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Sci-fi setting where cargo is being shipped to colony worlds. If a spaceship has to land and take off again, that costs a lot of money, so that mode of transport is reserved for people and urgent, high-value cargo; the plan is to get bulk cargo the last lousy few hundred kilometers in drop capsules, which aerobrake, parachute, splash into the ocean and get towed to shore by boat.

The maximum speed at which a capsule will hit atmosphere is 5 kilometers per second. (This is very hypersonic, but still slower than standard orbital velocity.) Their size will be on the order of a few meters, and they will be made of ordinary materials like steel and composites.

What shape should the capsules be?

Aerodynamic considerations suggest they should be round, perhaps ideally the quasi-teardrop shape (lens with short trailing cone) of the early space capsules.

On the other hand, that's a terrible shape for efficient packing aboard a freighter. Efficient packing would much prefer rectangular boxes like today's cargo containers. But sharp corners would probably melt when the capsule hits atmosphere.

Would it be viable to at least compromise on something like a cylinder shape?

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Use a separate disposable drop capsule. Pack each cargo container in a separate capsule. Then drop.

This way containers and capsules can each be of the optimal shape and material for their purpose. Since the two have separate function they should be separate objects with different shape.

Containers have efficient packing shape, probably the same as current cargo containers. They can be stacked and handled efficiently. The material is suited for near unlimited reuse and protects the content from normal handling and transport. It might be simply metal as with current containers on ships or something more light weight as with aircraft. With a separate capsule for protection during re-entry the containers does not need to compromise efficiency.

The capsule on the other hand is for the one way trip down, you do not need it on the way up and it has to handle much higher stresses on re-entry than are needed for other parts of transport.

So it is better to use disposable capsules that are not lifted from the gravity well at all but assembled on orbit when needed from ready components. This allows to use optimal shape and materials. It is probably much cheaper to have an ablative heat shield than to build a capsule from something that can be reused.

Down on the ground the capsule materials apart from the ablative heat shield can be simply recycled.

Note that since the capsule and container are separate things with different shapes there will be empty space between the capsule and the container within. This space would be used to isolate the container from the heat and vibration of the re-entry and the impact of the landing. Maybe it would be suspended with some sort of suspension system. It might be reusable (for vehicle suspensions for example) or disposable (very cheap) depending on design.

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  • $\begingroup$ Right, neither capsules nor containers if those are also used, will ever be lifted from the gravity well. But what is best shape for capsule? Teardrop shape cannot have a rectangular container inside. Cylinder could, though cylindrical capsules would take up a lot of space on the freighter. Would cylinder be aerodynamically adequate? $\endgroup$ – rwallace Feb 16 '18 at 20:26
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    $\begingroup$ My idea was that the capsules are assembled on orbit, presumably from ready components, and that the empty space between capsule and container is used to protect the container. It looks like I accidentally edited out the part about assembly on orbit. Oops. $\endgroup$ – Ville Niemi Feb 16 '18 at 20:43
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    $\begingroup$ @rwallace, if you're aerobraking, you want a blunt shape. A streamlined entry vehicle lets plasma flow smoothly along the surface, rapidly heating the vehicle up. A blunt shape, on the other hand, forms a shockwave that lifts the plasma flow away from the vehicle and greatly reduces heating. $\endgroup$ – Mark Feb 18 '18 at 6:07
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    $\begingroup$ @Mark Good catch. In theory there are alternatives but in case of prefabbed capsules to drop cargo cost effectively not really. Which makes your comment very important point. $\endgroup$ – Ville Niemi Feb 18 '18 at 13:25
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Use a MOOSE

Or, rather in this case, a COOSE, but that sounds less cool.

MOOSE stands for Man Out Of Space Easiest. The general concept is a teadrop-shaped device, but it's inflatable. Deflated, it doesn't take up a lot of space. Inflate it and shove it down from orbit, and there you go.

For this, you could do cubical cargo containers. One end contains the MOOSE gear, the other the parachutes. The cloth/flexible material of the MOOSE would guide the foam to the right shape, even around the sides of the cube. The flexible material is not the heat shield, though - That would be the foam itself, once it hardened.

It's something that dates back to the 1960s, so a sci-fi setting can easily have fancier foam that expands a lot and the like.

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keep in mind your space ships are going to be cylinders or rings, in which case truncated cones pack fairly well, also consider packing efficiency will be less important than minimizing material used, which those old capsule designs do. The more you change that shape the more shielding you need, and the shielding is all cost. Of course they could be a squat cylinders (more or less) on launch/release and end up as a rounded truncated cone after they drop the small rockets that get them to the atmosphere.

You also may want to look at the old Shenzhou and dragon reentry capsule, they are a lot closer to a cylinder in shape. Put a detachable* extension on the top (that would house the parachutes) and it gets even more stable.

*detachable saves space, store separately then just attach prior to drop

enter image description here

enter image description here

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  • Either the drop capsule is braked by an orbital tug or it must mount at least small engines and fuel tanks to decelerate until atmospheric drag takes over.
  • A minimum of powered braking means a lengthy aerobraking period. The drop capsules will spend relatively long time in an atmosphere. Small differences in high-altitude winds at the outset can cause large differences in the splashdown zone if the drop capsule cannot maneuver.
  • Aerobraking will cause serious heat, which requires shielding. The weight and cost of such specialized materials will have to be balanced against acceptable risks, especially for cargo capsules.

This suggests a somewhat streamlined shape with control surfaces, like the X-24 or the Nasa space shuttle.

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  • $\begingroup$ Ah, good point about the splashdown zone. Assume it cannot maneuver: how large a difference are we talking about, to an order of magnitude? If it's ten kilometers, that's a moderate inconvenience. If it's a thousand kilometers, that could be a showstopper. The capsules are expendable, so they have to be as cheap as possible, so they probably can't afford maneuvrability. $\endgroup$ – rwallace Feb 16 '18 at 20:14
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    $\begingroup$ @rwallace You may be interested in my old Space Exploration question How precise are our Mars landings? Make sure to also check out the sidebar links. $\endgroup$ – a CVn Feb 16 '18 at 21:29
  • $\begingroup$ The X-24 wasn't shaped for re-entry. The Shuttle and the X-20 were both designed to do re-entry in an anti-aerodynamic orientation, with the flat lower surface leading the way. There's a somewhat counter-intuitive phenomenon involved in re-entry: a blunt shape is better than an aerodynamic one, because the blunt shape creates a shockwave that lifts the superheated plasma away from the vehicle. $\endgroup$ – Mark Feb 18 '18 at 6:12
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Your suggestion about a cylindrical container is an interesting one. A cylinder will not be stable on reentry without some help, but I think spinning the capsule could be enough. Get it spinning fast enough and you'll start to get some lift. You can then use this as a means of controlling the landing zone (no left/right control but you should be able to get up/down control) by varying the rate of spin. The two ends of the capsule come off to allow the parachutes to do their work, and the parachutes should be mounted on bearings so that the cables don't get tangled up when the chute deploys. Just make sure that you make the capsule strong because the aero forces over such a large area could be an issue.

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Let's take it nice and easy.

enter image description here

I have mused upon the shapes of re-entering vehicles. They are aerodynamic to most reduce drag. They roar through the atmosphere, the little drag they do encounter heating them to glowing.

So that is alpha. What about omega? Maximum drag.

Your cargo would be a bunch of boxes, attached laterally by springs to their fellow boxes. Opened out they would form an enormous flat sheet, one box thick and many boxes wide and high. The sheet is folded onto itself in the cargo bay.

On being pushed out into space the springs cause it to unfold into the sheet. Then it falls like a sheet of paper, slowly tumbling, sometimes turning its momentum into upward movement, reorienting, tumbling again, drifting back and forth. It takes a long time to get down and substantial lateral movement is likely. But if you have cargo that does not mind going to and fro en route, this method will heat up minimally on the way down because it never gets going that fast.

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    $\begingroup$ You should muse a bit more on how re-entering vehicles actually work. Those streamlined space capsules you're probably picturing? They enter blunt end first, to maximize drag. Further, space isn't high, it's fast, and most of the re-entry speed you're dealing with is leftovers from being in orbit, not picked up while falling to the ground. $\endgroup$ – Mark Feb 22 '18 at 2:15

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