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In the film Interstellar...

...the Ranger shuttles are shown lifting off multiple times from gravity wells. Similarly, the Endurance main ship makes a long journey from Earth to Saturn, and then several trips about the Gargantua system.

However, neither ship has a visible fuel tank, with the one exception when a Ranger is launched into Earth orbit on top of a rocket. But...

...later on in the movie, a Ranger without external tanks launches from a planet with higher gravity.

Is there an in-universe explanation for this lack of apparent fuel source or reaction mass? (I realize that the film production explanation is likely: fuel tanks don't look cool) The movie otherwise made many efforts to get the science and engineering right, but this particular item irks me... I have found some attempts at explanation here, but these explanations seem to rely on highly advanced technology. If NASA had technology this advanced, it would seem conditions on Earth would be much better.

  • Given that the ranger can achieve orbit, I suspect that it has some kind of crude anti-grav. – Valorum Jun 8 '16 at 20:40
  • Did you actually see the Endurance start on top of a rocket? I assumed it was a transfer module - and the Endurance was already in orbit. – TomTom Jun 9 '16 at 8:01
  • @TomTom Not the Endurance, just the ranger shuttle. – Luaan Jun 9 '16 at 8:26
  • @Valorum: The entire point of the movie was that the heroes needed to discover how to counteract gravity, so this seems unlikely! – Lightness Races in Orbit Jun 13 '16 at 17:28
  • @LightnessRacesinOrbit - Well, my thought was that with an imperfect understanding, they may have been able to build an imperfect anti-grav. With a greater understanding would come the ability to build something more elegant and powerful. Having said that, the correct (in-universe) answer is "their engines were ridiculously efficient" which is actually far less satisfying to me, from a story-telling perspective. – Valorum Jun 13 '16 at 17:37
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If you go to the tie-in page Board the Endurance (which can also be found at the link Valorum gave) and click on the module labeled "main engine module", you get the following image (click to enlarge), showing that four of the modules are actually engine modules which presumably contain fuel:

enter image description here

The text reads:

The Endurance's engines feature advanced magnetoplasma rockets, which produce a distinctive blue exhaust plume of ionised gas when firing. Each module contains a compact tokamak power plant which generates electricity via magnetically-confined fusion. As well as generating electricity for use in the engines the tokamaks supply power to the rest of the Endurance's systems. Each engine module features small manoeuvring thrusters on its corners. These are standard hydrazine thrusters of the type use in present day space travel.

So the maneuvering thrusters (which would be used for things like changing orientation, spin, and docking maneuvers) are chemical rockets using hydrazine, the main thrusters (the three circles seen in the diagram) are magnetoplasma rockets, also known as ion engines or ion thrusters because they use magnetic fields to accelerate a plasma of charged ions which serve as propellant. Perhaps it would be similar to the proposed VASIMR (Variable Specific Impulse Magnetoplasma Rocket), a proposed type of ion engine which would be much more powerful than any of the ones currently used on space probes. This type of engine creates less thrust than chemical rockets (i.e. it accelerates the rocket more weakly) so it can't be used for takeoff from a planet, only for travel when the ship has already achieved escape or orbital velocities. But the advantage is that because it would have a much higher exhaust velocity (30,000-120,000 meters per second according to the table here, compared with 4,400 m/s for the best chemical rocket), according to the Tsiolkovsky rocket equation this means it can achieve a given delta-v (change in velocity over an extended period of time) using much less fuel mass.

As mentioned in this article, in the section "Webster, We Have a Problem", VASIMR would probably require nuclear power to generate the needed electricity for long-term spaceflight, so that may explain the reference to nuclear fusion above (unless they're suggesting the superheated plasma from the tokamak reactor is used more directly to superheat and ionize some exhaust as in the proposed fusion-driven rocket of the kind outlined in some NASA presentations here and here, but when they say the tokamak is used for "generating electricity for use in the engines", it sounds like the engines themselves don't rely on nuclear reactions for anything but electrical power). An article here has a quote from the movie's visual effects supervisor Paul Franklin which confirms it's a type of ion engine: "The engines are imagined to be something plasma electric, a super-advanced version of the ion engines already being used in space exploration. But of course, these [film engines] produce much, much higher levels of thrust than the ion engines of today. So the engine pods would have some kind of compact tokamak [nuclear] fusion generator for power, that sort of thing."

As for the Ranger, that can also be clicked on the Board the Endurance webpage, giving this screen:

enter image description here

The text reads:

This is a fast, agile reconnaissance vehicle with single-stage surface-to-orbit capability. It can also be launched in a double "belly to belly" configuration on top of a large multi-stage booster. The Ranger can carry a crew of four people and one robot. Each ranger has its own cryo-sleep tanks for long duration missions. The Ranger is driven by a combination of chemical rockets and plasma jets. Hydrazine thrusters are used for maneuvers in space.

The Ranger was probably inspired by the VentureStar, a real proposal by NASA for single-stage vehicle that could take off from the ground and achieve orbit (with the fuel carried in internal tanks), whose shape resembled the Ranger:

enter image description here

And here's a picture of the proposed Lockheed-Martin X-33, which was to be a 1/3 scale prototype:

enter image description here

Since your question was about where fuel is stored, page 8 of this report to Congress shows a cutaway of X-33, showing that its body is pretty much devoted entirely to fuel tanks; presumably that's just because it was intended as a prototype, and the larger VentureStar would have had room for human crew, but I'd imagine fuel would still take up the majority of the space onboard.

enter image description here

VentureStar would have been able to achieve orbit without boosters using an improved engine design known as the Aerospike, described on the wikipedia page:

The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes. It belongs to the class of altitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low altitudes, where most missions have the greatest need for thrust. Aerospike engines have been studied for a number of years and are the baseline engines for many single-stage-to-orbit (SSTO) designs and were also a strong contender for the Space Shuttle Main Engine.

A blog post on a visit to the Endurance prop here notes:

Other than its slightly fanciful shape, Ranger looks utterly authentic - as if it just landed. I can't really glean too much from the spacecraft's operation except to note that it seems to have something like aerospike engines.

Here is a page from another person who went to visit the prop which has a photo:

enter image description here

The two exhaust ports each have a central fin sticking out which looks similar to the "tapered nozzle ramp" on an aerospike (used to direct exhaust flow and increase efficiency) seen in the diagram here:

enter image description here

So I would say the combination of the similarity of the Ranger to the VentureStar and the fins seen in the prop make a good case that they were intended to be aerospike engines, though I'm not sure if this has been officially confirmed. The article here claims that "These ships, according to the producers, use an aerospike engine", but I can't really sure if the article's author actually heard this directly from producers or just saw aerospike engines mentioned on the Interstellar wiki page (or something similar) and assumed without checking that the information must have come from people involved in the film.

Also note that while the Ranger needs enough fuel to achieve escape velocity from planets, The Science of Interstellar by physicist Kip Thorne (who consulted on the movie) indicates that once away from planets, most of the maneuvering is supposed to have been done by gravitational slingshots around "intermediate-mass black holes" that orbit Gargantua (the supermassive black hole seen in the movie). See my answer here for details.

  • I was reluctant to mention aerospike engines since all of the available references to it seem to have been written by fans – Valorum Jun 8 '16 at 22:03
  • @Valorum - I added a paragraph on the case for them being aerospike engines after the last image, though I agree it's not absolutely certain I think it's a very strong likelihood that this was intended. – Hypnosifl Jun 8 '16 at 22:36
  • The answer has a lot of good facts and research, but a discussion of delta v required and the corresponding amount of fuel would improve it. – Stone True Jun 10 '16 at 3:37
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According to the film's official website, the Ranger craft carries its fuel internally, as evidenced by the panel showing the disengagement of the external fuel tank.

enter image description here

It also offers the following description of the ship's orbital capabilities. Apparently it uses a mixture of chemical rocket propellant and "plasma jets" within the atmosphere and "hydrazine thrusters" for extra-planetary flight.

This is a fast, agile reconnaissance vehicle with single-stage surface-to-orbit capability

The Ranger is driven by a combination of chemical rockets and plasma jets. Hydrazine thrusters are used for maneuvers in space.

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    No discussion of performance or fuel mass required is provided in this answer. The question is about how much fuel is needed, not what kind. – Stone True Jun 10 '16 at 3:33
  • @stonetrue - That is indeed true. And that's because no such information exists in the film. Now obviously I could have taken the approach you've used below and roughed out some numbers, but without knowing how efficient the craft's fictional engines are, those numbers would be largely worthless – Valorum Jun 10 '16 at 6:13
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    If this question were related to Star Wars or Star Trek, I would completely agree with you. However, the film's makers went to a lot of effort to keep the science and engineering realistic. The engines are not fully fictional (unlike, say, an oscillation overthruster) but actually have been well conceptualized and in some cases tested (see the links in various answers). I simply added a somewhat fictional (I admit it, but I believe it to be reasonable), performance extrapolation to see if they came close on the fuel requirements. – Stone True Jun 10 '16 at 11:47
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Both Endurance and the Rangers are reaction mass rockets, meaning they function by ejecting high speed propellant. Per Tsiolkovsky’s rocket equation (see link), the change in velocity, or delta-v, equals the exhaust velocity times the natural log of the rocket’s initial mass (i.e. with fuel for the rocket firing) divided by the final mass (i.e. after the fuel is expended). In order to determine how much fuel is required, we rearrange the equation to solve for ratio of initial mass to final mass, which is equal to raising e by the ratio of the delta-v divided by the exhaust velocity.

For a Ranger to lift off unassisted from an Earth-like planet to a low orbit, a delta-v of 9.4 km / second is required (see here). The exhaust velocity of the Aerospike engines is not provided, so let’s assume that it is 50% more as that proposed for Aerospike engines to allow for technological advancement, or about 600 seconds of specific impulse (see NASA link here). Specific impulse is related to exhaust velocity be simply multiplying be a standard gravity of 9.8 m/s, so the exhaust velocity is 5,900 m / s.

The ratio of initial mass to final mass is e raised to the power of (9,400 / 5,900 ) or 4.92. Therefore a Ranger must carry approximately 5 times its empty mass in fuel for a single lift off. The Ranger’s empty mass is not provided anywhere, but it appears to be smaller than a Space Shuttle. Let’s assume it is half the mass of a Space Shuttle, or 82,500 lbs, or 37,272 kg. Therefore, 161,360 kg, or 161.4 metric tons of fuel is required. The site Board the Endurance states that Rangers use chemical rockets. For ease of calculation, let’s assume hydrogen peroxide as the fuel. Liquid hydrogen peroxide has a density of 1.45 grams per cubic centimeter, so for a single launch, 111 cubic meters or 111,283 liters of hydrogen peroxide is required. This is equivalent to a cube 4.8 meters on a side. It would seem on visual inspection that a Ranger could carry enough propellant for one or two launches from an Earth like world. The propellant could be arranged in long tanks around the crew compartment, for example.

Endurance is much larger, but also requires less delta-v because it never has to descend into a gravity well. For Endurance’s trip from Earth orbit to Saturn, a delta-v of 7.3 km /s is required (see here). Presuming Endurance has advanced engines such as VASMIR or other advanced electric drives, then exhaust velocities of 50,000 m / sec are possible (see here). The ratio of initial to final mass for Endurance from Earth to Saturn is therefore 1.15, so Endurance only needs to carry 15% of its weight as fuel for this boost. Even if Endurance is assumed to weigh 10 times that of a Space Shuttle (this mass estimate is a complete SWAG), only 67,000 liters of hydrogen peroxide or a cube 4 meters on a side is required for the entire Earth to Saturn run.

Fuel for missions would therefore seem to be limited by that required for Ranger and Lander operations. If we assume 2 Rangers, each able to do 2 lift-offs each, that is about 500,000 liters of fuel, not counting whatever the Landers do. This seems like a lot of fuel for not many missions, but the mission planners probably intended to make fuel in the Gargantua system, a la Mars Direct.

Endurance making the equivalent of 5 Earth to Saturn runs is only 334,000 liters of fuel. Now we seem to be talking fuel tanks larger than seems reasonable as seen in the movie. We have over 800,000 liters of fuel to put somewhere. That would be a cube 9 meters on a side - it would seem that you could have this volume spread about Endurance. A denser fuel, such as liquid metal (mercury at 5.43 g / cubic ccm) or even depleted uranium (19.1 g / cubic cm) would require a corresponding less amount of space - perhaps vaporizing such a dense fuel is what the Board the Endurance site refers to when referencing "plasma" drives.

There does then seem to be an internally consistent in-universe explanation for the apparent lack of fuel tanks. It does require some extrapolation and assumptions as to making fuel in-system, but it seems to work.

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    The Board the Endurance link doesn't say they're exclusively rockets. It also mentions some kind of plasma-based engine. – Valorum Jun 10 '16 at 6:17
  • You are correct - "plasma" could refer to simply heating a reaction mass with an energy source (say a fusion reactor) and then expelling it from the back. In that case, the depleted uranium idea might work. Another option - the mission may have had the ability to MAKE fuel from local resources similar to Robert Zubrin's Mars Direct concepts. Basically take water, add energy, voila... rocket fuel! – Stone True Jun 10 '16 at 11:49
  • Why assume they need to carry enough fuel to do 5 lift-offs each? That seems like a lot, considering there were only 3 planets they got signals from; and as you say, it's quite possible they planned to make liquid oxygen and hydrogen fuel from planets with water on them (which is quite common in ice form in our solar system). – Hypnosifl Jun 10 '16 at 15:50

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