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Spaceplanes are cool. SSTOs (Single Stage To Orbit) are even cooler.

Imagine a spaceplane that is pretty much a 747. Imagine if Boeing decided to have a crazy new idea and made a space variant of the 747 (dubbed 747 Ultra or maybe 747-1000). Or maybe in a retro-futuristic universe (a 747 will likely be "the good old days" for space-faring civilizations) where they decide to make a spaceplane out of the iconic 747.

This 747 Ultra could be used to fly from planets to planets. It must be capable of taking off from a planet, get into orbit, travel to another one, reenter the atmosphere and land.

My main question however lies in the construction of such a thing. What are the challenges one would encounter when trying to transform the 747, Queen of the Skies and a giant iconic bird, into a (rather retro futuristic but still advanced) SSTO spaceplane... if it's possible?

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    $\begingroup$ It kinda feels too broad, or too short, because best answer is everything. Meaning: What obstacles would I face when transforming my paper plane into real aeroplane able to carry people? All. Of. Them. $\endgroup$ – Pavel Janicek May 30 '18 at 18:00
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    $\begingroup$ Please watch the first 30 minutes of Kubricks' "2001: A Space Odessey", you will find a "Pan-Am" shuttle that nearly completes your list - no 747 though. $\endgroup$ – Joe May 30 '18 at 18:13
  • $\begingroup$ The 747 and Saturn V have similar payloads: 248,600 lb / 112,760 kg vs 261,000 lb /118,000 kg to low Earth orbit. Depending on model, a 747 can carry about 48,000gal / 182,000 l of jet fuel, which it burns in about 10-15 hours. A Saturn V first stage carried 4 times as much - 203,400 g / 770,000 l fuel (plus 318,000 g /1.2 million l of liquid oxygen) - which it burned in 168 seconds: en.wikipedia.org/wiki/Saturn_V#S-IC_first_stage And that's just to get the second & third stages on their way. $\endgroup$ – jamesqf May 30 '18 at 18:46
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    $\begingroup$ As a note Victorbrine Cassini, it's usually good practice to leave off accepting an answer on this stack for at least 24 hours, there's a lot off timezone spread so it pays to give everyone a shot, saves you having to change your mind about the best answer later. $\endgroup$ – Ash May 30 '18 at 19:04
  • $\begingroup$ Compare Would a fighter jet be able to go into orbit from Mars surface? and also to a lesser extent on Space Exploration How far would the STS get without the SRBs. Full disclosure: I have posted answers to both those questions. $\endgroup$ – a CVn May 30 '18 at 19:10
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Building a SSTO spaceplane out of a 747 with current technology would just be flatly impossible. The key value is the mass fraction of your propulsion - which basically is what percentage of the takeoff weight is not fuel, and what part of that weight is payload as opposed to vehicle. An SSTO vehicle needs to be about 90% fuel by mass. (Staged rockets can be less fuel by mass, which is why we use them.) Needless to say the 747 is not designed to hold nine times its dry mass in fuel.

So let's assume this is your second scenario: that a futuristic space place is for whatever (presumably cultural) reason shaped like a 747. Because you don't have a specific technology in mind, I think it's more reasonable to look at what you would need.

First, thrust (and fuel). As mentioned above, you need your thrust mechanism - whatever it is - to be a lot more fuel-efficient than any existing design. (Glancing over its specs, the 747 only carries about its own weight in fuel at maximum load, so you need to be ~10x as efficient as current drives... or some kind of science-fictiony reactionless drive.) Making the engines small enough to fit in its engine cowlings is probably going to be the easy part.

Second, the underlying structure. By that I mean: the 747 is propelled from its wings; all of the thrust originates there, and the wings transfer it to the body. An SSTO design generates vastly more thrust; the wings need to be strong enough not to snap. (They also need to be strong enough not to be broken off on reentry, but I suspect that's a lower threshold than liftoff thrust.)

Third: it needs to be able to maneuver. Given everything else, a small version of whatever your thruster is to serve as an RCS system probably isn't a big deal. The 474's own flight surfaces of course won't work in space.

Fourth: on reentry, you need to soak an immense amount of heat (unless you have such a phenomenal amount of fuel that you can reenter slowly). Whatever the skin of the plane is made of, it will need to be tough enough to withstand that. It will also need protection from micrometeorites and what have you. It'll need strong materials for the cockpit windscreen, etc.

All of this has been assuming that you want the internal layout to be basically identical. If you were going to hollow out the cabin and cargo hold and turn them all into fuel tanks, it's not quite so implausible, but I think the wings are still shaped wrong, and the heat shield will still be an issue.

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  • $\begingroup$ Even if you manage all this, it's still completely the wrong shape for supersonic and hypersonic flight, which I imagine would cause further trouble. $\endgroup$ – Elukka Jun 3 '18 at 10:06
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Physics

Lift

The 747, like all fixed-wing powered aircraft from the Wright Brothers' Wright Flyer I depends on lift from wings flying through atmosphere to fly. This means generating lift as air moves over the wings. At some altitude, the thinner air means insufficient lift. So wings don't help you above that maximum. You'll notice that there are no large wings on any space craft that is now or has ever been in service. There's a reason for that.

Yes, the Space Shuttle had wings. But they were stubby and didn't provide much lift compared to the 747's.

Thrust

Your engine must provide thrust. As you climb, you lose thrust. Especially from propeller or jet engines. So you have to compensate with a different form of thrust. Today, that means rocket engines instead of jet engines. Your 747's jet engines are useless above some maximum altitude, because there's simply not enough air.

(See this post which covers thrust and lift in greater detail.)

Fuel

Rocket engines consume a great deal of fuel. Basically every machine humanity has sent to space has carried more fuel than payload at the time of launch. Wikipedia has details to expand on this.

Steering

Space ships steer by either rotating to point their main thrusters in a new direction or by using smaller, engines to point and fire, shifting the direction of flight. Your 747 flies by moving control surfaces to change direction. Those control surfaces have absolutely no use once you rise above some altitude. For the same basic reason that you lose lift.

Pressure

Airplanes maintain cabin pressure by bleeding air off from the engines and circulating it into the cabin. This, by definition, means the cabin is not airtight and that it must, by definition, have access to external air. Your space-747 must have oxygen on board and a sealed environment to maintain internal air pressure against the vacuum of space. That's a completely new set of systems that can't just be welded onto the fuselage; it's a complete redesign of the whole system.

Shielding

Space craft must be shielded against radiation that simply isn't a threat within the earth's atmosphere. Your 747 would need some serious upgrades to handle this.

Literally every exterior component of your 747 needs to be replaced, from the glass in your windows to the aluminum of the hull. Nothing is of sufficient strength to handle the new stresses you're wanting to introduce.

Thermal Control

Your plane can regulate temperature because it is flying in an atmosphere. It's a relatively trivial task to heat or cool the outside air as needed before feeding it into the cabin.

But the vacuum of space is literally the best insulator possible. Wikipedia describes the array of thermal control systems required to maintain temperature in space. A 747 needs none of this complexity. Each of these systems would need to be factored into your 747-like ship.

Electronics

The 747 was introduced in the 1970s. Even with upgrades, the onboard electronics in it are not going to cover the needs of a space ship. You will need to pack in far more complex power generation, communications, navigation, life support, avionics, and other computer sub-systems. Other than whatever onboard entertainment, the phone that lets flight attendants talk to the cabin or to the pilots, and overhead lighting, I suspect there's literally nothing in the onboard electronics that would be usable in the new space role.

Safety

A simple across-the-lap safety belt, barf bags, and flotation devices are not going to cut it. You need pressure suits. Which means you need seats with enough room for an average human in a pressure suit. There's not a 747 in commercial service that has the room you need. And I seriously doubt that flotation seat cushion is any use at all.

Summary

Basically, there's no advantage to a 747-style plane for space flight. You'd have to strap on rocket engines, massive fuel tanks, and all the attitude jets required for steering. Your wings can never create sufficient lift at low altitude to get you above the atmosphere. They would just create drag once your massive rocket engines begin to accelerate. They probably couldn't be made strong enough to overcome the forces experienced during reentry, either.

Basically, the only way to make your 747 into a space plane is to add handwavium in massive doses. You need some sort of anti-gravity engine to negate the weight of your plane, some sort of engines that require little or no consumable fuel to provide lift, and you need steering engines added, too. Oh, and radiation shielding and other environmental concerns. And even then, the final product probably wouldn't look anything like an actual 747 at all.

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  • $\begingroup$ Perhaps the wings could be shaped like delta wings, and the engines could be some scramjets and closed cycle engines, like what Skylon would use. $\endgroup$ – Victorbrine Cassini May 30 '18 at 18:45
  • $\begingroup$ Regarding wings, you might want to compare the design of the Space Shuttle (and its Russian counterpart, Buran) with Concorde (and other supersonic aircraft). They aren't all that different, except of course for the fact that the Space Shuttle was deliberately designed to be approximately as aerodynamic as a brick with some control surfaces, in order to shed speed on reentry. $\endgroup$ – a CVn May 30 '18 at 19:07
  • $\begingroup$ Space shuttle wings are quite stubby compared to a 747. $\endgroup$ – CaM May 30 '18 at 19:12
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    $\begingroup$ @Victorbrine Cassini: But if the wings are short and stubby, it's not much like a 747 any more, is it? Really the best way to turn a 747 into a spacecraft is to melt it down, and use the recycled aluminum to build your spacecraft. $\endgroup$ – jamesqf Jun 3 '18 at 6:14
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As mentioned above, an SSTO spaceplane out of 747 is not possible at the current technology stage. But let's look at what is possible – or at least was projected and more or less feasible.

  • Silbervogel (30s-40s) is probably the first project of a spaceplane, obviously never implemented. A suborbital Nazi "Amerika Bomber", ow...
  • DynaSoar (50s-60s) was probably one of the first realistic spaceplane projects. However, the launch was planned by a "normal" rocket, it was the spaceplane that should glide (and dyna-mically soar, yep). It was planned to be much smaller than 747.
  • The Spiral project (60s-70s) in its full form aimed at having a hyperspeed heavy launcher plane on top of which the booster with the actual spaceplane resided. Models of the spaceplane were tested with the conventional start (with US analogon being the DynaSoar project above), the carrier plane never came to be. The characteristics of the spaceplane made me remember this project in the context of your question, a 50 ton Mach 6 jet! MAKS is the same idea with subsonic An-225 as a carrier, that seems to be canceled in 1991. Launch mass of MAKS was planned at 275 ton, 747-8 is 220 ton.
  • SpaceLiner (2005) is a modern take to a spaceplane. It appears to me that it is nothing more than just a project.
  • White Knight Two (contemporary) is a custom-build carrier plane for SpaceShipTwo. The latter should reach lower space (110 km) in a suborbital flight. (X-15 did a similar feat earlier.) VirginGalactic folks are slowly working into the direction of carrier-based space launch. The actual carrier does not go into space.
  • The Pegasus launcher (since 90s) is a similar take, an airborne rocket start.
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I am going to reuse some material from another question that I have answered about SSTO's.

The Space Shuttle weights 78 tons when empty, and can take a payload of 27 tons to a low earth orbit. But in order to to that, it needs the help of a 756 tons external tank, and two solid-fuel rocket boosters which weight 571 tons each. The bulk of the weight of those beasts (~92%) is fuel.

The 747 has many models, with maximum takeoff weights ranging from 320 to ~448 tons.

It simply does not have the capacity to hold enough fuel to reach a low earth orbit.

In fact, to make such a thing capable of SSTO would require making a much larger, frankeinsteinian vessel that would have so many design problems attached that the answer might be as big as a book on aerospatial engineering. The main problem has to do with the rocket equation - TL;DR, your fuel requirements will tend to grow exponentially.

You would have an easier time fusing the external tanks of the shuttle into one piece and attaching a crew cabin at the tip of the larger tank.

If you wish to accomplish SSTO, you need to either:

  • Work with a much smaller aircraft. Something the size of a modern fighter jet. These aircraft are build to have more favorable thrust-to-weight ratio, and if I remember correctly their engines have better specific impulse. Or...

  • Delve into and allow for things such as "anti-gravity drives" or whatever. Spaceships in the universes of Star Wars, Star Trek, Marvel Comics etc. are fueled by disbelief - as disbelief is lifted, so are the ships.

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The first things that come to mind are the Material construction issues.

First off the steel and aluminium hull of a traditional airliner is right out it lacks the strength and gas sealing needed for going into a full vacuum. It also lacks the heat resistance for a re-entry vehicle. Materials with that combination of strength and heat resistance are going to be thicker and/or heavier than a traditional fuselage which means that the shape can't stay the same. The second reason the fuselage has to be redesigned is that a cylindrical tube hitting the atmosphere during re-entry is going to make the trip more violent because it's not shaped for effective aerobraking, re-entry is violent enough that a long cylinder will tend to buckle under the best case scenario conditions and it shifts conditions away from best case to a frightening degree.

In summary given the severe disadvantages to the plan form and construction methods used for airliners a full redesign is needed. That's to say nothing about thrust ratios etc... which are a different and even more restrictive set of problems, have a look at Winchell Chung's Atomic Rockets page some time, he explains the problems with energy/mass restrictions that mess up most Sci-fi designs far better and more simply than I can.

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Let's list some of the problems that would need to be solved, following the hypothetical trajectory from JFK to Moon base.

  • Reach space (>100 km height): airplanes flight thanks to wing lift. The more rare the atmosphere, the less the lift. Any commercial airplane reach at best 12 km above sea level. To reach 100 km and above wings would need to be enormous. Also the engines could not work by just sucking in external air.
  • Stay in space: once you reach LEO, you need to reach orbital velocity or escape velocity. This means going around 10 km/s. And your motors lack air (see above).
  • Steer and slow down in space: airplanes use air drag to achieve this. Guess what? In space with no air the plane would fly straight, guided only by gravity. If you are heading for the Moon you will hit it really hard.

And now that your plane is just a chunk of scrap metal scattered on the Moon it will be pretty difficult to make it fly back to Earth...

Assuming you manage to take off again with a somewhat bumped airplane, now you have to face the last problem:

  • braking for orbital velocity to flight velocity: once you enter atmosphere you will have to take rid of that large amount of kinetic energy you have (remember the 10 km/s?). Few millimeters of aluminum are a joke for the heating caused by air compression. The airplane will become a cloud of plasma, and the passengers will follow shortly after.
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