7
$\begingroup$

With the increasing amount of sci-fi using drop-pods to get their soldiers on the ground I would like to look at the design and how a proper drop-pod should look.

The look and shape of such a drop-pod would depend on available technology, its function and cargo. A drop-pod that carries large construction equipment would be different from one that carries just one person, and usage for frontline drops or dropping in friendly lines is also going to need different from each other. So for the purpose of this question:

  • Fuel available in this sci-fi universe has an energy density 10x that of current rocketfuel. This to emulate most sci-fy where fuel is often of little concern. Otherwise normal physics apply.
  • Drop-pods need to be inexpensive enough to leave on a battlefield. If they are re-useable that is a bonus. This means limited use of rare materials and construction but it is assumed that construction has become a lot cheaper to begin with.
  • The drop-pod needs to be more effective than a landing craft at dropping into combat
  • The drop-pod is used for combat drops close to enemy positions. Key is high velocity and as little preparation time on the ground as possible before the infantry is ready to engage.
  • In case of erroneous landing (IE the drop-pod lands on a slope and falls over) the occupant still needs to be alive and able to exit. Maximum slope to land on will be set at 45 degree angle.
  • It is a single-seat drop-pod.
  • The occupant is wearing a pressure-suit against G-forces.
  • G-forces may not exceed 9 G's, which is what airforce pilots can endure without extended loss of consciousness. If the G-forces are downwards relative to the body (blood is pushed out of the brain) then 3 G's is the maximum.
  • Drop-pods are used both in-atmosphere and on planets without an atmosphere.
  • Drop-pods need limited steering capabilities.

Question:

How would a drop-pod be designed based on the above constraints? Things I want to know are:

  • Occupant's space. Orientation of his chair (or other arrangement), cushioning, airbags(?), type of restraints (easy to take off, but won't disengage on landing), placement of additional equipment occupant might need.
  • Shape of the outer shell. What is the most efficient shape trade-off between going through the atmosphere (or no atmosphere) and landing on a surface? Does it have landing struts, does it have parts designed to break off or expend themselves?
  • Visuals and steering. A monitor and steering equipment might seem simple but when 9 G's are pulling on you it'll be pulling on your steering equipment as well.

Edit: For a look at potential methods to slow down, look here: Orbital Drop Pods: Useful?. However, this question is about the entire design, not just the methods to slow down.

$\endgroup$
  • $\begingroup$ As this is for use in no-atmosphere conditions (amongst other uses) any one-way thingy (aka drop pod) will be cheaper than the normal(?) alternative, because without atmosphere cheap atmospheric brakes/chutes/wings are out. So i do not really get your second point. Could you expand on what you see as a problem to be overcome, or what you value in an answer? 'cheap reentry capsule with retro rockets, not exeeding 9g' is a given, but what more do you want to know? $\endgroup$ – bukwyrm Mar 1 '19 at 8:27
  • 1
    $\begingroup$ Actually, current reentry capsules are as small and cheap that we (humanity) can make them. Except for first three points you describe normal ways to get back from orbit, so being cheaper than normalwway sn't really possible — cheaper way will become the norm. Only way around it would be to allow G or failure rate high enough to kill some % of your troops. $\endgroup$ – Mołot Mar 1 '19 at 8:46
  • $\begingroup$ @bukwyrm A landingcraft can take off and land anywhere after reentry, a drop-pod lands and if it's ever used again needs to be collected and transported by something else back to orbit. Added some criteria that I want to focus on in the question. $\endgroup$ – Demigan Mar 1 '19 at 9:05
  • $\begingroup$ @Molot there's a variety of reentry modules with various techbologies and shapes in that link. As for expenses against landingcraft I now realize that they are different in function and that my aim was to prevent extremely expensive drop-pods. I'll edit the question. $\endgroup$ – Demigan Mar 1 '19 at 10:16
  • $\begingroup$ Each and every one of these pods was, at its time, cheapest and lightest their creators could get with the technology and economy they had, for the number of people they needed to land. If you want to have more modern ones, you could use the knowledge of how people did it in the past, when they had really similar design goals you do. I like your edit, it makes answers more possible. $\endgroup$ – Mołot Mar 1 '19 at 10:35
8
$\begingroup$

As a pure physics question, you're unlikely to do much better than what various space programs have come up with. The Apollo Command Module, Soyuz Descent Module, and the new Orion Crew Module all have the same basic shape and configuration, and for good reason. For reference, this is the Apollo CM:

enter image description here

The basic shape is essentially driven by physics. The gumdrop shape (or the wider bell shape of the Soyuz) will naturally keep itself oriented during reentry and allows you to limit the heat shield to only the bottom of the craft. Putting the heat shield, heavy equipment, and ballast (if necessary) along the bottom also keeps it rightside up when it lands. The top surface isn't affected by reentry heating, so the hatch goes there, along with any delicate specialist equipment you want (like a grenade launcher for those pesky enemy forces). Anything on the bottom surface, like landing struts, will be a) melted and b) crushed against the ground, and so should be avoided.

The seats, which you can see more clearly in the full-sized image at the link above, are oriented so that the pilots reenter more or less on their backs, which is more tolerable in terms of G-forces. It also means that the hatch is conveniently right in front of them when they land. The seats aren't directly attached to the hull but are held by a shock absorbing system. The restraints don't seem to have been anything out of the ordinary; a typical crash harness will handle any kind of G-force that you want to subject your passenger to.

In the Apollo, cargo spaces were under the seats and on the upper walls of the craft; the lunar missions carried all sorts of consumables and life-support equipment there, but of course you won't have to do that because you're using your pod for a matter of minutes rather than days.

Since your drop pods intend to drop on targets without an atmosphere, they'll need retrothrusters. The Apollo actually has RCS thrusters, ten of them, arranged around the rim just above the heat shield. Yours will need to be quite a bit larger and more powerful, but you've provided for that in your question so all is good there. The thruster exhaust is unlikely to do anyone on the ground any favors; aim well away from friendly troops.

On that note, aiming control is provided by the same thrusters. Control is probably computerized, with manual backups. Guidance is a bit tricky, because you can't receive radio signals from the ground - the so-called reentry blackout. You may be able to transmit and receive through your wake, though, which would allow the launching ship to guide you in. Otherwise you're likely going to have to rely on inertial guidance which is, to put it mildly, not that great. (Most accelerometers are not designed to withstand those kinds of forces, let alone measure them with any accuracy.) Visual guidance is going to be dubious at best once you're in the atmosphere.

Cost is going to be a bit of a sticking point. At the very least, fuel is expended, although not more than you would use in a conventional landing craft on an airless surface. The heat shield, assuming you land in atmosphere, is probably ablative and will therefore need to be replaced. The other components are in theory reusable; I wouldn't be caught dead plummeting to earth in a reused pressure hull, but the computers and rocket engines can probably be refurbished.

On the other hand, militaries do fantastically expensive things all the time. The AIM-9 Sidewinder, perhaps the most popular anti-aircraft missile in the world, renowned for being relatively cheap and mass-producible, cost $600,000 each in 2015. Drop pods should not be used lightly, but that's not the same as never being used at all.

|improve this answer|||||
$\endgroup$
  • 2
    $\begingroup$ Worthwhile to note, it took about 15 minutes from initial re-entry to landing, maximum G of around six. That does not seem long, but in a modern battle that is an eternity. For comparison a battle tank might be expected to go about ten to fifteen kilometers. A fighter jet might make 800 to 900 kilometers under full (unsustainable) throttle - or New York to Chicago. $\endgroup$ – Justin Thyme Mar 2 '19 at 15:29
  • $\begingroup$ @JustinThyme That's a good point - I wonder how much of that duration could be shaved off with a more rapid descent and a more violent rocket-powered landing. 6 Gs isn't all that much, so there's room for improvement, but not a ton. $\endgroup$ – Cadence Mar 2 '19 at 22:03
  • $\begingroup$ To go faster than free fall, as you suggested, takes thrust ti increase your descent speed, not to diminish it. Either that, or you come in at a high velocity, like a meteor. The most pertinent problem, I posit, would then be heat. And eventually that speed as to be reversed. When you land, your velocity WILL be zero. Recall that when a meteor lads, thee is no need for perimeter defenses, The perimeter doesn't exist., $\endgroup$ – Justin Thyme Mar 3 '19 at 14:58
  • $\begingroup$ I also don't think that the reusability-aspect will be that big. dropping stuff into a warzone from space, and then later hauling that same stuff back up for refurbishing does not sound like something the military will relish. --- The command modules that you show as examples are much too big for what OP has in mind. Single-use, single-occupant drop pods would need just about nothing that is shown in the annotations of the modules - no water management, no hatches... Thereby a lot of space and mass can be trimmed. $\endgroup$ – bukwyrm Mar 4 '19 at 7:30
  • $\begingroup$ @bukwyrm The main details I wanted to show with the diagram were the overall shape and where/how the seats are mounted. You're right that life support isn't necessary, although storage (for weapons, etc.) is a plus. As for reusability, the OP asked about it in the question so I tried to address it. $\endgroup$ – Cadence Mar 4 '19 at 8:08
3
$\begingroup$

Modular thrusters with thrust vector steering, sized so one to three of those will suffice for the smallest projected drop-pod (small equipment drop, 20-40kg of stuff) - For higher loads, and higher 'glide' time and higher redundance, more of those modules get fitted. There is also a modular scales-like heat shield with active cooling that can be expanded, layered, or left off, according to the atmosphere at the target (or maybe the expected laser-based defenses). Thrusters can be mounted under, and obliquely behind the sides of the heat shield. Oblique thrusters are used for orbital and entry-level maneuvering and braking. completely shielded thrusters only get activated after the shield is scuttled.

Piloting is done extremely high-level, basically just choosing from a computer-provided list of locations - everything below that is automated, as the necessary thrust and shield management is beyond the reaction times of humans anyway. Interaction with the autopilot via the HUD that the soldiers have anyway.

The soldiers lie curled up on a matress that is formed from an elastic sack filled with elastic (and gas filled) grains. Basically a vacuum matress, but built so you basically are completely sunk into it when the vacuum is applied - (the vacuum in the matress will stiffen it, gving you support, at the same time 'inflating' the grains. When the vacuum is turned off, the grains loose volume, making it much easier to get up from the matress. Equip is also vacu-formed secure.

Soldiers have their own air supply anyways, so the cabin is flooded with nitrogen to keep fire-hazards small.

The capsule is for one-time use, and the retro-rockets obviate any attemps at secrecy anyways, at touchdown a spike is shot into the ground, anchoring the capsule. Opening the capsule is accomplished by explosive bolts that open the capsule like a flower, the leaves opening by material-spring-action. (Getting into the capsule involves lying on the g-matress-flight-computer combo, and then being transferred, en bloc, into the capsule that is welded shut in an automated process taking a few seconds.)

The capsule is blown free from the carrier, brakes as much as neccessary to drop from orbit (that initial delta-v may also be given by a launcher on the carrier, too, to save fuel). Further braking is accomplished by shield, or via bursts from the thrusters, in a deceleration sequence that is input by the carrier's nav department. Any corrections beyond that are done on-the-fly by the pod. 9g is ~90m/s^2, meaning you can loose 90m/s every second. Orbital velocities of a few km/s thus will take a minute or more to burn. The available fuel and acceleration-restricions mean that the soldier is provided with a map outlining reach, from which she can choose landing locations dynamically, up to the last second. As there is no budget and possibility for real-time sensors on the pod, evasive maneuvers will be restricted to pre-planned twitches in the flight path (and woe unto those whose random-number generator falls into enemy hands!)

The thrusters (and in atmosphere - shield) will be visible in even the most basic thermal imaging equipment, and will at the same time preclude any last minute recce by the pods inhabitant - all maps will be preloaded. The last 450m/s (Mach 1.5) will take 5s to burn off (at 9g), in this time the pod will travel 1.2km, quite vertically, so it will be easily spottable by everyone around. Going at an angle is possible without aerodynamics, just using well-timed thruster bursts, but any velocity in the horizontal is bought with much more fuel, and the overall goal would be to get boots on the ground as fast as possible, so i'd guess there will be no real hovering (although there could be!)

Shape will be a flat pinecone (with shields) and a flat egg with thrusters on struts below it without shields (though for cost reasons this will be the capsule that is inside the pinecone version).

There will be no planned survivability for direct kinetic hits, just the shields that can double as laser-armor and against the most puny of kinetics. If the enemy can spare a guided missile or 1000+ shots of AA cannon per pod, the time to fire them is after landing, anyways, and individual grunts getting orbit-dropped will not make a difference to the strategic situation.

|improve this answer|||||
$\endgroup$
  • $\begingroup$ aviation.stackexchange.com/questions/1971/… It doesn't seem like a predictable (directly vertical)flight path is a good idea for a contested landing, and (apparent)lateral motion can be used to bleed off acceleration forces on occupants, if not the actual velocity of approach. might propose a shape more like a finned dumbbell, to allow the armor to cover the rockets whilst the pod is inverted, otherwise almost any kind of hit is a kill on final descent path.(whilst still preferring aerodynamics) $\endgroup$ – Giu Piete Mar 1 '19 at 11:26
  • $\begingroup$ @GiuPiete You are right, i'll add something about automated (non-responsive, but random) evasive maneuvers. My guess is that any hit by kinetics will kill the vehicle. It's just too small and un-redundant to survive. Lasers might be survivable with enough ablation shield to burn. --- My take is that drop pods are the story-equivalent to parachutes, though, so i'd expect a certain resigned-ness to losses.... $\endgroup$ – bukwyrm Mar 1 '19 at 11:44
  • $\begingroup$ sure & whilst that's all true, it doesn't mean you don't make the best effort you can to ensure they get down to the dirt. aside from that, if you make them (too) vulnerable to 'soft kills' the enemy will just adopt air-burst shells that knock out bunches at a time. $\endgroup$ – Giu Piete Mar 1 '19 at 16:47
  • $\begingroup$ you know, you could use the drop pod as a weapon.. your descent reaches x feet the drop pod tilts and the trooper and the mattress are dumped out the (perhaps with some final-final-final approach assistance,) the pods engines fire one last time on a terminal path to some target/location/whatever. It would probably be dangerous anyway having a pod with any amount of 10x propellant remaining on board landing next to you in a combat zone :) $\endgroup$ – Giu Piete Mar 1 '19 at 16:54
1
$\begingroup$

I do not believe they will use the same drop pods for atmosphere and airless landings as the requirements are too different.

A pod for an atmosphere landing has very minimal use for rockets and has no need of them at all if it not going to be reused, whereas a pod for an airless world needs a considerable landing rocket.

A pod for an atmosphere landing needs a heat shield and the shield should be impregnated with chaff and other things to confound enemy sensors (the reason to use drop pods over landing craft is enemy fire, thus you want to make that fire as ineffective as possible.) The pod for the airless world will simply eject it's chaff and decoys.

For an atmosphere landing the soldier is probably not going to ride it all the way down. Once through the fire leave the pod and let it fall ahead of you (make sure it falls faster) still shedding chaff and decoys. The soldier has something akin to a wingsuit to give him good steering without slowing him too much (don't worry about ending up on a steep slope, the soldier picks his landing spot) and then a chute for the landing itself. On an airless world the main job of the pod is the landing rocket and you want to be on your back when that lights, there's nothing to be gained by leaving it.

For the atmosphere landing I don't see it being sensible to make it reusable. It's basically a frame, a seat, the heat shield and the countermeasures, and a chute if it is to land. The countermeasures are expended, the cheapest, most foolproof way to deploy them is to embed them in the burning shield and a shield that burns off is a lot cheaper than one that is reusable.

|improve this answer|||||
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.