes from the reaction to a mass of air being moved down, diverted by the airfoil and angle of attack of the wing.