# What material should be used for a spaceship that doesn't need to re-enter?

What is the best material to use for a near future spaceship. Assume that most materials in the solar system can be mined and used.

The spaceship is 700m long, 500m wide and 500m high. It travels at around 1,212,062 m/s over long distances, say around 63 AU. The ship has the design shown below.

The material must be capable of keeping safe a crew without them suffering any ill consequences and should be as light as possible to save on fuel as well as being able to remain in space with only minor repairs for as long as possible (preferably at least 5 years).

The ship does not enter the atmosphere at any point but will pass through most conditions encountered in the Solar System.

• 1,212,062 m/s doesn't sound very approximate to me... – a CVn Aug 26 '17 at 17:12
• Also, your numbers don't quite add up. The volume of a box with sides 700 m, 500 m and 500 m indeed is 175,000,000 m³, but your spacecraft design is anything but a box shape. Traversing 63 AU (63 × 150M km) at a constant velocity of 1212 km/s will take about 90 days (7.8M seconds), and by the time you are at those velocities, the effects of gravity within the solar system will be next to zero; even the Sun won't slow you down appreciably. If you intend to remain within the solar system for five Earth years, then you're going to need to shed all that velocity by propulsion. Pluto is ~4.67 km/s. – a CVn Aug 26 '17 at 17:21
• @MichaelKjörling Nor does 63 AU. – jdunlop Aug 26 '17 at 17:27
• @Bellerophon - are those thrusters on the front ring pointing towards the back ring? Are the two rings equally sized? Because if so, I see an engineering problem that goes beyond materials science. – jdunlop Aug 26 '17 at 17:30
• – jdunlop Aug 26 '17 at 18:55

## Anything you hit hits hard

Travelling at 0.4% the speed of light any small rock that happens to be in your path will pack quite a punch. Sure you want something strong but on the other hand you can't avoid damage entirely so I'd suggest Carbon NanoTubes both for their strength and their ability to combine with nanotechnology. I would propose a sensing skin for the hull of your ship which detects any damage.

You want to ensure you can repair any damage (even minor) as soon as possible to maintain the atmosphere within the ship.

You particularly want to strengthen the struts between the two circular parts since any break here (especially whilst accelerating) would distribute the forces unevenly and cause further stresses and damage. Though, unless the space has some function, I would suggest moving the two circular parts together.

• The gap is unfortunately a crucial part of the function. – Bellerophon Aug 26 '17 at 16:33
• Where did you get 40% of the speed of light from? OP specifies the speed as 1,212,062 m/s (though doesn't specify in relation to what, so I guess we can assume that this is the total delta-v capacity of the craft), and the speed of light in vacuum is 299,792,458 m/s. That's 0.4043% of the speed of light; it's still substantial, but it's a very, very, very far cry from 40% of the speed of light. – a CVn Aug 26 '17 at 17:14
• I've no idea how but I must have I missed out a few zeros in my head! Thanks for pointing this out. – Lio Elbammalf Aug 26 '17 at 17:46

If it is just a matter of surviving in vacuum, you can make your spaceship out of aluminum foil. The LEM was built out of very thin materials because the spaceship needed to be as light as possible, but also because it was not subjected to the various stresses of reentry.

LEM in lunar orbit

For longer term occupation of space, you should consider building the spaceship out of ice. Ice is inexpensive, readily available throughout the solar system, absorbs radiation, doubles as a heat sink if needed and can act as the reservoir of hydrogen and oxygen or water for the life support system.

So consider the mission and the economic resources of the building company or nation. The vast majority of people are going to engineer the vehicle to be as economical to build and operate as possible (within the mission parameters), so once you have made these considerations then you have your answer.

Ice ship as described by NEOfuel

Building spaceships out of other materials would have to be considered on the basis of costs, the demands of the mission (are you pulling high energy manoeuvres outside of atmospheric reentry or aerobraking) and possible threats (obviously if you plan to cruise through the rings of Saturn or are on a military mission in a shooting war, your ship needs to have a great deal of protection).

Ultimately, it is going to be based on the demands of the mission and the resources available to the builders. They will, under most circumstances, build the ship in the most economical manner to meet the mission demands.

• This make me remember a short history where they first shielded the front of the ship with tons of ice before setting off to conquer the stars! – Tridam Aug 29 '17 at 20:59
• If the fabric tore on a small piece of space debris... wouldn't the ice just sublimate into the vacuum? Mission Control, we have a problem. – Cloudy7 May 24 at 15:25
• @Cloudy7 Only if you were close enough to a source of energy, like a Sun. – BMF May 25 at 18:08

I think Titanium and its alloys (including ceramic forms) would be a good material for your space ship if it was combined with insightful structural design.

First, Titanium is very plentiful in our solar system. And, it is light and strong, providing it doesn't get really cold. And, in space, no one can hear you say "Damn, its hot." So the first insightful design is to keep the hull warm -- above 0 C.

Second, The forward facing sections of the ship might be made of harder alloys of Titanium to improve resistance with collision, and shaped to deflect the impact away from the body.

Third, double or triple or quintuple hulled, in case a high energy collision punctures one hull, the redundant hulls stand a better chance of keeping the air inside the people tank. Because, in the vacuum of space, no one can hear you ask for a puncture repair kit.

Fourth, fill the gaps between the hull with a combination of low-density plastic and sealing goo. Low-density materials are good barriers to beta radiation. And, the sealing goo will seal small punctures. The crew can go out and repair them during regular maintenance because in space no one can hear you ask for overtime.

Fifth, lots and lots of compartmentalization -- any places with people or dangerous material need to be isolated from each other to save other peoples lives if one section is too badly damaged, because in space no one can hear you ask for a do-over.

Many of these ideas are incorporated into the modern submarine design, albeit for other reasons.

These are only a few ideas, I am sure there are more.

I know you want a lightweight solution but for safety and longevity go hard or go home, my answer is a Tungsten-Gold alloy, for the front end at least, Tungsten for structural strength and impact resistance Gold for its radiation absorption. This gives you a ship which can go head-on with debris at the kind of speeds you're talking about for the slightly over 80 hours of mission time you're looking at with integrity to spare, which you want in case anything goes wrong. While it's doing that it's also protecting the crew from the majority of the radiation coming their way.

• Tungsten wouldn't be especially helpful as it would make the ship incredibly heavy (and acceleration in space is purely A=F/M). Lighter materials would likely give you better impact protection for the same weight - but at over 1M m/s, hitting almost anything is going to be pretty destructive. Better to make the ship light and manoeuvrable, and look at alternative solutions to avoid smaller collisions (how about a laser to destroy small items?) – Matt Bowyer Oct 18 '17 at 14:16
• @MattBowyer Yours is one of the two big schools of thought on this, mine is the other. Small and nimble vs big and durable, whether it's motorcycle vs car or space ship design I favour the big and durable option because when something goes wrong and we get hit to spite all efforts to avoid it the big durable object affords me a margin of survival. – Ash Oct 18 '17 at 14:25

For impacts with tiny space rocks at enormous velocities, what you want is a whipple shield: several very thin layers with big empty gaps between them, held away from your hull on sticks. Whichever metal foil is cheapest would be fine. The rock will be vaporised on impact with the first layer and punch straight through. The resulting hot gas will spread out before it hits the next layer, reducing its penetrative power.

Whipple shields are used in real life because of the very light weight for the amount of protection you get.

For long journeys, pack some rolls of spare foil so you can replace damaged sections of the shield as part of your routine maintenance.

For large obstacles, you have to spot them on radar and dodge; no reasonable amount of shielding would allow you to hit a bus-sized rock at interplanetary speed without it drilling a hole through your ship from front to back. If there are rocks that are too small to show up on your radar but big enough to punch through the shield, I guess they just add drama to your story.