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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?

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    $\begingroup$ 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). $\endgroup$
    – Charles Staats
    Jun 9, 2021 at 11:54
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    $\begingroup$ 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. $\endgroup$
    – Charles Staats
    Jun 9, 2021 at 12:02
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    $\begingroup$ @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. $\endgroup$
    – Moo
    Jun 9, 2021 at 23:26
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    $\begingroup$ 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. $\endgroup$
    – J...
    Jun 10, 2021 at 14:23
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    $\begingroup$ Peter, you should play Kerbal Space Program - you can test out your ideas and see how they work :) $\endgroup$
    – Aaron F
    Jun 11, 2021 at 10:11

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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.

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  • $\begingroup$ 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. $\endgroup$
    – Peter - Reinstate Monica
    Jun 9, 2021 at 10:44
  • $\begingroup$ -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. $\endgroup$
    – Charles Staats
    Jun 9, 2021 at 13:43
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    $\begingroup$ 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. $\endgroup$
    – Slarty
    Jun 9, 2021 at 17:07
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    $\begingroup$ > 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. $\endgroup$
    – Tejas Kale
    Jun 9, 2021 at 19:44
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    $\begingroup$ @Tejas Kale yes I think we are in agreement. $\endgroup$
    – Slarty
    Jun 10, 2021 at 15:30
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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.

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  • $\begingroup$ That is an interesting concept, I was not aware of it. $\endgroup$
    – Peter - Reinstate Monica
    Jun 10, 2021 at 8:14
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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.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    Jun 14, 2021 at 14:02
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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).

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    $\begingroup$ 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. $\endgroup$
    – armand
    Jun 10, 2021 at 9:40
  • $\begingroup$ @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) $\endgroup$
    – BrendanLuke15
    Jun 10, 2021 at 11:21
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    $\begingroup$ Yeah, the point is being in the same plane, not in different planes with the same inclination. $\endgroup$
    – armand
    Jun 10, 2021 at 14:04
  • $\begingroup$ 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? $\endgroup$
    – jwdonahue
    Jun 11, 2021 at 20:34
  • $\begingroup$ @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. $\endgroup$
    – armand
    Jun 12, 2021 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.

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  • $\begingroup$ 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? $\endgroup$
    – Ng Ph
    Jun 13, 2021 at 16:55

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