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Inspired by another question asking why we can't use faster rockets to Mars I came up with a naive question of my own ;-).

The answer to the first question is that the pesky rocket equation makes faster rockets which need longer burns and hence more fuel prohibitively large and expensive.

One solution would be to send fuel tanks on the way earlier, then rendezvous with them at successive points en route and refuel mid-flight. Obviously, you cannot send that fuel very fast because, you know. This unfortunately implies a large delta-v latest at the second rendezvous point unless somebody comes up with something really smart, like slingshot maneuvers for the fuel or some other method to align the trajectories better. This would accelerate the actual payload flight even if the preparation might take years.

Has such a possibility been explored? Anything wild? Like, dunno, little fuel bubbles that will be slurped up, delta-v be damned?

Kyle Delaney
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Peter - Reinstate Monica
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    Seems like a better approach would be to send a caravan of tankers. The entire caravan starts a burn, stops at the same time (when they all still have some fuel left), and then some of the tankers refuel the the others and are discarded. Then the remaining parts of the caravan continue the burn. Rinse and repeat. This is basically staging, but with different difficult parts (rather than building one ginormous rocket, you have to built a fleet of merely huge ones and solve in-space refueling). – Charles Staats Jun 09 '21 at 11:54
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    But it is true that you can often achieve the same result with less fuel given additional time, so your original idea does have merit. I think the easiest approach might be to have your tankers lower their perihelion, then burn at perihelion to take advantage of the Oberth effect. The advantage is that unlike a slingshot, it's not dependent on the position of the planets. But you'd need to do a bunch more simulation to figure out if that approach can actually be used to less expensively rendezvous with a fast earth->mars transfer. – Charles Staats Jun 09 '21 at 12:02
  • @CharlesStaats Yes, "staging separately". May indeed be easier to build but harder to execute. Advantage: No single point of failure if you have some redundancy in the chain. – Peter - Reinstate Monica Jun 09 '21 at 12:28
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    @CharlesStaats sounds very similar to the approach the RAF took in Operation Black Buck in the 1980s. One bomber, 14 tankers refuelling both the bomber and each other to get the bomber on target. – Moo Jun 09 '21 at 23:26
  • @Moo With the added difficulty of an ever accelerating bomber. – Peter - Reinstate Monica Jun 10 '21 at 00:57
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    Everything in space moves. Space is not a highway. You can't just put a tanker "halfway" and leave it there for you to meet up with later. – J... Jun 10 '21 at 14:23
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    @Peter-ReinstateMonica - but without the pesky need to be able to get back to base – MikeB Jun 10 '21 at 14:41
  • @J...: Though if you consider Mars as halfway, it would make sense to put fuel & other consumables in orbit and on the surface, before you even launch a manned mission. – jamesqf Jun 10 '21 at 17:04
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    @jamesqf But... fuel at Mars doesn't help you get to Mars. So we can't consider it to be "halfway". – J... Jun 10 '21 at 17:08
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    Peter, you should play Kerbal Space Program - you can test out your ideas and see how they work :) – Aaron F Jun 11 '21 at 10:11
  • @J...: That depends on your point of view. If you're just sending a robot lander, one-way is fine. Not so fine if you're doing a sample return, even less so if you are sending (non-suicidal) humans. Having the fuel sent separately means you can build smaller ships (but more of them), thus economies of scale... – jamesqf Jun 11 '21 at 15:22
  • @jamesqf But the whole point of this question is to hack a method to get to Mars faster... – J... Jun 11 '21 at 19:15
  • @J... not carrying the return fuel would mean the outbound trip is faster, because theres less mass for the outbound fuel to accelerate and decelerate at the other end, so more velocity can be imparted. – Moo Jun 12 '21 at 11:49
  • @Moo Fair, but it's not a huge difference. An extra 50% in the delta-V budget can save you maybe up to six weeks off of a nine month trip, or a 17% reduction in travel time. To make any serious gains needs factors of improvement, not fractions. – J... Jun 12 '21 at 12:10
  • @J..: But getting to Mars faster is only relevant if you have humans on board. Machines can wait around without problems. Since any non-suicidal human will want to return, a return journey is part of the plan. So you send return fuel, consumables for the stay, and other equipment on slow, low-energy orbits. Once they're in place, then you can send the humans (and the much smaller mass of supplies needed en route) on a faster orbit. – jamesqf Jun 12 '21 at 15:58
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    @jamesqf Yes, I understand - my point is that it doesn't save you a lot of time for all that extra work. A whole second mission to turn a 9 month journey into a 7.5 month journey - to shave down the trip to Mars to something more like weeks instead of months would need orders of magnitude more delta-V, not just a minor, incremental efficiency gain. – J... Jun 12 '21 at 18:52

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Unfortunately sending propellant ahead to pick up on route will not help. The same amount of propellant will be needed regardless if both are accelerated together or separately. Also if launched separately it adds to the complexity of the mission as a docking will be required.

Any type of docking requires both ship and propellant store to be traveling on the same trajectory and at the same speed. It is not possible to “grab” some propellant as you go past as it would lead to disaster considering the speeds involved. Even if it were possible the act of grabbing the propellant would accelerate the propellant but decelerate the ship proportionately according to the laws of motion which are as unforgiving as the rocket equation.

Edit: Gravitational assist might help in the sense that propellant could be sent via gravitational assist on a much longer but more energetically favourable trajectory. But ultimately the whole ship could have been sent on the same trajectory anyway saving even more propellant.

The benefit would be in limiting any crew exposure to zero gravity and radiation. There would be no energy advantage in sending the ship and extra propellants separately if both use gravity assist.

Slarty
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  • Well, as I tried to outline, the fuel would not originally be accelerated as much, just because. (Because of what you and I said about fast travel.) So I wondered whether one could do some clever slingshot trick or perhaps something I couldn't think of to align the trajectories. – Peter - Reinstate Monica Jun 09 '21 at 10:44
  • -1 because the OP understood this and was suggesting a more subtle approach -- launching the propellant tankers years ahead of time, and using gravity assists and other time-consuming but less-expensive maneuvers to get them into a good trajectory to rendezvous with the crewed mission. – Charles Staats Jun 09 '21 at 13:43
  • @Charles Staats Well perhaps you have a point, I have edited my answer. Ultimately there is no energy advantage in sending crew and propellants separately if they both travel on the same trajectory, whatever it is. If different trajectories are used then could be sent on a long time consuming but energy efficient orbit whilst the crewed ship could travel via a more direct route – Slarty Jun 09 '21 at 16:59
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    One problem would be matching trajectories in deep space. Not impossible but would probably require some exact timing. Another possibility (for example) would be to send propellant on a long slow low energy trajectory to Mars when the transfer window was particularly favorable and send the crew via a higher speed trajectory. Both crew and extra propellant would need to aerobrake into Mars orbit and dock there. – Slarty Jun 09 '21 at 17:07
  • @Slarty Matching both orbits by aerobraking from very different trajectories will be very difficult. To save fuel, the planes of both orbits should be identical. Aerobraking may not be used to align the planes. Changing an orbital plane requires fuel, not aerobraking. – Uwe Jun 09 '21 at 18:03
  • @Uwe yes it might be very difficult, but it is definitely possible. It would also be possible to send fuel via a hohmann orbit whilst sending the crew via a much higher energy orbit in the same plane. – Slarty Jun 09 '21 at 18:55
  • Downvote removed in response to edit. – Charles Staats Jun 09 '21 at 19:18
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    But ultimately the whole ship could have been sent on the same trajectory anyway saving even more propellant.

    Humans go faster, with some fuel waiting for them at Mars for the return journey. This fuel could have taken orders of magnitude more time to arrive there.

     – Tejas Kale Jun 09 '21 at 19:44
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    @Tejas Kale yes I think we are in agreement. – Slarty Jun 10 '21 at 15:30
  • @Slarty Where does "… sending propellant ahead… will not help…" come from? Isn't the very wording "the same amount of propellant will be needed regardless if both are accelerated together or separately", the real clue?

    If the propellant is already on board, that propellant itself has to be accelerated. If it's not, it doesn't. How could you change that?

     – Robbie Goodwin Jun 12 '21 at 23:01
  • @Robbie Goodwin If the propellant is already on board, that propellant itself has to be accelerated. If it's not, it doesn't. How could you change that? You can't. May point is that on a given trajectory it doesn't matter if you split up propellant deliveries or send them all at once it amounts to the same thing in energy terms. – Slarty Jun 14 '21 at 20:24
  • @Slarty That doesn't mean launching a rocket with all the fuel needed to deliver the largest cargo in the shortest time is the best way to go.

    You might load your sledges with everything needed for a polar expedition but where will you find such juggernauts, or the dogs to drag them?

    You might want longer-range replacements for the Vulcan bombers that flew 8,000 miles to the Falkland Islands, and where will you get those behemoths?

    Total energy does not equal practical efficiency.

     – Robbie Goodwin Jun 15 '21 at 21:47
  • @Robbie Goodwin There is an optimum size for a rocket depending on the intended use and in the case of a crewed Mars landing the optimal size is a behemoth like the SpaceX Starship. That will require re-tanking in orbit and will arrive empty and require re-tanking on Mars from locally produced Methalox. Like successful polar explorers, you minimize what you take with you and live off the land as far as possible. – Slarty Jun 16 '21 at 12:42
  • @Slarty "Intended use" is far too vague a term to guide optimization. Of what? Fuel? Life support resources? Time?Even orbital re-tanking - is that the same as re-fuelling? - is distinguished from stashing fuel-dumps along the route mostly by the size of the gravity wells at each end.

    I thought all polar explorers used stashes. They can only minimize so much and living off the land sounds like a hope, not a plan.

    How is SpaceX Starship a behemoth? Isn't it scarcely twice the size of the old Shuttles?

     – Robbie Goodwin Jun 16 '21 at 20:02
  • @Robbie Goodwin Intended use in the case of SpaceX is the human exploration of Mars and the optimisation is for cost/tonne to orbit. That is what Starship is optimized for. Note re-tanking means the replenishment of fuel and oxidizer. Check out Roald Amundsen v Sir John Franklin and the search for the Northwest Passage – Amundsen lived off the land and survived, Franklin took mountains of supplies with him and didn’t… – Slarty Jun 16 '21 at 22:14
  • @Robbie Goodwin The tyranny of the rocket equation means any practical plan to send human explorers to Mars more or less has to “live off the land” by manufacturing propellants on Mars. Even NASA accepts this these days. Starship is planned to be the tallest and heaviest rocket ever launched with twice the thrust of Saturn V. I would call that a behemoth, but you may think otherwise (behemoth is an inexact term). – Slarty Jun 16 '21 at 22:15
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If you think less about fuel and more about the other amenities for a long distance mission, you are awfully close to the concept of a cycler.

The basic idea is that you take a big vessel, on which astronauts can live for a long time and accelerate it on a trajectory that regularly passes by the places you want to travel in between. You do this without a crew or anything, so maybe you are able to use gravity assists, slow ion engines and other tricks which take a long time to get you onto this trajectory but save fuel. If you pick the right orbit, you can also reuse the ship. You then pack the actual crew and the payload into a tiny shuttle which rendevous with the cycler as it passes by and similarly drop them of at the destination the same way.

As stated by the other answers, you still need to use the same amount of fuel for the actual mass you are transporting to your destination and there is no way around it. But you save on fuel for accelerating the mass that is only needed during transit.

mlk
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The real problem is that, in space travel, your speed determines your trajectory, and therefore two objects can't follow the same trajectory at different velocities. Precisely, a faster interplanetary trajectory will give you a more elliptic, elongated orbit around the sun, while a slower one will be more round (and I don't even consider the case of gravity assists, which makes the trajectory even more convoluted).

To summarize, if you send the fuel slow and the crew fast, their respective trajectories won't have the same shape. And if you send them on the same path, they must have the same speed, so the crew will never catch up with the fuel.

You could theoretically time the launches so that the trajectories intersect (a big challenge in interplanetary space already...), but at this time the crew and the fuel won't have the same speed nor move in the same direction. In order for them to rendez vous and dock, you need one of them to match the other's velocity and direction. Presumably, have the fuel match the crew's velocity.

But now, if they are in the same place at the same speed, that mean they are on the same trajectory. So you might as well have launched the fuel at the same speed than the crew in the first place, because all we did is only waste a lot of fuel launching in the slow trajectory and then correcting to end up on the fast one.

Consider also the risk that the crew and fuel miss their rendez vous because of some technical glitch. Having two vessels matching position at a few metres from another among billions of cubic kilometers of interplanetary void is no trivial task, and if anything goes wrong the crew is stranded with no fuel, and dead.

With this in mind it's much more sensible to have crew and fuel rendez vous in earth orbit, where the mission can always be cancelled in case of trouble, and send all at once on the fast trajectory, where your fuel will have to end up anyway.

armand
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Rather than having fuel tanks you meet on the way the better way would be preposition fuel tankers at the start and end of the trip i.e Earth and Mars orbit. You put fuel tankers in Earth's orbit as its the easiest place to refuel (in this scenario) and means you can make use of small rocket launches (crew, ship, and fuel separately) instead of needing one giant rocket to launch crew+ship+fuel together. You then stage a tanker at Mars so your slow down burn (at Mars) can use as much fuel as possible. Then you refuel your spaceship for the trip back. The tanker to Mars can be sent on some slow orbit that does not need to be that fast, to conserve its fuel. Effectively you optimize to make the crewed trip faster, but make the overall mission (tanker trip plus crew trip slower).

Rob
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    One could even land with hydrogen, produce some methane on Mars and store it in orbit. The main difficulty would be to have the incoming crew vessel match the orbital inclination of the fuel at Mars. Launch windows are constrained and it might not be possible to approach Mars in the correct plane. And inclination changes are very expensive. But it clearly makes more sense to catch up with the fuel around Mars than in deep space. Earth orbit > Mars Orbit >> deep space. – armand Jun 10 '21 at 09:40
  • @armand inclination is easy to match for an incoming hyperbolic trajectory, it is the other orbital elements that are difficult, longitude of the ascending node and argument of periapsis* (*for an elliptical orbit at Mars) – BrendanLuke15 Jun 10 '21 at 11:21
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    Yeah, the point is being in the same plane, not in different planes with the same inclination. – armand Jun 10 '21 at 14:04
  • Couldn't you save even more on fuel, by boosting it to an appropriate Langrange point (could take years or months), rendezvous crew and fuel there, then the boost to transit orbit would take a lot less fuel? – jwdonahue Jun 11 '21 at 20:34
  • @jwdonahue L4 and L5 are the stable Lagrange points, and they are as far from us than Mars (60 degrees in front and behind Mars on the same orbit). So getting there takes just as much fuel and time than going to Mars. And then you're not even at destination. – armand Jun 12 '21 at 11:53
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As in the other answers, your ideas won't save fuel and time, but they may in the future vastly improve likelihood of mission success and crew safety, at the cost of more fuel and time.

An unpeopled lander, or a sequence of unpeopled landers could, for instance, establish a fuel depot on Mars to allow for a lighter craft to land, and then refuel with the propellant needed to escape Mars gravity. This sequence of missions would be highly expensive, but it doesn't need to be foolproof, since one can take higher risks with each drop and simply repeat the mission if one fuel drop fails.

Similar benefits could result simply from refueling in low Earth orbit, or assembling a larger spacecraft piecemeal in low Earth orbit to save on the heavy lifting through the atmosphere.

These options are all massively inefficient and costly, but it's very possible they could be part of future Mars missions, precisely because they improve likelihood of mission success without risking life.

As for what's actually planned, ideas along these lines (i.e. piecemeal deployment of hardware into low Earth orbit before actual crew joins the mission) can be seen in the ideas of the Deep Space Habitat, which would be deployed in LEO before the crew joins it in the Orion spacecraft atop the SLS launch vehicle.

Selene Routley
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  • That's my feeling too (cf discussions with User "armand"). Another possible "use case" is to have more launch opportunities to Mars, breaking the 26-month wait for a favorable alignment or Earth-Mars. Basically, if you just miss the window for direct-to-Mars, you head to one of the depots. There you re-supply and go to final destination. Is there any obvious flaw in the reasoning? – Ng Ph Jun 13 '21 at 16:55