Would it be better to build a Mars-fairing ship on land or in space?

Question

We do not have a human-rated rocket which can send a sizable payload to Mars. The Falcon Heavy is certainly capable of sending a payload there, but the rocket would need to be human-rated, which basically means tested far more extensively than it has been now.

And that got me thinking - for a vehicle to launch from the surface of the Earth it needs to be pretty substantial. But you can avoid a lot of the requirements of the stresses of launch by becoming a payload (e.g. satellites wouldn't survive a launch if they weren't inside of a rocket. And if it were economical to build satellites that could be self-launching, they'd already exist, right?).

So would it be better to just build a ship in pieces and just send it up on a (couple) of Falcon Heavy Rocket(s)?

I think you mean "Mars faring" rather than "Mars-fairing," but I'm unsure since you seem to be referencing fairings indirectly in your question body ('wouldn't survive launch if they weren't inside of a rocket."). Also, asking if something is better begs the question, "better than what?" That Falcon Heavy is not man-rated is an issue; how does launching a ship in pieces on the non-man-rated Falcon heavy deal with that issue? Do you mean to build a ship in orbit (remotely, I guess), and then use a man-rated vehicle to get the crew up to the ship? — Wayne Conrad, Jun 22 '18 at 20:25
Or send more astronauts up to the ISS or something aboard a Soyuz - basically, is it cheaper/easier/better to ship a ship up designed solely for interplanetary flight, that perhaps can tow a lander module, than it is to build a ship that's designed for take off from Earth, landing, re-entry, and take-off from Mars, and finally re-entry and landing on Earth. — Wayne Werner, Jun 22 '18 at 20:45
The thoughts you are having are many of the same thoughts that went through the minds of the designers of the Apollo mission. Single ship to the moon and back? Assemble a ship in orbit? All of these ideas were considered before LOR (Lunar Orbit Rendevous) was settled on. — Wayne Conrad, Jun 22 '18 at 21:24
This is exactly the method considered by Reactions Engines' "Project Troy" - a mission plan to get people to Mars and back using their Skylon launch vehicle concept. — , Jun 24 '18 at 15:35

score 4 · Answer 1 · answered Jun 23 '18 at 19:51

Better by what criteria? It's a tradeoff. Assembly from parts makes less efficient use of mass, due to the need for mating structures and a larger area of hull wrapped around smaller bits of volume, and greater complexity due to the need to design all the separate parts and assemble them in orbit.

A modular spacecraft is also unlikely to be able to withstand reentry and landing on Mars. This means that it will need enough propulsion to brake into Mars orbit on its own power, and then send a separate lander/return vehicle, which itself will have to operate as an independent spacecraft. SpaceX's BFR instead uses a direct entry and lands the entire spacecraft after using the atmosphere to remove almost all of the kinetic energy of the spacecraft: http://www.spacex.com/sites/spacex/files/making_life_multiplanetary-2017.pdf

The modular approach is inefficient and complex, but it can be done with existing launch vehicles. SpaceX initially proposed the even larger ITS, but could not fund the development of a system that would find little use outside of Mars missions. BFR is that system scaled down to be useful as a commercial launcher on Earth.

score 0 · Answer 2 · answered Jun 23 '18 at 20:10

As several people have said there are lots of options and lots of trade-offs. To give you an idea, here is SpaceX's presentation of their ideas for Mars. To summarise.

BFR takes off from Earth and leaves BFS (upper stage) in orbit
Repeated (maybe 6) BFR launches take tanker-loads of fuel to the BFS and partly refill its tanks (it can hold about 1000 tons of propellant, but uses about 900 of them to get to orbit).
At some stage the crew go aboard. More launches maybe necessary to carry supplies for the crew.
Using the fuel that was carried aboard, the BFS flies to Mars, enters the atmosphere, and lands, Taking nine months or so.

After this there are two ways it could pan out:

5a The BFS is refueled with propellant made on Mars and stored there by earlier robot (or manned missions). It can take off, fly back to Earth, reenter and land. It might be wise to transfer the crew to a nice new BFS before reentry, in case theirs has taken damage on Mars or in space

5b A much smaller launch vehicle (carried to Mars as cargo) launches the crew to Mars orbit, where the rendezvous with yet another BFS that has been waiting, to take them back to Earth

5b leaves a ship stranded on Mars, but doesn't need propellant manufacture there. It also means you have much less cargo space, so your mission can do much less.

This isn't the only approach, but it one. Empty spacecraft are fairly light, so launching the spaceship in one piece and then launching fuel and cargo afterwards is quite appealing if you have enough launchers and the ability to send them up frequently

So the spaceship is brought to orbit in one piece. There is much more fuel mass than the dry mass of the space ship. But refilling the tanks should be done in zero gravity. — Uwe, Jun 23 '18 at 20:29
The transit times being discussed by SpaceX are much shorter, using high energy trajectories that take around 3-4 months. The BFS is also incapable of entering orbit around either Earth or Mars from the transit trajectory, so there is no transfer to a new BFS, and no launch with a smaller spacecraft to rendezvous in orbit. Refueling the BFS and flying it back is the only way the system works. — Christopher James Huff, Jun 23 '18 at 21:49

Would it be better to build a Mars-fairing ship on land or in space?

2 Answers2