I'm working on a science fiction story which includes the following premise: Tony Stark invents an unobtainium reactionless engine, and the protagonists want to build a spaceship around it, big enough to carry a nontrivial amount of cargo. They have a company similar to SpaceX, enough money, political authority to bypass red tape and a mandate to waste anything but time.

The current idea is to basically take the design of the C5 Galaxy, cut the wings a lot shorter and replace the landing gear with helicopter style skis (the unobtainium engine is VTOL-capable), make a few other tweaks and that should be pretty much it. (Planned flight times are only a day or two, so no worries about extended life support.)

How long would it take to build the first model?

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    $\begingroup$ The crew section of your C5 Galaxy is going need a lot of work to make it space worthy. It will need increased insulation, radiation shielding and presumably enhanced computers and navigation systems. If you are in a hurry, you might be better off just gutting the entire cockpit to increase cargo space and then replacing the plane's nose with a SpaceX Dragon capsule. Then all you need to do is wire up the new engine and plan a launch date. $\endgroup$ Jun 20, 2017 at 3:45
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    $\begingroup$ @HenryTaylor Save time. Just put a Space X Dragon capsule inside the C5 and wire it to the reactionless drive. You're definitely right about making it spaceworthy. This prototype could be used for short hops into orbit and back again. Though it ought to be great for resupplying the International Space Station. About 800 cubic metres of cargo would do that easily. $\endgroup$
    – a4android
    Jun 20, 2017 at 5:01
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    $\begingroup$ In any project the effort is calculated in manhours or manyear. If you want a time estimation, care to give at least how many people are going to work on it. I.e. 20 manyears with a man would take 20 years, with 20 men only 1 year.... $\endgroup$
    – L.Dutch
    Jun 20, 2017 at 5:12
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    $\begingroup$ @o.m.: Adding manpower to a late software project makes it later (Fred Brooks). But in general yes you are right, the interchangeability of people and time is not perfect. One woman makes one child in nine months, and you cannot put nine women to the task to have it in one month; but if you want two children you don't have to wait for 18 months. $\endgroup$
    – AlexP
    Jun 20, 2017 at 10:46
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    $\begingroup$ Don't start with an aircraft, start with a submarine. It's much more suitable. $\endgroup$
    – Mike Scott
    Jun 20, 2017 at 14:45

7 Answers 7



Not long at all. Your engine is so incredibly over-powered that its not rocket science anymore.

Scrapheap Challenge/Junkyard Wars

Firstly, lets look at your engine.

  • VTOL capable, this means it has incredible Thrust to Weight ratio
  • Reactionless, means that it is light-weight and not dependant on heavy reaction mass for range
  • Reactionless, means that the engine can be mounted anywhere (and not just on the front/back)

When one considers that the Falcon 9 carries about 500 metric tonnes of reaction mass, just to get 5.5 metric tonnes into GTO. You can appreciate how much more leg room your magic engine gives you.

Quite frankly, once you have a working engine, the problem is very similar to an episode of Scrapheap Challenge/Junkyard Wars.

Your engineering margins dropped to such low levels, you could just weld parts to the magic engine and it would still work fine.


The approach I would do to make your space truck would be simply to bolt 4 of your magic engines onto the ends of a X frame. Then use a modified quadcopter controller to remotely control your space-copter.

Iterating like a Kerbal, you should be able to get the thing working in a few weeks.


You have unobtanium, you have reactionless drives, said drives invented by an individual, you are in the Marvel comicbook universe.

The answer is dependent on your plot, not on any real-world building issues because nothing in your story follows real-world physics or project planning and management.

If this is a device used within the story, then it will take “one act” to get working well enough to do its job. If the story is about the craft, than it will take the entire novel or arc.

Consider the first Iron Man movie, where he built the first thing in a cave with scraps, and building the finished suit was not a significant passage of time where other things happened and people got older, but one dramatic all-nighter of planning and design and a few meetings to explain his decisions with other company people.

I suggest the ship follow the same idea. Review the movie and notice how much time passes, and how they handle the logistics of material handling and industrial equipment on that scale: Basically, not. Instead the screen time was taken with essental points needed by the character.

Now a written work may have more attention here than a graphic work. But it’s also easier in writing to ignore time and have an unspecified duration take place.

To move things along quickly,

  • bring in a big team of people
  • write scenes where stuff needs to be shipped/delivered now
  • have on-site rapid prototyping including building metal and composits directly from computer files. Have them “build more fabs first!” to get it done quickly.

In real life, testing and planning is slow (and dull). Think Tony Stark will put up with that? no! He’ll stay late, assemble the last components, and take it for a spin himself.

Your universe is driven by drama and plotting and has all manner of “super” stuff and features (literally!) comic-book physics. Asking about realism with your stated set-up is the wrong thing. You should look at the purely writing problem of pacing and plotting.

Decide how long it takes (say, a week). Then find ways to justify that, to the little extent needed.


Very much a question of risk and red tape. Lots of things to go wrong with that conversion.

  • Turning a STOL aircraft into a VTOL aircraft is going to be extremely complicated. Consider the R&D effort for the Harrier or the Osprey. It might actually be easier to design a new one.
  • Getting the fuselage airtight is going to be another challenge. The C-5 operates at altitudes where the air is rather thin, but it doesn't have to worry about small leaks.
  • Life support is the next problem. CO2 scrubbers and bottled oxygen, or some way to split the CO2 again?
  • Where does the power come from? The C-5 has air-breathing engines and a fuel system for them.
  • For reentry, the C-5 might try to come to a stop relative to the upper atmosphere before it starts the descent. That means it might get away without a heat shield. But what if the pilot miscalculates and things get hot?

The modified C-5 might fly, but the first couple of attempts might kill the test pilots. My gut feeling is that a new design would be easier.

  • There is the proverb that with enough thrust, even a brick will fly. You have magical engines (err, beyond science as we know it), so take a solid steel box and mount engines on the side.
  • Add a submarine-style power plant (with modified coolants) and an space-shuttle-style life support system.
  • Quarters that are habitable both under thrust and coasting in zero-G. With a C-5, gravity is at right angles to the axis of thrust. Imagine the mess from the toilets.
  • Don't forget the airlock, and a cargo bay that can be opened without evacuating the cockpit.
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    $\begingroup$ I've only a few small nitpicks with this answer. "mount on the side" No need to do that, its reactionless, just bolt it directly to the powerplant in the middle of the steel box. Air lock, no need for that, just unhook the cargo container from the power plant. It would basically be a space truck. $\endgroup$
    – Aron
    Jun 20, 2017 at 9:23
  • $\begingroup$ @Aron - a reactionless drive system is likely to be manipulating some hitherto unknown physical field. As a passenger of such a craft, I'd like it to be doing so as far away from me as possible, at least until we've proven the process safe through decades of experience. Mount it outside, on as long a strut as you can get away with without sacrificing structural integrity. $\endgroup$
    – Jules
    Aug 10, 2018 at 10:30

If you have a super powerful reactionless drive most of the constraints of spaceflight are negated. Spacecraft are generally built to balance two conflicting requirements;

  • They have to be light, based on limits of the rocket launching it into space.
  • They have to be strong enough to survive a high acceleration to orbital speeds through atmosphere resistance.

Both of these could be negated through your magic drive. If it has the capacity to lift very heavy things it is trivial to make it stronger, just weld on some more steel.

Similarly if it can accelerate at slow constant rates, you don't need to survive massive acceleration or care about high Mach aerodynamics, you could have a box shaped space ship that slowly rises above the atmosphere and accelerates to orbital speed slowly over several hours.

If all you need is cargo go for a box and don't worry about pressurizing it. Depending on the amount of cargo, I would recommend either a large steel shipping container or a large cargo ship, with your drive(s) welded onto it in structurally supported location(s). Both containers and ships are readily available for purchase and would only require your drive to be attached and provided with power, this could happen very quickly if needed.

For man rating it would take slightly longer, but if you start with a pressure vessel like a boat, submarine, or airplane, it should take very little to make it air tight. I would personally choose a boat over an airplane as they tend to be much bigger with larger structural safety factors and your drive doesn't need the aerodynamics.

Adding atmospheric controls again is difficult if you are mass limited and require automated controls, if not just throw a whole bunch of Oxygen pressure cylinders and some Carbon Dioxide scrubbers inside your pressure vessel and have someone monitor and manually adjust gas mixtures in your vessel, if you start with a submarine these systems would already be in place.

If you don't care how it looks and your drive can handle the mass you could have a large ship in orbit within days.

  • $\begingroup$ Indeed, in this classic story the toy ship built by kids just has the magical guts installed in a matter of minutes. In Ringworld the reactionless drive made a building fly just by anchoring it firmly in a closet. There’s a series of novels about a sub turned into a space ship. $\endgroup$
    – JDługosz
    Jun 20, 2017 at 18:28

This isn't an overnight build.

Phase 1, design

Someone has to design the craft to withstand the forces your engine provides. Otherwise, your pilot takes it out for the first test-flight, hits the accelerator, and either becomes part of the engine or watches as the engine rips forward through the fuselage and leaves him and the debris behind.

So your engineers needs to get busy with AutoCad (or equivalent design systems). Then send their rough designs over to be tested in simulations; some digital, some physical. Simultaneous to that, some group will be building various sizes and designs of this new engine, attaching it to rocket sleds, and firing it up to accurately measure it's acceleration and other forces.

Phase 2, tests

After a design is approved, various unmanned miniatures are going to be built, wind-tunnel tested, safety-tested, and then test-piloted. Each of these tests will consume time and materials. Each will result in new data that goes back into the design, so each new test is a refinement on the previous designs. It won't be perfect the first few designs. Heck, the first mock-up would probably just explode on the runway if you went straight from "back of envelope" to manned prototype.

Phase 3, test flights

At some point, you'll feel confident the system is safe enough to stick a person in. You'll find a volunteer test pilot. They will fly a prototype that's (we hope) got a working design. The pilot will discover any lingering design flaws. There may be a few changes and re-tests.

Phase 4, FLIGHT!

A production prototype will go out, for even more test flights. At this point, your design is solid. This is more fine-tuning and discovering the limits. The final build will have some build-up phases as raw materials are converted into finished parts on spec, then assembled into a working system.

Real World Examples

Boeing 787

According to this post, it took almost 9 years for Boeing to build the 787. That's an iterative update to jet planes, so no new physics involved. Your new engine will probably have a slightly higher learning curve in the early stages than this. But the larger budget will reduce your time.


Elon Musk began hiring staff for SpaceX in 2002. The goal was to get to Mars in 10-20 years. As of 2017, they have not yet achieved that goal. This timeline is closer to your OP, since Elon Musk comes closer to "Tony Stark" than most real-world peope (at least with regards to available cash). Your question assumes more money and more resources, so the time gap will be lowered. More engineers and a harder schedule is possible, but only to a point.

Space Shuttle

The US Space Shuttle program chose a design in 1972 after starting up the project in 1969. The first flight was in 1977. The first orbital test was 1981. In 1982, the first non-test flight took place. Thirteen years from the formation of the task group to the first non-test flight. Your group will move faster, thanks to CAD, availability of supercomputers for rapid initial testing, and because budget is no constraint on the project.


Engineering for a project like this always falls back on the project management triangle. You have 3 inputs: Time, Quality, and Cost. The old maxim is "Cheap, fast, or good; you can pick any two." You want Fast and Good. So your cost is going to be quite high.

I imagine this project could not be safely completed in less than five to ten years. But some of that preliminary design work would (we hope) be happening while Mr. Stark was off in his bourbon-fueled manic design state. Hopefully, while he's tweaking the engine design, his engineers would also be building the boring old plane to carry that engine.

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    $\begingroup$ I completely disagree with this answer. Yes, engineering in general is not simple or easy. The reason being is that engineering is always about striking a perfect compromise. However a reactionless propulsion system with high T/W ratio (VTOL capable) removes so many constrains off the block, it is literally not rocket science anymore. Its like asking someone to design a viable electric car around a Mr Fusion. $\endgroup$
    – Aron
    Jun 22, 2017 at 5:28

A few more points to consider:

You wouldn't use a C5 for a space ship, because it is specifically designed to operate as an aircraft - where a great deal of weight is held up by two enormous wings. That's a lot of mass in the airframe, for stresses that your space ship will never experience, and probably isn't stressed for different engines mounted in different locations.

Think Apollo/LEM - that's what a pure space ship would look like. No consideration for aerodynamics at all. Or aesthetics... but that adds authenticity to your story.

Because a space ship has to maintain pressure internally, and because your space ship will be quite a bit larger than the LEM, it will probably be tubular in shape, as that is the most efficient pressure vessel.

How long would it take to build this? How long to paint the Mona Lisa... as long as it is expedient to support the rest of your story.


Boeing and Lockheed had a bake off to win the Joint Strike Fighter contract. The task was, starting with a blank sheet of paper, to design and build an aircraft that was capable of both supersonic flight and VTOL (a combination which had never been previously achieved) including many test flights to prove that the prototype(s) worked.

They were given 18 months to complete the task. Both achieved the goal, and both used 100's of top engineers and used the full time quota.

The JSF has similar complexity, technology and context of your craft, so...

18 months.


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