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John D. Clark's Ignition! says that a frequent problem with choosing propellants has been that they can be very challenging or dangerous to handle, or that they have a high melting point or a low boiling point. How is this going to change when the rocket is in a vacuum, so everyone working on it is wearing airtight spacesuits anyway, and there isn't an atmosphere to take away or add heat?

How will rockets be built if they don't have to worry aerodynamic drag? How will they be shaped? Can fairings and interstage rings be done away with? Will things that are covered in rockets on earth be exposed? Will they be launched differently, perhaps starting out at an angle instead of vertical?

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    $\begingroup$ Have a look at the Lunar Excursion Module to get an idea how spacecraft can be designed for a lower gravity and zero atmosphere. This may be a bit broad $\endgroup$
    – GdD
    Jun 9 at 7:59
  • $\begingroup$ A rocket launched from a moon without an atmosphere does not need a fairing to protect the payload. $\endgroup$
    – Uwe
    Jun 9 at 19:39
  • $\begingroup$ Although an interesting and unique example, the lunar module was definitely influenced by earth-bound construction methods and constraints. It's put together with thin sheet metal which is somewhat difficult to create in a zero-g environment. It's also a one-time use vessel in very many ways, shedding its main engine, legs, and being designed for a very limited lifespan (batteries). Ultimately, a primary design focus was weight reduction, which isn't going to be a massive concern for craft being constructed in space. Elements like its small windows and standing posture are entirely for weight. $\endgroup$
    – Innovine
    Jun 10 at 16:39

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It's likely we will see a return to hydrogen peroxide as a propellant. Creating it from water ice is possible with vacuum distillation. It's not quite as effective per kilo than other propellants, but it's one of the easier ones to create in-situ. You just need to import silver from Earth (perhaps other catalysts exist too). I can imagine deep space vessels having their own small hydrogen peroxide refinery onboard, and carrying a giant chunk of ice. No need to wrap it all in metals, just attach the spacecraft to a large ice block and let it convert the ice to fuel as needed.

Regarding metal structures, vacuum-deposition techniques lend themselves well to creating lumpy, sphericalish pressure vessels, where a custom balloon would be inflated and then coated in a mist of metal particles to build up the form. Using this technique in vacuum, zero-g environments isn't very well researched afaik so lots of speculation on what may emerge. I doubt there is any need for aerodynamic shapes, and even cylinders are more difficult and less efficient to construct than inflated spheres and blobs. See https://worldbuilding.stackexchange.com/questions/180460/would-inflating-hot-spheres-of-metal-be-a-viable-zero-g-vacuum-ship-building-te for some more discussion on forging and construction in space. It's not impossible to construct flat sheet metal and build craft out of it, but with a suitable mold you might possibly just deposit the metal right into the final form, skipping sheet metal and welding entirely.

There is little need for exact symmetry. If you put a gimballed engine under, and stick RCS thrusters around, it's quite possible to control any odd-shaped spacecraft. The symmetry you see in todays spacecraft is entirely due to aerodynamics, manufacturing processes, and structural strength. Little to none of this applies for spaceship created in space, so it's quite likely you'll see bulbous pressurized compartments, with a bunch of stuff stuck on, seemingly randomly, all around the outside. The deeper into space you go the colder it gets, so expect a good degree of thermal blankets wrapping the ship.

It's not likely that ships will experience intense g-forces (unless maybe going to and from the surface of Mars), so spindly, thin structures will be possible. Radiators, antennae, delicate instrumentation and solar panels could stick out at all kinds of angles, although use of solar panels isn't super practical at the asteroid belt and further, at least with todays efficiencies.

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    $\begingroup$ There is little need for shape symmetry, but there is some need for mass symmetry. The thrust vector of the main engine should point to the center of mass. The RCS thrusters should be point symmetric to the center of mass. $\endgroup$
    – Uwe
    Jun 10 at 19:52
  • $\begingroup$ That is not symmetry, though, and the rcs don't need to be symmetric. $\endgroup$
    – Innovine
    Jun 11 at 6:48
  • $\begingroup$ the rcs don't need to be symmetric, but they are easier to use if they are symmetric. $\endgroup$
    – Uwe
    Jun 11 at 8:17
  • $\begingroup$ @uwe i can imagine a scenario where a giant, irregularly shaped block of ice is attached to a spacecraft. Astronauts could stick rcs thrusters around it in roughly symmetrical locations, but let computer software work out the torques created and how best to fire them to achieve fine attitude and translational control. Perhaps even give recommendations for placements. Constraints for tracking every single bit of mass onboard and working out the exact CoM like on Apollo isn't going to be practical for long-term space work, especially when propellants are plentiful. $\endgroup$
    – Innovine
    Jun 11 at 15:35
  • $\begingroup$ If a giant, irregularly shaped block of ice is attached to a spacecraft, propellants could not be plentiful. Only if the block is not heavier than the spacecraft itself. $\endgroup$
    – Uwe
    Jun 11 at 18:38
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John D. Clark's Ignition! is a fascinating book, but I doubt that many of the exotic propellants that he describes would see much use in space. Factors such as availability and cost of materials are likely to become more of an issue in the future and cryogenic storage less of an issue without an atmosphere (although still causing problems especially with long term storage of liquid hydrogen).

Some rockets will still have to worry about aerodynamic drag, namely those that will return to Earth or land on Mars. But others won't. Fairings will not be necessary and different factors will come into play for structural consideration. Spherical tanks provide the greatest volume for a given surface area so might be used more. Attention will need to be paid to energy and heating especially around cryogenic tanks where sunshades and even Earth shades might be of use.

Crew compartments might be better constructed as a cylinder rather than a sphere as unless very large, a spherical space does not lend itself to human habitation and subdivisions, whereas a cylinder can at least have decks. Square or very angular shaped pressure vessels are far from ideal as they would require a far more massive construction to restrain the pressure.

Anything that lands on the Moon would require support on the surface and anything large may also require mitigation for the rocket exhaust blasting debris into the base of the rocket and or excavating a hole under the rocket as it lands. So landing engines might be mounted higher up and angled out to prevent this until such a time that a landing could be made onto a pre-prepared pad.

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