Stealthy Orbital Re-Entry for Combined Arms Warfare

[johnandersoni]

I'm working of a short story of the sci-fi nature, and I'm indecisive about one of the finer details. The plot of the short revolves around combined arms warfare in the future. The particular operation in the narrative involves an insertion of men and materials from orbit. (Kind of like paratroopers, only from much higher.) Capsules would carry men, equipment, vehicles, UAV/UGV's, etc. from orbit into the area of operations on the ground.

So, the problem I'm running into is heat, heat shields, and stealth. I find it pretty implausible that any kind of heat shield material or shape could be made in such a way that it would be radar absorbent, refractive and stealthy. Also, if the targets on the ground see a batch of bright falling stars, I think they'd surmise what is happening, and begin calculating possible landing zones based on the trajectory of the falling objects. So far, I'm thinking that an insertion from orbit cannot be completely stealthy. So I'm looking for the best insertion path, a wide shallow entry where your fall rate is less than your linear flight rate, or a steep entry where you punch right through the atmosphere until you slow down? Perhaps somewhere in-between? I think the shallow entry would reduce the amount of heat required to be dealt with, but the steep entry would get the window of vulnerability over with much quicker. The answer may be a bit subjective, but the ultimate goal would be to enter dark, atmospheric/slow flight as quickly as possible, so that the radar reflective heat shields could be jettisoned, the re-entry capsules aren't bright flaming orbs in the sky, and the stealth design of the re-entry vehicles can hide the trajectory of the craft from radar.

 

[Vanadium 50]

Apollo had an angle window of only 2-1/2 degrees. So I don't think you have the flexibility with re-entry that you want. Perhaps you can solve the problem another way - many dummy reentries, for example, to overwhelm the response.

 

[johnandersoni]

That may well be the most realistic approach, and it well might be used later as a form of tactical feint. Though we can assume great leaps forward in materials science between now, and orbital insertion of troops, I still don't think we'll have any materials that will stand up to something like a 90 degree, nearly uncontrolled re-entry. I'm also looking into buoyant re-entry, and other methods of coming in "slow" so that heat shock is not a problem. I'm not making very much sense of all this, though feathered re-entry a la SpaceShipOne looks promising. Wikipedia states this wouldn't be useful for orbital re-entry though. I'm trying to figure out if the usefulness of feathered re-entry is limited to materials science, or if its limitations are a fundamental problem that can't be fixed by more suitable materials.

 

[DaveC426913]

How advanced is the propulsion? Do they need aerobraking? Why not simply slow down outside the atmo to subsonic speed? At 500mph, you'd be on the ground in 6 minutes.

 

[johnandersoni]

The men and equipment landing would be part of a larger force in orbit. In order to remain in orbit, the group of ships carrying the larger force would need to have a significant amount of speed in order to remain orbiting, right? So, once "detached" from the large force, the landing craft could simply slow, under the power of their engines, as they descend and ensure a low maximum speed before they de-orbit and hit the atmosphere?

I didn't plan on the landing craft themselves to have any kind of advanced engine, because they are disposable craft. (Still working on returning the troops to safety, and recalling them). However, they might still not need any kind of advanced propulsion to do this. They could remain attached to a landing craft of sorts which would detach from the main body, slow down, drop it's payload, then return to the main group to reload for another drop in the next pass or whenever.

Unmanned pre-positioned drops, like reconnaissance UAVs, autonymous combat systems could detach and aerobrake. They wouldn't be dropped in one big group all clustered together. They'd be dropped one at a time, over a preparatory time span, and they wouldn't need to sustain life within their drop systems. If the enemy noticed and engaged one of them, they still wouldn't know the location of the main invasion force, or the other pre-positioned machines being dropped. The enemy on the ground would have a whole lot of sky to monitor for a prolonged period of time prior to any invasive force dropping in.

 

[mfb]

Everything in low Earth orbit has an energy of roughly 30 MJ/kg. For 150kg (human+re-entry equipment), that is the equivalent of 1 ton of TNT. This energy has to go somewhere, and such an amount of heat is easy to detect.

In order to remain in orbit, the group of ships carrying the larger force would need to have a significant amount of speed in order to remain orbiting, right?

Right, about 8km/s.

So, once "detached" from the large force, the landing craft could simply slow, under the power of their engines, as they descend and ensure a low maximum speed before they de-orbit and hit the atmosphere?

"Simply". With current rocket technology you would need at least 10 times the payload mass as fuel to slow down. That is 1500 kg of fuel - with even more energy than the atmospheric re-entry from above. And that is with a disposable rocket that enters the atmosphere. Getting this to orbital speed again would make the rocket even more massive (let's say again a a factor of 10, and that is optimistic). Velocity changes need a lot of fuel.

You could try to build giant mass drivers in space that "shoot" the humans backwards at some relevant fraction of those 8km/s, but such a structure would have to be huge (at least tens of kilometers to have an effect).

 

[johnandersoni]

You could try to build giant mass drivers in space that "shoot" the humans backwards at some relevant fraction of those 8km/s, but such a structure would have to be huge (at least tens of kilometers to have an effect).

Who would have thought slowing down could be such a pain.

So, if you put an object inside a shell, and shot them down a gun, which hit 4km/s in the opposite direction of travel by the end of the barrel, you'd still have to figure out how to loose almost 4km/s before re-entry. So, where the "fiction" part of science fiction comes in is to reduce payload mass, or increase braking force on the payload. Some kind of nuclear pulse propulsion on the braking vehicle isn't exactly my idea of stealthy. I was planning for the main power source aboard ship to be He3 fission. So, turning electricity into propulsion without requiring the landing craft to haul around a huge amount mass is the next hurdle. Maybe after being fired opposite the direction of travel, the drop ship descends to the highest reaches atmosphere where an advanced scramjet and scoop can create propulsion from highest, sparse gases?

 

[johnandersoni]

What about a ribbon, or cable, spanning a long distance between two ships orbiting in the same direction. The drop ship is fired along this cable in the opposite direction of travel, and a motor grips and pulls on the cable to further decelerate. Once the drop ship reaches the low velocity required, it separates from the cable. The trailing ship would have to use reverse propulsion to keep the cable taught as the drop ship pulls on the cable in order to lose velocity.

My other thoughts have to do with ships orbiting in opposite directions, the drop ship separates from one, and then is captured by a cable from the other and then "towed" to a near stand still. As the opposite orbiting ship moves further away, more cable is let out a rate slower than the drop ship was traveling. So the drop ship is reeled to a near halt.

 

[mfb]

Your main ship has enough time, I guess - you can use very efficient ion drives or something similar. They need a lot of electricity but only tiny amounts of fuel.
Unmanned payload is easier, as it can withstand larger accelerations. A 1km or 2km space gun might be sufficient to slow it down so much that the re-entry is very gentle.

The cable idea would work - if the cable is ahead of the spacecraft you don't even need a second spacecraft , the deceleration of the stuff that is supposed to land would give the required tension.

My other thoughts have to do with ships orbiting in opposite directions, the drop ship separates from one, and then is captured by a cable from the other and then "towed" to a near stand still.

That would need a cable with a tip velocity of ~10km/s or more, similar to the requirements for a space elevator. There are some concepts with cables - designed for the opposite direction, but they could be useful for re-entry as well.

 

[Pete Cortez]

Pretty much echo what others have said. Commit to tethers and then deal with the problem of counterweights, length and arrangement.

 

[johnandersoni]

Thank you all very much for your responses, they've really helped me to flesh out this idea. I'm going to go with the mass accelerators (coil guns) for unmanned, sturdier payloads, and tethers for manned and sensitive payloads or payloads which require a more delicate placement relative to the area of operations on the ground.

The idea is to pull off a surprise attack from orbit on a barren, rocky planet with little to no infrastructure while using the fewest amount of humans possible. Most of the ship-to-shore force is going to be autonomous. Software systems are going to be acting as the S-2 (intelligence), S-3 (operations), and S-4 (logistics) shops. Once engagement commences, 'boots on the ground' would then be receiving packages of options put together the shops, and making decisions based off their first hand knowledge combined with how software perceives enemy's threat priority and disposition. I'm trying to write a scenario which is at the apex of Col. John Boyd's ideal of maneuver, with maybe some 4G elements from Lind & Hammes. I'd like to use as few Green Rocks as possible.

Using the coil guns, tethers, and even life support itself is going to be a complex plot line, because all of that produces enormous amounts of heat. If the orbiting party is caught in the shade radiating huge amounts of heat, the defending party should easily spot them. Likewise, if all the anonymous parts and pieces broadcast to one another via radio, again the defending force is going to be clued in. So all of the preparatory work of inserting supporting units is going to have to be done under a heat deadline, and all done using aimed laser, burst and relay communications, between the various points of contact.

 

[DaveC426913]

It occurs to me that perhaps you're being too subtle. If they carry out an attack on a ground base, why are they throwing away one of their biggest advantages - huge amounts of free potential energy. Simply dropping things from orbit would deliver huge a amount of destruction. Especially if it's bombs they drop.

At those speeds they'd have a big surprise element, who cares if the defenders see it coming, they'll have no time to react. No need for sneaking around, no need to put troops in harm's way, no need for tricky deployment, maneuvering and stealth tactics in get in position.

 

[Ryan_m_b]

Apollo had an angle window of only 2-1/2 degrees. So I don't think you have the flexibility with re-entry that you want. Perhaps you can solve the problem another way - many dummy reentries, for example, to overwhelm the response.

This reminds me of Old[/PLAIN] Man's War by John Scalzi. There's a scene where humans need to insert a team onto the surface of a planet that is heavily defended. To do so their ship engages its jump drive and appears right above the atmosphere, only to be immediately destroyed by orbiting ships and defences. They were counting on that though and the soldiers all ejected in one man pods designed for reentry and to look like debris. From the alien's perspective a ship appears, is destroyed and debris rains down on the planet. Unbeknown to them an army is sneaking down.

 

[johnandersoni]

This particular scenario has hostage rescue, and intelligence gathering components. Thus the reason for the ground assault on a fortified position, and why it needs to become instantaneously overwhelming. The hostage takers on the surface have assets in orbit, but not total orbital supremacy. The orbital assets would certainly spot a Normandy landing coming their way, and that would be the end of the hostages. It would be possible to insert a small strike force ahead of the main body which will take up the engagement once the hostage rescue succeeds or fails.

Another plausible scenario for not "nuking them from orbit" would be 4th generation (asymmetrical) warfare. A non-state aggressor could be mixed in with a local populace. Wiping out the position from orbit would result in high collateral damage or loss of civilian life which may not be politically feasible. If the conflict were symmetrical, then the force defending a plot of a planet's surface would be useless without also controlling the orbit above and approaches to that plot of surface. In that sense, controlling the planet surface would be more of a political or, maybe industrial position. Controlling the orbit would be the martial objective, as controlling just the surface would be like controlling a castle under siege. You may own the castle, but you'll probably starve in it. If the enemy is employing asymmetrical strategies, then controlling the orbit wouldn't be enough martially. Without an occupation of the surface, and arguably in spite of a surface occupation, an asymmetrical enemy could do plenty of moral and political damage without ever engaging the military forces.

 

[DaveC426913]

This particular scenario has hostage rescue...

mmmOK, I can see how that sort of makes 'bomb them from orbit' more of a plan B option.

 

[johnandersoni]

mmmOK, I can see how that sort of makes 'bomb them from orbit' more of a plan B option.

Yeah, it's the 2nd objective in order of importance. Primary objective is make a best effort attempt to get the hostages out, third is an intelligence sweep of structures if the force of action opens a time window long enough for the ground forces to do so. The reason the extraction/ground component is mostly unmanned and automated is because if the hostage rescue fails, the main force is still going to have to engage the enemy and wipe them out.

I got the idea for this series of short stories when I was in the Marine Reserves. I was an intelligence analyst (0231) for the USMC, and a systems engineer for my day job. While deployed to Iraq, I also did a lot of work for the battalion's operations shop (S-3) because they were short handed. What really struck me after a while was the total lack of useful tools. We had some mapping software, blue force tracking software, and I wrote some various software tools to help, but other than that we didn't have any kind of software tools. I received folders and boxes of paper reports from group (G-2) every day, and had to manually read through and filter those reports in order to find the things that were relevant to our battalion, and then generate a brief for our C.O. and Ops. O. I kept thinking about what kind of software tooling would be most useful to make sense out of all the raw intelligence I had to manually sort and filter. That grew into thinking about what kinds of software systems could bridge the gap between S-2 & S-3, and so-on, until I realized that nearly my whole job, and most of the "prep-work" of S-3 could be automated. That's what gave me the idea for some short stories involving automated command structures in a military environment. The "from space" aspect to the story is really just to make it more interesting, add more dimensions to the battle space, and to provide more varied challenges for commanders to solve with their tool sets. So the focus of these shorts isn't the missions or the action, but on the technology which enables maneuver warfare principles to be applied at speeds that not even Boyd could imagine.

 

[Czcibor]

I see one idea for being "stealth", which may work pending on setting.

Are monitoring stations scattered around all planet or just above a few populated continents?

My idea is the following - you start descend above some unmonitored area (like on Earth - Antarctica or southern part of Indian Ocean) and finally with remnants of your starting velocity glide to your intended target.

Possibly drop part of your heat shields somewhere around the way. Because of that mentioned v=8km/s, I think that one point of atmospheric entry and another of final landing is quite plausible.

 

[Pete Cortez]

Is there any reason the bad guys--who apparently can set up shop on another planet and easily track objects reentering the atmosphere--can't detect your ships approach in the first place?

 

[GTOM]

Is there any reason the bad guys--who apparently can set up shop on another planet and easily track objects reentering the atmosphere--can't detect your ships approach in the first place?

Maybe one could disguise the transport ship as a merchant ship, or take out orbital recon, and come from the direction of the Sun.

 

[Pete Cortez]

Maybe one could disguise the transport ship as a merchant ship, or take out orbital recon, and come from the direction of the Sun.

johnandersoni apparently wants to set this scenario on some barren rock with little to no infrastructure, so don't imagine there will be a lot of traffic. Even if there was, the bad guys apparently have the wherewithal to cross interstellar distances and track the sky. Tracking objects in conjunction is doable, too; they cast shadows. Taking out orbital eyes is another way of announcing "hey, we're here!"

 

[GTOM]

johnandersoni apparently wants to set this scenario on some barren rock with little to no infrastructure, so don't imagine there will be a lot of traffic. Even if there was, the bad guys apparently have the wherewithal to cross interstellar distances and track the sky. Tracking objects in conjunction is doable, too; they cast shadows. Taking out orbital eyes is another way of announcing "hey, we're here!"

If they have some interstellar, FTL stuff, that makes stealth easier, if they can jump near to the planet, waste heat only need to be contained for hours, probably they could have recoilles drives too.

If they take out orbital recon, yes they announce their present, but they might not know, where exactly the troops land.

 

[GTOM]

Hmm, i thought about the following things and questions : so, when the fleet appears, it can't be disguised, but the missiles and fighters kill orbital recon, and big surface radar dishes and laser domes, the problem is digged-in anti-sat silos. (With enough firepower, nothing is impossible of course, but if a missile targets the silos directly, it will be so heaten up by reentry, that lasers easily ruin the electronics, then shatter it)

So they want to land surface units with not so big death ratio. The mothership should have a radius of a 100m at least, it can give a significant delta-V to landing craft with a launch loop. I suppose they can reach LEO from GEO with minimal use of rockets, so they should be able to reach LEO stealthy...
Now, if they erase horizontal speed with a big enough rocket (horizontal speed on GEO is around 4km/s will they become very much faster if they spiral in after slinged?) could they descend with a big enough parachute?

I thought the rocket could drag the reentry capsule with a carbon nanotube cable, so even if the rocket got hit, the capsule itself can survive and still aerobreak. If troops can withstand 3g, then it takes less than five minutes to decelerate it don't give too much time to intercept.
I also wondered, whether a decoy could create a similar plasma coma as the reentry craft, for a few seconds to misdirect a missile? I thought about a solid metal rod, that evaporates fast, and while its mass is much lower than the reentry craft, but air resistance is also weaker, so it slows down at a similar rate.

 

[DaveC426913]

If troops can withstand 3g, then it takes less than five minutes to decelerate

They definitely can. Fighter pilots withstand 8Gs but black out at 10Gs.

Then again, pilots must be upright and in-control. Troops could be prone, which means they could withstand a fair bit higher, especially if it's only for a few minutes.

 

[GTOM]

Thanks. I wonder, could that space sling idea possibly work to reach the atmosphere from GEO without much rocket power?
If the launch loop radius is 100m, than a km/s speed results in so big g-force that it surely kills people...
But what if they extend a 100 km nanotube cabel, and accelerate that?

Not as if rockets are bad until exact course can't be calculated from the surface.

 

[mfb]

Current untapered cables can give a tip speed of about 4 km/s (optimistic). To limit acceleration to ~3 g they need a length of 500 km. Everything falling down from GEO to Earth will impact it with a high velocity and a bad angle (short deceleration distance) - doesn't work. Better start in not-so-low orbits and get rid of 4 km/s via a rotating tether. That reduces the kinetic energy by a factor of 4.
Significant tapering might allow to get the speed down to nearly zero.

They definitely can. Fighter pilots withstand 8Gs but black out at 10Gs.

The time is important here as well. You cannot sustain 8 G forever.

 

[GTOM]

Thanks. By not-so-low orbit, should i think about a few thousand kms?

Something falling down from GEO approximately what would be the vertical speed? (That is a serious integration.)

 

[mfb]

Something sufficient to have the cables rotating, probably not higher. Tidal gravity will increase their tension and lower the possible tip speed a bit, but going higher means more acceleration on the way down.

Something falling down from GEO approximately what would be the vertical speed? (That is a serious integration.)

Close to escape velocity, there is no large difference between GEO and "very far away".

 

[GTOM]

Something sufficient to have the cables rotating, probably not higher. Tidal gravity will increase their tension and lower the possible tip speed a bit, but going higher means more acceleration on the way down.

Close to escape velocity, there is no large difference between GEO and "very far away".

Thank you.(Than I guess i'll write my martian scenario with a combo of tether propulsion from 1000-2000 km (outside laser range thanks to Rayleigh scattering, even if the thin atmosphere don't swallow much UV), short rocket brake, then parachute descend.)

 

[DaveC426913]

To limit acceleration to ~3 g they need a length of 500 km.

It is my expectation that, in a military setting, the troops would be pushed a lot harder than 3Gs for the sake of tactical advantage.

Fighter pilots are actually a lousy analogy here. They're nothing like the passengers in this scenario, and I listed them as an example of the lower end of the limit of g's. As I mentioned, troops are essentially cargo until they make landfall, so they can be prone (or better yet, supine), which dramatically increases their tolerance to G forces (no danger of black outs, since the heart and head are at the same level.) But above and beyond that a lot of resources would be put into the landers to facilitate the best - yet still within safety tolerance - mission plan. In fact, thy don't have to even be able to move for those few minutes of planetfall. You could essentially encase the troops in tanks of incompressible water.

I think 3g's is dramatically underestimating the likely levels they would be put through. Frankly, I'd say even 10gs is not out of the question, considering how much freedom and resources and motivation designers have to max out tolerable g's.

 

[DaveC426913]

Here's a little blurb that gives some nice details about prone acceleration limits:
http://aviation.stackexchange.com/q...sition-for-the-pilot-minimize-g-force-effects
Keep in mind that
- this data is a half century old and technology has come a long way, and
- your troops-as-cargo situation is markedy different than this study's purpose.

 

[GTOM]

It is my expectation that, in a military setting, the troops would be pushed a lot harder than 3Gs for the sake of tactical advantage.

Fighter pilots are actually a lousy analogy here. They're nothing like the passengers in this scenario, and I listed them as an example of the lower end of the limit of g's. As I mentioned, troops are essentially cargo until they make landfall, so they can be prone (or better yet, supine), which dramatically increases their tolerance to G forces (no danger of black outs, since the heart and head are at the same level.) But above and beyond that a lot of resources would be put into the landers to facilitate the best - yet still within safety tolerance - mission plan. In fact, thy don't have to even be able to move for those few minutes of planetfall. You could essentially encase the troops in tanks of incompressible water.

I think 3g's is dramatically underestimating the likely levels they would be put through. Frankly, I'd say even 10gs is not out of the question, considering how much freedom and resources and motivation designers have to max out tolerable g's.

Even if they are in water, won't they experience the same pushing force, after they reach the wall of the tank? For significant delta-V, it isn't enough to withstand high G-force only for seconds.

 

[DaveC426913]

Even if they are in water, won't they experience the same pushing force, after they reach the wall of the tank? For significant delta-V, it isn't enough to withstand high G-force only for seconds.

The problem isn't in the magnitude of forces, it's in the disparity between different parts of the body.

If immersed in a tank of (saline) water, the body will be essentially neutrally buoyant, no matter what the G forces are. The body will not press against the walls any more than water in the tank will.

Look at an extreme example: a small water-filled balloon, in a gallon jug of water, can be bounced down a hill and it will not pop. (If the gallon jug is rigid so that it does not deform, even better) There's virtually no differential forces acting on the inside or the outside of the balloon. It just floats there, unperturbed.

Now, the one problem humans have is that they are not undifferentiated bags of water. The lungs are a large air cavity, and that will result in differential forces. The lung cavity is probably the limiting factor in g forces the body can experience.

 

[mfb]

Getting the spacecraft to the tip of the rotating tethers would need some time - of the order of minutes. More than 3 g could be tolerable in the right position and/or immersed in water, but the crew certainly has to be able to breath for a while during the process.

 

[GTOM]

Getting the spacecraft to the tip of the rotating tethers would need some time - of the order of minutes. More than 3 g could be tolerable in the right position and/or immersed in water, but the crew certainly has to be able to breath for a while during the process.

Secret of the deep had the idea of breathing fluid (so fill the lungs with fluid that can give enough oxygen) could this possibly work?

 

[mfb]

Possible. I don't think it is worth the effort. You save a bit of time and a bit of cable material.