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In the novel "The Mars Project" by Wernher von Braun humans fly in convoy to Mars. Is it being considered seriously at all today, to send several spacecrafts together?

Most proposed missions to Mars require pre-deployment of some payloads on Mars before humans arrive. If instead sent together with the crewed spacecraft, I can imagine some potential benefits. In an emergency the crew might evacuate to the surface habitat in flight. They could sacrifice one of the spacecrafts if they have better need for the fuel meant to land it. At Mars, they could teleoperate the landing of the spacecrafts, or maybe land in formation to make sure they all land within reach for the crew.

Drawbacks might be the risk of collision, difficulties of docking spacecrafts to transfer fuel, crew or cargo. And it doesn't seem possible to reliably make several simultaneous launches from the ground (although the military has that ability with solid launchers) so the convoy would have to get together in orbit.

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    $\begingroup$ In The Case For Mars Robert Zubrin made the point that by launching cargo missions to Mars, that will be for a crew arriving 2 years later, it can be confirmed that the cargo landed safely before risking the crew. That way you eliminate one of the biggest risks before the crew even leaves. $\endgroup$
    – kim holder
    Jun 6, 2015 at 15:22

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That's an interesting idea. There are several considerations to bear in mind here:

  1. Ability to launch several rockets into the same plane in quick succession. Very expensive: you have to double everything and more than double quality control since you are working with tighter margins and in the atmosphere of let's say undue haste. You'll have to pay severance packages after the surge which strains the budget.

  2. Trans-Martian orbit Insertion accuracy: a second's worth of delay would mean transit modules won't be anywhere near each other at engine cut-off. Travel between the modules won't be easy.

  3. Ballistics nightmare. Instead of aiming solely for the entry point, all midcourse corrections will have to keep the modules together along the route.

  4. Entry, Descent and Landing difficulties - as you said, collision is a real possibility if you want to land the modules within a certain distance (20-30 meters) from each other.

  5. Lack of any clear requirement to keep modules together. Transit is the least risky phase (galactic cosmic rays and solar flares notwithstanding), and life support systems are more efficient with larger crews. If one module fails in flight, there's no spare capacity in other modules. You definitely want to keep all crewmembers in one craft. You also don't want to hold cocktail parties for guests from other modules.

  6. Pre-positioned cargos can be launched on more efficient trajectories, lowering total mission cost.

TL;DR - formation transit raises costs and doesn't lower risks.

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  • $\begingroup$ One might speculate that multiple crew vehicles in formation would offer some measure of fail-safe. Let's say 3 vehicles each with 50% surplus crew capacity, so if one vehicle suffers some sort of catastrophic event, its crew could be evacuated to the remaining vehicles. Of course this raises questions about what sort of events might befall a vehicle to require an evacuation while allowing it to be accomplished successfully, and whether the likelihood of such an event would justify the added cost and complexity of building a fleet of vehicles and launching them into a formation. $\endgroup$
    – Anthony X
    Jun 6, 2015 at 22:02
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    $\begingroup$ Why on Earth would a sensible person think of launching a Mars mission directly from Earth surface? Instead assemble the spacecraft in orbit, and have them undergo a shakedown/checkout phase. As for having multiple modules, it seems obvious that a major benefit would be in having two or more modules rotating around each other at the end of cables (perhaps with a central service core) in order to provide artifical gravity. $\endgroup$
    – jamesqf
    Jun 8, 2015 at 18:55
  • $\begingroup$ @jamesqf - if the modules are tethered they are no longer flying in formation/convoy. In-orbit assembly leads to propellants' boiloff and increased mission risk should one launch fail. $\endgroup$
    – Deer Hunter
    Jun 8, 2015 at 19:00
  • $\begingroup$ @Deer Hunter: I suppose whether tethered modules are a convoy is a matter of definition. However, it does seem the only way to provide necessary artificial gravity for a long-duration mission. In-orbit assembly in fact decreases launch risk: if the launch of one module fails, just launch another. As for propellant boil-off, if that problem is not solved, then (unless you have Mars-based propellant extraction plants) you are going to have real problems getting your mission back to Earth. $\endgroup$
    – jamesqf
    Jun 9, 2015 at 0:53
  • $\begingroup$ @Anthony X Your comment reminds me of the parallel with reliability analysis. i) Two items, both of which has to work = lower system reliability than one item because of the increased chance of failure. ii) Two items, either one of which has to work = increased reliability. iii) as (ii) but with a switch to activate the second item = not as good as ii). Thus the rescue is a bit like the switch in iii) and has a reliability figure which may (or may not, lets be positive) offset the gain. $\endgroup$
    – Puffin
    Nov 4, 2015 at 18:02
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Space "convoys" will, in my fondest dreams, one day be extraordinarily common. The reason has very little to do with the traditional advantages of traveling together, i.e. Companionship, redundancy, etc. Docking spacecraft is hard enough to make that impractical. Instead you will see the convoys as a natural consequence of orbital mechanics.

To get from Earth to Mars or any other planet, you will probably use a Hohmann Transfer orbit.

These are highly elliptical orbits that stretch way out so that your aphelion reaches the target planet's orbit. Missing something as large as a planet is very embarrassing, so you plan a Hohmann transfer so that the planet arrives near your aphelion at the same time as the spacecraft. The lowest delta-V/thrust/fuel Hohmann transfers occur on a repeating "transfer window" cycle based on the orbits of the source and destination planets.

For Mars, the low delta-v transfer window occurs every two years. For Venus, it's every 15 months*.

Alex Moon's Launch Window Planner is a launch window calculator for Kerbal Space Program that illustrates this nicely. Getting from Kerbin to Duna can take as little as 1.7 km/s of delta-v or as more than 10 times that for the same travel time, all depending on when you do your trans-Duna-injection burn.

If we become a truly space faring civilization I expect you will see many craft that launch to LEO in the weeks before a colony transfer window. They'll shake down each craft, testing all the knobs and widgets before their long voyage. (Hopefully they'll fuel up from an orbiting depot too.) Finally, as the magic time draws close, you'll see them each light off one-by-one and slowly disappear from your window/viewscreen/telescope.

  • Check the value for Venus before swearing by it.
  • All of this is moot if EmDrive technology can scale up by a few orders of magnitude.
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  • $\begingroup$ Might be worth noting that it kinda already happened with ISRO's Mars Orbiter Mission (MOM) and NASA's MAVEN, with the latter overtaking the former on the way to Mars and both orbiters inserted at Mars less than 2 days apart of each other. $\endgroup$
    – TildalWave
    Nov 8, 2015 at 21:33
  • $\begingroup$ @TidalWave The $64,000 question is "With all the talk about a robot uprising, is it really safe to let them have whole planet to themselves?" $\endgroup$
    – ElizabethGreene
    Nov 9, 2015 at 23:17
  • $\begingroup$ Where can I collect the check if I answer that? ;D $\endgroup$
    – TildalWave
    Nov 9, 2015 at 23:42
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    $\begingroup$ +1 (also) for using KSP to make your point. Orbital mechanics is now accessible to gamers, thanks to that exceptional game. $\endgroup$
    – Ricardo
    Sep 30, 2016 at 14:12
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Travelling through space in a string of vehicles relatively close together, in the same trajectory, is expensive and unnecessary, as Deer Hunter points out.

But travelling to Mars, say, in a series of ships that are spread apart, maybe very widely, might make sense. One vehicle could carry people, and the others could carry supplies or fuel. They could be launched from several different pads around the world, which could be one way to spread out the cost. It might also let us spread out the cost over time. It might allow us to use existing rocket boosters like Delta or Atlas instead of having to develop new, more powerful ones (like the planned SLS.) The supply craft could be launched months or even years in advance, and take slow, cheap routes, while the manned ship might take the fastest trip possible, to minimize the crew's exposure to radiation. I believe that is actually one scenario for a manned trip to Mars. Robots might arrive first to build the habitat, probably underground, so that people could arrive and move in quickly. Robots would also start producing fuel, water, oxygen, maybe even food, so that it doesn't all have to be carried there from Earth. (On the other hand, why send people at all?)

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    $\begingroup$ Another advantage of sending supplies first is that one may be able to confirm the success of the supply mission before sending the crew. The Antarctic explorer Ernest Shackleton planned a trans-Antarctic mission which was supposed to take advantage of supplies left by another team, but the other team failed its mission; fortunately for Shackleton and his crew, their ship got stuck in the ice and destroyed before they could set out across Antarctica; had that not occurred, they would have likely found themselves in the middle of the continent, in desparate need of non-existent food... $\endgroup$
    – supercat
    Jul 24, 2015 at 15:50
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    $\begingroup$ ...to replace their own exhausted supplies. To be sure, losing ones ship outside Antarctica would hardly seem to qualify as "good fortune", but despite that misadventure all of Shackleton's men did make it home safely. Of course, if they'd know the supply mission was going to fail Shackleton's men could have saved themselves a lot of trouble. $\endgroup$
    – supercat
    Jul 24, 2015 at 15:53
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Interplanetary launch windows

The efficiency of interplanetary trips - the fuel and time required - varies significantly in time due to orbital mechanics. This means that if we assume some far future with significant traffic between, say, Earth and Mars, then we would expect to see most of ships in one direction being launched in the same few days where the trip is efficient, and then a pause for many months with trips made only if absolutely neccessary, as they require far more fuel and thus a different ship.

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    $\begingroup$ And still, starting a day apart means distance on the order of a million kilometers. Not exactly a tight formation, and no way to move between the vehicles. $\endgroup$
    – Deer Hunter
    Nov 4, 2015 at 11:09
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    $\begingroup$ This scenario is a major plot point in the Heinlein novel "The Rolling Stones". $\endgroup$
    – Organic Marble
    Nov 4, 2015 at 12:27
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One additional risk from a convoy: If you do suffer a nasty strike a convoy means debris would be more likely to hit another ship.

On the flip side, a convoy means you have a better chance of rescuing someone from a damaged spacecraft.

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