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I remember gravity in both the planets in Interstellar being comparable to that of Earth's. 80% of Earth gravity in case of Mann's planet and 130% in case of Miller's planet. So, why did it take a rocket to lift off the Earth surface and get to the Endurance orbit, but just Ranger shuttles to lift off from Miller's & Mann's planet?

I do see an explanation here

But if that is the case, why use a rocket to lift off from earth?

Napoleon Wilson
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rusk
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6 Answers6

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The same question was also asked on Quora, with various interesting answers, even if it's hard to say which one is correct and they all sound quite plausible. Everyone seems to agree though, that both Lander and Ranger are very much capable of planetary lift-offs and could as well have left earth on their own. Remains the question why they chose a rocket-aided lift-off for earth. The most-compelling in-universe answers are:

  • As Victor also says in his answer already, they might have to carry a lot more cargo and supplies for their initial trip to the Endurance in preparation of their long journey. For this the additional boost might be required.
  • They might want to save fuel, which is made clear to be a very valuable resource. So why not rather waste a whole rocket on earth, where they could easily built and setup one and save the fuel for the, likely fuel-intensive, lift-offs for the planets where it's definitely needed.

Yet there is also a very good out-of-universe answer mentioned, which I'd like the most (since I appreciate analysis more than fan-fiction, but that's just me ;-)), even if it's probably to be seen in connection to the other explanations:

  • The usage of a classical multi-stage rocket lift-off and its whole depiction in the movie pretty much evokes reminiscences of the good old space missions, especially the Apollo missions, we experienced in the past. And this is definitely intended, since it is that pioneering attitude of space exploration that Cooper complains to have gone astray in humanity's pessimistic society of "care-takers". And it is this hope and inspiration humanity took from those missions that the movie wants to recall with this depiction.

As a little addendum, after a fruitful discussion I indeed managed to get together a computation for the escape velocity on Miller's planet in an answer to a related question over on Physics.SE (similar to jld's computation in his answer here), giving that the planet has only about 96% of earth's escape velocity. So it is indeed a little bit easier to leave it than earth (and even more likely for Mann's planet, which following the computed constraints would have to have only more than 64% of earth's density to be easier to lift off). But I still stand to the above assumption, that they could also have left earth without a rocket, hadn't they had the additional cargo, fuel constraints and obligation to show us a good old rocket launch.

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Napoleon Wilson
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  • Whilst I agree the rocket lift off evokes memories of old, there's no reason to believe that's the only reason it was featured. For a movie packed with scientific accuracy, this seems to be a stretch. Whilst it was a fun scene, it was certainly a required one. There was no way the Ranger could transport them and all their cargo. – Andrew Martin Nov 23 '14 at 19:25
  • @AndrewMartin Sure, that's why I said in conjunction with the other answers...But ok, rephrased it. – Napoleon Wilson Nov 23 '14 at 19:28
  • @AndrewMartin If you undelete your answer and incorporate the results from your [physics.se] question into it (which indeed support your assumption that lift-off is easier), I'd freely undownvote it and remove those remarks from my answer (since, while interesting, I'd still deem them irrelevant for my answer anyway). Afterall it's your [physics.se] question and it shouldn't have been for nothing, you were right about lift-off being easier, just not about its exact relation to planetary density. – Napoleon Wilson Nov 24 '14 at 00:03
  • To be honest, I think @jid's answer covers everything my did, in much greater detail. So I think mine's probably better deleted. Don't worry about the downvote, I'll just go on a serial downvoting spree of your answers :) (that was a joke, I promise!) – Andrew Martin Nov 24 '14 at 00:04
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    Although gravity is a cardinal factor for determining the escape velocity, I feel the Aerodynamic Resistance (ADR) is also a significant factor. Earth's atmosphere could be assumed to be much more denser due to the presence of Ozone layer, the other planets may have much less dense atmosphere leading to much more less ADR. I am strictly contemplating without any foundation and I may be very much wrong. But just wanted to put forth this point and invite some constructive criticism. Feel free point out if I am wrong :) – Cool_Coder Nov 25 '14 at 03:10
  • @NapoleanWilson: Haven't been to quora in a while but I do like that out-of-universe answer. ;) – rusk Nov 25 '14 at 07:32
  • @NapoleanWilson: Although there is still one thing that's bugging me. As mentioned in the link given by me in the question, there are Lander shuttles as well which can carry heavy payloads. Couldn't those have been used instead of a rocket? – rusk Nov 25 '14 at 07:35
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    @rusk: I think the out of universe answer is logical - it's an epic scene, wonderful take off, etc. But it's also a heck of a lot more than just evoking old memories. There was no way they were leaving Earth on a Ranger, with all their supplies. It was just not going to happen. They needed the propulsion of the rockets. – Andrew Martin Nov 25 '14 at 09:39
  • @rusk Just that they (the Landers) were already up in space. Coop and the others were riding a Ranger from earth to the Endurance. So bad planning, maybe? But honestly, in this case I think you can still take the fuel conservation argument. It is not entirely explained and I guess all the possible explanations might work to some degree, even if not one alone for every possible side question one might have. – Napoleon Wilson Nov 25 '14 at 10:52
  • @Cool_Coder That indeed sounds reasonable (even though I'm by far no expert in physics and already had a hard, but fun, time getting that escape velocity together). But anyway, I already deem that escape velocity consideration not too relevant for this question. But still thanks for the added information, in fact given that we know to some degree how Mann's atmosphere looks, one might be able to reason about its ADR a bit more. – Napoleon Wilson Nov 25 '14 at 10:56
  • It's also worth pointing out that there have been real-world proposals for single-stage spaceplanes which make use of a scramjet engine to achieve very high speeds in the atmosphere before rocketing into orbit, see the Rockwell X-30 for a design that NASA was considering in the 1980s. And for getting a plane off the ground without a runway, there's various types of VTOL. – Hypnosifl Nov 26 '14 at 19:30
  • BTW, the wiki page says the Rockwell X-30 was expected to be able to use its scramjet in the atmosphere to achieve a speed of about 25 times the speed of sound, which would work out to about 8500 m/s. So such a craft would already be much closer to the needed escape velocity (which is 11200 m/s on Earth, and jld mentions it'd be around 9930 m/s on Miller's planet), with the extra kick achieved by firing rockets rather than using the scramjet. Since the scramjet works by taking in air and heating and expelling it at high speed, not as much fuel needs to be carried. – Hypnosifl Nov 27 '14 at 14:40
  • Considering they were in hypersleep/cryo stasis most of the trip, they wouldnt need enough supplies to justify the extra power/thrust of the rockets, which in turn have to compensate for their own weight. Since they only came up in a bare Ranger, which is shown to be very small, Victor's answer and your first point are moot. – cde Sep 28 '15 at 09:54
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This is what I imagined after watching the movie:

The shuttles could carry people and little (or no) cargo in and out of a planet's atmosphere, but a big rocket was needed to carry all the cargo needed for the long journey to Saturn and into the wormhole.

Victor Diaz
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  • I did think of this at first but then I saw explanation about Lander shuttles(as given in the link) which can carry heavy payloads and hence could have been used instead. – rusk Nov 25 '14 at 07:28
  • They were in hypersleep/cryostatus for most of the trip. Minimal supplies needed. They were not awake, eating, drinking, breathing, for 2 years. – cde Sep 28 '15 at 09:50
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Okay, so several people have mentioned that the planet's density will have an effect on its escape velocity. This is true. The total mass of the planet is: M=(4π/3)R³ρ, where ρ is its average density. The escape velocity squared is: v² = 2GM/R = (8πG/3)R²ρ = k²R²ρ, where I've defined k²=8πG/3. So we see that the equation for escape velocity, in terms of its radius and its density, is given by:

v = kR √(ρ)

The acceleration due to gravity is given by g = GM/R²= (4πG/3)Rρ = (k²/2)Rρ

Kip Thorne gives an estimate for Miller's planet's average density: ~10,000 kg/m³, compared with Earth's value of ~5,500 kg/m³. Additionally, we know Miller's planet has 1.2 times the acceleration due to gravity on Earth: g₂=1.2g₁.

R₂ = 1.2R₁ρ₁/ρ₂

This gives us a radius of ~4200 km. The escape velocity can be obtained by plugging this into our first equation, giving us a value of of ~9930 m/s. This is about 90% of Earth's escape velocity, so indeed it is easier to take off from Miller's planet than Earth, however not by much.

Jold
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  • So, now the same for Mann's planet. ;-) – Napoleon Wilson Nov 23 '14 at 22:43
  • @NapoleonWilson I don't have any information about Mann's size or density, so I can't do anything very rigorously, but it's a good bet that the escape velocity is smaller than both Earth's and Miller's. – Jold Nov 23 '14 at 22:50
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    @jld: Isn't earth's escape velocity 11.2 km/s? – rusk Nov 25 '14 at 07:44
  • @rusk Sorry for the typo, I meant 9930 m/s. I edited my answer to reflect this. – Jold Nov 25 '14 at 09:29
  • It's actually 1.3 instead of 1.2. Sorry for the confusion, but doesn't change too much in the relevance anyway, escape velocity is still lower. – Napoleon Wilson Dec 02 '14 at 01:15
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There are two types of ships used with Endurance: the Ranger and the slightly larger Lander. If you go to the Endurance model site, it is stated that the lander can carry down entire modules of Endurance. So they could be used to bring up any amount of supplies from Earth. http://www.gannett-cdn.com/experiments/usatoday/2014/11/interstellar/interstellar.html

So the fuel/limited capacities explanations cannot be correct,unless....

All three ships Ranger, Lander and Endurance use some form of very powerful plasma thruster, that needs to be fed by a Tokomak fusion reactor. The fusion must be one of the aneutronic reactions (no neutrons), or else the ships would melt or require gigantic radiators, since neutrons are waste heat in fusion reactions. He3+Lithium is a good candidate for aneutronic fusion. But Helium3 is excessively rare and extremely expensive to produce. So that might be the answer. NASA can't afford to waste helium3 for routine work, so they use antiquated chemical rockets to launch from Earth, where chemicals are very cheap.

A spreadsheet with the numbers: http://bit.ly/1vlpaVD

Napoleon Wilson
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Michel Lamontagne
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Actually, the rocket launched TWO Rangers AND the docking sub-assembly between them while they are attached to the central hub of Endurance. During the first docking sequence, you can see the sub-assembly and the second Ranger attached to the bottom of the Ranger Cooper is flying.

So it's very likely that they couldn't lift all this with just a single flying Ranger and needed the rockets for that amount of cargo.

Napoleon Wilson
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Jano Xavier
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  • I tried to add a little conclusion (which I suppose you were actually after) to make the answer adress the actual question more. If that does not reflect your original intention, then feel free to revert it. But in that case you still might want to adress the actual question a bit more in the answer. – Napoleon Wilson Apr 14 '15 at 11:43
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Best answer I've seen is here: It reframes the movie as a conspiracy by the Brands (and a few others like Mann) that is unwittingly foiled by Cooper. Brand has already developed his anti-gravity theory and equipped the shuttles with anti-gravity drives by the time Cooper leaves on the mission.

When they take off from Earth, they use a big rocket to reach orbit. Miller's planet has 1.1 times Earth's gravity, and they don't need any rockets at all, the shuttle is taking off on its own. The same happens on Mann's planet twice. That's only possible if they already have the anti-gravity drives installed and working.

Plan A was never an alternative, probably because if humans didn't have food on Earth they wouldn't have any in space either. And on top of that, all crops were infected on Earth with the Blight, so Brand wanted a fresh start.

feetwet
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