ISRU Atmospheric Entry

JoshNH4H · 2013-07-29 22:24:56

I've been thinking a lot lately about building up an economy in space that would prove profitable in the traditional sense. Obviously this entails launching a lot of material and workers up into space. However, at some point it will also require a downmass capability. Whether it be to bring back people, Platinum Group Metals (PGMs), Aluminium (The profitability of Aluminium production hinges directly on whether it's possible to produce electricity in space for a lower cost than producing it on Earth), science sample return, or even dropping rocks to the ground just for the hell of it.

For the sake of this discussion, there is already a colony on Mars with significant manufacturing capabilities, as well as a good amount of industry happening on the Moon. There are also fuel production facilities at the lunar poles and depots scattered around the Earth's hill sphere, as needed.

My question is this: What will Earth Entry vehicles look like?

I'm sure someone (probably Louis ) will suggest deceleration with rockets. This is a bad idea for two reasons: Firstly, it's very wasteful of fuel. Although production in situ (or at least in Space) reduces the cost significantly, the fuel still has to be brought from the Moon to LEO. Using H2/LOX with an exhaust velocity of 4.5 km/s, a rocket will need a mass ratio of around six. Given that rocket transportation is likely to be the dominant method of getting stuff from the Moon to elsewhere for a good while, I would judge this to be excessive. Secondly, and more importantly, rocket engines are hard to build and will probably have to be brought from earth. This is a huge cost, and even if you build them in space it's incredibly wasteful to send them down to Earth.

Aerobraking maneuvers are therefore absolutely necessary. However, they are made much simpler by the use of in-situ production: One need not worry quite so much about the mass of the entry vehicle. I propose that a glider with a very low ratio of mass to frontal area be used; I propose that the underbody of this glider can be made with Basalt fiber (m.p. ~1500 K) woven into a cloth, and I propose that it can be made rigid by use of inflatable struts, which will themselves probably be made from basalt fiber cloth and sealed somehow. An Iron-basalt fiber composite created by deposition of Iron from the Mond Process, which amounts to chemical deposition of Iron*.

If the wing shape is chosen properly, the glider will have a significant range and high accuracy with choosing a final landing point. For shock resistant payloads, a landing may be chosen whereby the landing vehicle simply skids to a stop in a stretch of deserted desert. For less shock resistant payloads, a helicopter is (in my mind at least) a good way to soften the landing.

What does everyone think? Do you think there's a better way? Am I perhaps missing something important?

*Point for research: Is there a way to make Steel from the Mond Process, for example by admixing the Iron Pentacarbonyl with another compound, which decomposes at appropriate temperatures to Carbon and something else? A Steel-Basalt fiber composite would presumably have some incredible properties.

RobS · 2013-07-29 22:49:24

I think it depends on what you are bringing to Earth. If you are bringing gold and platinum group metals, I'd just make a big metal sphere with a spray on ablative heat shield on one side and put the cargo inside. Push a group of them toward earth with a reusable booster and release them on their final trajectory a few thousand miles up, then fire the booster's engine so it misses earth (and ideally is put on a free return trajectory back to where it came from). Let the big dumb spheres hit the atmosphere, get slowed to a few hundred mph, and smash into the desert somewhere. Then you head in with a bucket loader and recover the pieces.

For people, interplanetary transportation would involve a different vehicle than trans-atmospheric transportation. You'd come from Mars, aerobrake, rendezvous, transfer, and head to the Earth's surface in a shuttle designed just for the surface-to-orbit phase. A similar shuttle would provide transportation between the Martian surface and orbit as well.

JoshNH4H · 2013-07-30 06:43:11

Would spheres be optimal, though? Spheres are difficult if not impossible to steer and that makes landing in a precise spot really difficult. A delta-wing, on the other hand, is very steerable and can function, at least to some degree, as a glider. This makes the logistics much easier on the team tasked with "catching" the payload, because they don't have to go anywhere.

Also, I'm not familiar with any spray-on ablatives. Did you have one in mind?

Terraformer · 2013-08-01 09:14:12

Don't rule out retrorockets entirely. Coming in at 6km/s is a lot easier than coming in at 8km/s,

But why do they have to be manufactured in space? Is Terra going to be running a trade imbalance in terms of mass, importing more than it exports? Why not launch a shuttle from Terra and use that?

JoshNH4H · 2013-08-01 15:22:57

Is Terra going to be running a trade imbalance in terms of mass [...] ?

The simple answer: indubitably. The somewhat longer answer: the space based economy will have the resources at this point to manufacture pretty much anything it needs except for labor. I would expect that the cargo going up will be mostly people, and going down will be almost devoid of people. Given that technological advance is likely to make launchers much safer, and given that it's something that can be manufactured in space, it seems unlikely that any significant downmass capability will be launched from Earth. If you must, take it as an axiom. I'm more interested in the technical aspects right now.

That's a reasonable point about retros, but I would counter with this question: How much does it cost to make a rocket engine, and how much does it reduce the costs of reentry by to use one? In my opinion, a lot and not very much respectively.

GW Johnson · 2013-08-05 08:58:05

I've been having serious difficulties signing-in, to discuss things here in the forums. So, I put a posting on "exrocketman" that addresses some of the issues with re-entry dynamics and heat protection. It's titled "Entry Issues", and right now it's the latest thing posted. A lot of that posting might be germane to the discussion here, especially as regards shipping cargo from the moon to Earth.

GW
http://exrocketman.blogspot.com

JoshNH4H · 2013-08-06 15:56:36

GW-- Very interesting post. It sounds like we're kind of hitting the same points, especially with regards to spending more time in the upper atmosphere.

When you talk about the increased difficulties of coming back from the Moon as compared to coming back from LEO, I can't help but wonder: Do the same considerations apply to aerocapture from the Moon? If you split the orbital change from LEO transfer to LEO, and the entry from LEO to Earth into two parts, do you still have the same issues? Intuitively, I feel that you would, but perhaps if you just want to aerocapture into LEO it makes more sense to choose a different trajectory that remains almost entirely in the very upper reaches of the atmosphere and thus eliminates significant heating.

GW Johnson · 2013-08-07 08:41:42

I honestly don't know much about incorporating aero-capture or aerobraking into any cargo transport from moon to Earth. Launching rockets off the moon could conceivably be replaced by catapults, since the delta-vee is so low.

Yet, the approach to Earth from the moon is inevitably 11 km/sec. Without men on board, you could design for aero-deceleration levels approaching 100 gees, as long as you provided a heat shield. Low-density ablatives like PICA-X offer the best near-term potential for that, which means shipment from Earth. I guess some sort of mineral-wool ablative might be made from local lunar rocks and dust, but that's a material development yet to be done.

If you enter steep enough to pull 100 gees (with the associated high heating), then you pretty much eliminate the risks of bouncing off the atmosphere into deep space. So there is an advantage to it. Whether that aero-deceleration is for aerocapture into orbit, or for direct entry, wouldn't seem to me to be much different. It's still quite hot, either way.

GW

JoshNH4H · 2013-08-08 19:32:53

I don't know about catapults. 1.6 km/s (lunar orbital velocity) isn't very much compared to Earth, but it's still pretty hard to accelerate to this speed with reasonable sized structures at reasonable accelerations. Long term, I would expect that some kind of magnetic launch, or even a cannon ala Jules Verne would be used to transport bulk items into space from the lunar surface.

Right now I'm concerned about the inbound leg of the trip from LMO to LEO. For bulk cargo, maximum deceleration isn't that important. For people, it is obviously much more so. My proposal is a delta-wing with a very low ballistic coefficient, made from simple materials that can be sacrificed.

Your intuition is in line with mine on this issue, but one can hope that orbiting sensors with a stronger ability to measure the density of the atmosphere at different levels, in combination with more precise ship targeting, should make it okay to aim higher in the atmosphere with more confidence. In this case, as long as you burn some momentum you can try again.

GW Johnson · 2013-08-09 10:02:22

Hi Josh:

Now I understand, your glider is for the people. To take full advantage of the reduction in peak heating of a low ballistic coefficient / low wing loading design, you will need to do your entry from LEO. That's because the velocity dependence is cubed, and that's a huge effect going from 11.2 km/s down to 7.7 km/s. To reduce the heating, you need to hit at the inherently-very-shallow angle afforded by minimal retro burn from LEO.

Getting that deceleration into LEO is going to be some sort of tradeoff between mass ratio required (and the associated propellant logistics) and the total transit time. You could use a very minor burn to put the craft into an elongate ellipse with a perigee below 135 km in the upper atmosphere. That's for aerocapture braking, perhaps multiple passes, but the time to make that orbit is measured in the same sort of days as the lunar transit. That's a lot of days in a capsule for people, not an objection for cargo.

Winged craft have very strict AOA limits, as we saw with shuttle. Plus, hypersonic designs like that have very poor subsonic approach and landing characteristics. I kind of like the old Lockheed idea from the early 1950's of folding the wings (and tails) dorsally, and entering dead broadside belly-first, with aerosurfaces "hidden" in the wake. You could use a straight-wing design with a subsonic airfoil in a folding design like that. The people will feel the peak deceleration eyeball-down, but it might be around only 5-6 gees. That's doable, even by civilians.

The other difference with folding wings is, you no longer need ablatives to protect stagnation zones on leading edges and nose caps, if you can get the stagnation peak heating below about 25 W/sq.cm. The low-density ceramic approach would then work, even at stagnation (belly of the craft). Build a structurally-tough version (like I did 3 decades ago), and you just might have a fully-reusable heat shield that resists damage and is easy to repair.

It's easier at Mars to use this material, but there is a niche at LEO for this stuff. That's my paper next week at the convention.

GW

JoshNH4H · 2013-08-09 20:43:48

I'm very much looking forward to it! I think that part of the problem here is that I was being unclear about what I wanted to do with the entry craft, both in my posts and in my thoughts. My apologies.

As I said in the original post, in my hypothetical scenario we have a need to bring things from the Moon back to the Earth. My question is about how to do this with a manufacturing capability based on the moon, and what configuration the vehicles should take.

That said, I suppose we can split anything that could be sent from the Moon down to LEO or Earth's surface into two basic categories: Replaceable materials and machinery, and non-replaceable humans. For the bulk items, vehicle reliability is a matter of a cost-benefit analysis. Nobody wants the vehicles to fail, but it's certainly not the end of the world if they do, because everything on them can be replaced at some cost. People, on the other hand, are much more important, and they cannot really be replaced; treating them as replaceable is likely to result in poor outcomes.

Anyway, in addition to the differences in safety requirements, most bulk cargo can withstand much higher gravitational loading than people. It's my understanding that you're not supposed to subject people to more than 3-4 gs. It is possible that the entry craft will have to ferry sick or injured crewmembers down to the Earth's surface, so this function should probably be kept in mind while designing the vehicle. Bulk cargo, on the other hand, and maybe even most machinery, can handle much higher gravitational loading.

However, it is my opinion that these two sets of constraints result in convergent designs.

For bulk cargo, you want to minimize costs (let's say for the purpose of this exercise that insurance is combined with transport fees so that loss of cargo manifests itself as a simple cost). For human cargo, you want to minimize gravity loading to within an acceptable range while making the trip absolutely as safe as possible.

I am strongly of the opinion that it makes the most sense to transport cargo from the Moon to LEO by having two separate crafts. One is to be a reusable launcher/lander from the Lunar Surface to LLO (Low Lunar Orbit) and back. This will probably be replaced by some form of non-rocket spacelaunch at a later stage. Next, there will be a reusable rocket that transfers from LLO to LEO and back, aerobraking in LEO to reduce the round trip delta-V. The system is designed to piggyback on fuel depots that will be placed wherever necessary in the Earth-Moon system, fueled from electrolysis products of the water ice at the lunar poles.

We now have four cases: Bulk cargo and people for aerocapture, and bulk cargo and people for aerobraking. In all cases, lower thermal loading is good for safety, because you can use stronger materials with lower temperature tolerances.

For cargo that is ultimately Earthbound, a disposable heat shield can be used for entry to LEO. Again, it should have a low ballistic coefficient and be constructed from cheap but light materials. I think that delta-wing is a strong contender because it enhances your ability to steer the re-entry craft. Ideally we would be able to land each payload on a dedicated landing strip somewhere out in the desert. This may require additional control in the form of inflatable wings or other components, which is no issue to me. Given that your material is coming from space, if your craft has a higher mass it's not much of a big deal.

For the inbound to LEO phase, it might make sense to do multiple passes and use the rocket structure (With H2/LOX fuel there should be a good deal of it on an area basis) potentially augmented by additional "wings" if needed, to heat sink and cool the rocket down each time. Fortunately coming in from the Moon you can't bounce off into deep space. The period of the transfer orbit from the Moon is about 7 days, and I would imagine that the first aerocapture pass should cut this down, perhaps easily down to one day (the energy difference between GEO and the inbound transfer from the Moon is small). Because of the strongly positive relationship between speed and heating, it might make more sense to shed the least velocity in the first pass and more in following passes. This strategy may be problematic insofar as the hydrogen needed for the return trip might boil off. If heat sinking is not sufficient, even for lower heat loadings like these, I'd stand by low ballistic coefficient. And again, coming from the Moon it's always safer to aim high and do another pass if you need.

GW Johnson · 2013-08-09 23:13:58

What I found playing with that oversimplified LEO entry ballistic analysis is that low ballistic coefficient lets you decelerate higher up where density is lower. Doing a minimal de-orbit burn from LEO inherently gets you a very shallow entry angle, which also helps you do the peak deceleration higher up.

Peak heating rates occur close to, but not concurrent with, peak deceleration. They are directly proportional to density^0.5, inversely proportional to "nose" radius^0.5, and directly proportional to velocity^3. Choosing the right trajectory and a low ballistic coefficient get you the lower density effect. Choosing a very blunt shape gets you the "nose" radius effect, and both effects are quite strong at 0.5 power.

But that velocity effect is so strong, it kind of rules out clever peak heating reductions for near-escape entry speeds at Earth. You're basically stuck with piling on the ablatives, and trying desperately to shallow-out enough to reduce peak gees, while not bouncing-off into deep space. (Yep, I know for a lunar return, you're barely sub-escape, so you don't bounce-off on a one-way trip. But, the return time is measured in many months to a few years.)

At Mars it's different. The velocity effect is scads weaker. 5.6 km/s for the typical direct entry from an interplanetary transfer orbit, about 5 km/s escape, and about 3.6 km/s from LMO. It's a whole lot easier to get away from sacrificial ablatives there. Really tough to do that here, with escape at 11.2 km/s. (BTW, an interplanetary free return from Mars is about 15-16 km/s at entry.)

I'm kind of surprised to see 3-4 gees as the recommendation for people, when there have been roller coasters pulling civilians to 5-6 gees for decades now. The coasters are a 10-15 sec transient at peak gee, and so is the peak gee pulse during entry. Recline the seats a tad, and it's not very tough, even for the sick or injured.

I'd consider a winged spaceplane for some people with demanding circumstances, maneuvering capsules for "general use" (people and cargo). A desert capsule landing with a mile or two circular error probable is actually a very inexpensive recovery, not much different from recovering an airplane that landed in the desert. I don't think I'd attempt a very large spaceplane; we sort of found out that was a bad idea, with shuttle.

As long as the spaceplane has fixed wings, you're faced with low AOA during entry, so as not to rip the wings off, and so also the related need for ablatives on the nose and leading edges. No way around that here, not with the materials and technologies available at this point in history.

You're also faced with the very poor landing characteristics of delta wings: 200-300 mph, and AOA near 40 degrees, at touchdown, while riding the rather-"iffy" leading edge vortex to prevent immediate catastrophic stall. Sink rates are inherently high because subsonic L/D is so very poor. And, basic stability is quite difficult to achieve, due to massive cp shift across the speed of sound.

There's a very good set of reasons why the F-102/F-106 fighters and the B-58 bomber were not in service very long. I just gave them to you in the previous paragraph. Shuttle suffered from the same difficulties, in addition to all the other difficulties that are better known.

GW

JoshNH4H · 2013-08-10 19:11:39

Something else that I was meaning to ask you in the last post, but forgot to: What's AOA? Angle of Approach?

I understand what you mean now. The cubic velocity effect is so strong that entering higher in the atmosphere with a lower ballistic coefficient just isn't enough to make the heating situation nicer. For the same craft, entering inbound from the Moon will result in peak heating 2.8 times higher. This corresponds to entering approximately 8.3 km higher in the atmosphere, increasing the "nose" radius by a factor of 7.8, or some combination of the two. It seems like a better idea to use an increase in height more than an increase in size to reduce the loading.

I don't quite understand how atmospheric trajectory modification (which can't add energy to an orbit) of an orbit with a period of 7-8 days can result in an orbit with a period measured in months. It seems impossible to me. Is there some physical explanation for such a result, or was it just a (potentially erroneous?) estimate? The worst case scenario in which the craft bounces off the atmosphere on a similar orbit as the one it approached it on, thus adding eight days to the transit time, doesn't seem that bad to me. I would expect that some course corrections would be necessary, but not major ones if done at the right time.

According to NASA, Apollo 8 entered the atmosphere at velocity of 24,696 mph which corresponds to 11.04 m/s. Just to get some experimental values.

Now, in the interests of reusability, ablative heatshields are not ideal. As I mentioned in another post, heat sinking with multiple passes is one way to go about things, if it works. Alternatively, a higher Lift to Drag ratio, in combination with more drag (resulting from a high ballistic coefficient) will make it possible to stay in the upper atmosphere for longer and shed the velocity there where the heat fluxes will be lower.

Velocity may increase the heat transfer in proportion to its own cube, but fortunately for heat shields that re-radiate the flux, they will do so in proportion to the fourth power of the temperature. This means that even a 2.8-fold increase in heat transfer can be cancelled out by a 1.3-fold increase in temperature. That's not to say that this increase is negligible, because it's not.

You make a reasonable argument about heat shield shape; Can you suggest a better one?

On the whole, the issue here seems to be that I don't know what I'm talking about. Can you point me towards a reference that I can use to educate myself on the matter?

GW Johnson · 2013-08-11 09:12:10

Hi Josh:

I’m sorry, AOA means “angle of attack”, which is aircraft engineering and pilot’s jargon. It’s the angle between your craft’s reference axis and the relative wind vector, zero when you are streamlined to the flow. For an airplane, this reference is usually the fuselage body axis or the main deck line. For a capsule or gun projectile shape, it is usually the axis of symmetry.

What you say about not “bouncing-off” into a longer-period ellipse is quite true of a two-body problem, considering only the Earth and the moon. The trouble with near-escape speed problems is that the orbital mechanics is really a multi-body problem. This situation is adequately explained as a three-body problem: Earth – moon – sun. At near-escape speeds, the sun’s perturbation can “strip” the craft free of the Earth-moon barycenter and place it into a sun-centered orbit, which would have a period grossly similar to that of the Earth-moon system in its orbit about the sun. I remember “them” (both the TV news and the NASA spokespersons) talking about this during the Apollo years.

As for educating yourself about entry dynamics and heating, do what I did. Go find the Justus & Braun EDL paper on the internet. It covers multiple related topics, and is very useful for descriptions of “average atmosphere” conditions on several bodies of interest. That paper also describes the old Julian Allen entry ballistics model, and gives lots of sample results with it. I also remember seeing that model in class when I was a college student at UT engineering school decades ago. But, those notes disappeared long ago. I had to re-learn it from Justus & Braun, and other sources on the internet.

As for Justus & Braun, their version looks OK for the dynamics, but they did not understand Allen’s heating correlation properly. They use the wrong value for the proportionality constant, and inconsistent values by orders of magnitude between the closed-form peak rate equation and the closed-form integrated-total equation. I fixed this when I put that model into spreadsheet form, and added a numerically-differentiated timeline, and then used it to numerically-integrate total heat absorbed from the instantaneous rates. I used the old 1950’s Imperial-units constant, and very carefully converted it to metric for this spreadsheet. The heating rate equation is a dimensionally-inconsistent empirical correlation. It’s pretty reliable, it was the closed form estimate equations that I could not trust.

I’ve got that stuff posted somewhere on “exrocketman”. If you want, I can email you a copy of that spreadsheet model. It’s a 2-D Cartesian thing that gets remarkably close to Apollo’s 11 peak gees when coming home from the moon at the max allowable 2 degrees below horizontal. So I presume, since I avoided all the closed-form estimates, that the instantaneous-peak and numerically-integrated total heating numbers I got for Apollo are also reasonably accurate. At least to around +/- 20%. Ballpark.

The heating in that model is really intended for non-lifting craft. When you fly a capsule off-angle for L/D 0.1-ish lift, the stagnation point moves off-center of the heat shield, and the stagnation heating is a little different than you would predict from that old correlation (which is only an approximate to begin with). With a winged craft or a lifting body, things are so different that the old simple model is the wrong model to be using, mainly because the trajectory dynamics are wrong. Although, at the actual dynamics, the heating correlation still predicts a rough-but-good approximation to nose cap tip, and leading edge, heating rates. Your peak and integrated totals are wrong with the Allen model, because the trajectory is wrong.

This old Allen model was developed for warheads coming in fairly steeply, and it did a really good job. At shallow angles, you get into the mismatch between 2-D Cartesian and true spherical coordinates, and (worse) you get into the error induced by ignoring gravity as “small”. That second one means the trajectory angle at end of hypersonics (Mach 3-ish for a blunt capsule) is predicted way too small (actually, it was assumed constant); in the real world, post-peak-gees is where you start bending significantly downward.

But to zeroth/first order, it’s a pretty good model, even at shallow angles, as long as you are pretty close to zero-lift ballistic. I take its horizontal range prediction, and simply wrap that around the circular girth of the Earth, Mars, or whatever body. I just use its predicted end-of-hypersonics altitude as-is, and (for capsules) then assume a trajectory angle “near 45 degrees downward” at end of hypersonics. Rough-and-ready, but not that bad a prediction. End of hypersonics is local Mach 3-ish for blunt shapes. Closer to Mach 5 for “pointy” things.

That simple entry model is completely inadequate for aerocapture or aerobraking applications. That stuff is just too far away from “steeply-entering warheads”. For that, one needs a real 3-body computational orbital mechanics model, and a real computational trajectory program in spherical coordinates.

It can be 3 degrees of freedom for nonlifting ballistic “particles”, but you need the full 6 degrees of freedom for lifting craft, because they can roll and yaw, as well as pitch. You’ll need a sequence of aerodynamic models from hypersonic to subsonic, and in the case of lifting craft, some model of how you will “fly” your entry. That said, I don’t have any such models in my possession, although I used one like that when I worked on the “Scout” launcher at LTV Aerospace back in the 1970’s.

So, that’s why I seem clueless when aero-capture or aero-braking questions come up, yet so informed when ordinary ballistic entry questions come up. I have the tools to figure one, but not the others.

GW

tahanson43206 · 2019-05-17 21:19:40

For JoshNH4H ...

In a recent post in Louis topic about Lunar Nickel Mining, you closed a post with this:

Launch methods are a topic for another thread. As far as entry vehicles, I favor woven basalt fiber as a thermal protective material

I asked FluxBB if you had discussed this before, and learned that there are numerous posts which contain the words woven and basalt.

I looked at several of them, and noted the exchanges you had with several forum posters, including GW Johnson

I selected this one as an anchor for what I hope will be an interesting digression on the theme of heat shield design.

JoshNH4H wrote:

Something else that I was meaning to ask you in the last post, but forgot to: What's AOA? Angle of Approach?
<snip>
According to NASA, Apollo 8 entered the atmosphere at velocity of 24,696 mph which corresponds to 11.04 m/s. Just to get some experimental values.
<snip>
You make a reasonable argument about heat shield shape; Can you suggest a better one?
<snip>

Traditional heat shield design (as far as I can determine) takes as a given that when molecules of gas encounter a moving solid object, such as a heat shield for a spacecraft returning from orbit, the individual molecules interact with one or more molecules of the oncoming solid object in such a way that the atmosphere molecules bounce away from one or more molecules of the solid object. As a matter of conjecture on my part, I would imagine the individual molecules also interact with other atmosphere molecules as all of them experience random accelerations with respect to the solid, and with respect to each other. The sum of all the interactions is observed by humans as heat and light, and acoustic manifestations of one sort or another.

A portion of the momentum of the oncoming heat shield is given up to the atmosphere molecules as they are accelerated away from the solid.

The force at work in all these interactions (as I understand the physics of the situation) is the electrostatic force, as maintained by electrons in stable shells around the nuclei of the atoms comprising the respective molecules.

The purpose of this windup is to set the stage for the question I would like to pose:

Is it possible to capture the molecules of gas so they do not bounce away from the heat shield?

For example (as just one possibility) can a heat shield present to molecules of gas a matrix of tiny tubes into which the molecules can enter, but from which they cannot exit?

(th)

SpaceNut · 2019-05-18 08:17:04

Friction heating is the need for the heat shield.

Trying to keep a boundary layer requires that the area would have a greater volume and pressure than you are forcing against.

The transpiring heat shield is that type of concept in that it is using a porous skin which has water molecules under pressure being forced out of the holes to keep a boundary of evaporation of heat occurring away from the heat shield.

tahanson43206 · 2019-05-18 08:41:11

For SpaceNut ...

Thank you for sharing a common misunderstanding that would prevent progress if it were allowed to continue.

The term "friction" is most applicable to a situation where two solids are interacting with each other.l

In an interaction between a solid and a gas, each and every collision is perfectly elastic. That is because the electric fields created and maintained by electrons in shells around nucleii are themselves perfectly elastic. When a molecule of gas collides with an atom in a matrix of a solid, the collision is perfectly elastic up to the point that the energy of the collision breaks through. My understanding is that for most collisions of this type, starting with water flowing over rocks in a stream as an example of liquid interacting with solid, through the most extreme collision of a meteor with the atmosphere of a planet, the forces at work are insufficient to allow any atom to penetrate the electron shells of any other.

The purpose of THIS initiative, within the Atmospheric Entry topic, is to invite consideration of an idea that is not present in the literature I have seen, because we humans tend to go with the observations we make about nature. In the case of interactions of solids or gases with solids, those observations go back thousands of years. It was only recently that humans discovered (worked out) the mechanics of those observations.

To repeat the question: Is it possible to interact with atmospheric molecules in such a way that the molecules are (a) captured and (b) NOT allowed to bounce away and contribute to heating of the atmosphere in front of the moving solid?

SpaceNut wrote:

Friction heating is the need for the heat shield.
Trying to keep a boundary layer requires that the area would have a greater volume and pressure than you are forcing against.
The transpiring heat shield is that type of concept in that it is using a porous skin which has water molecules under pressure being forced out of the holes to keep a boundary of evaporation of heat occurring away from the heat shield.

(th)

kbd512 · 2019-05-18 08:58:15

tahanson43206,

Short answer is "no". The collision is what generates the heat. It's conversion of kinetic energy into thermal energy. That is also what happens when a bullet punches through armor. A very thin layer of the armor is melted / liquefied by the incredible heat generated by the force of the penetrator impacting the armor. Even if the armor is strong enough to stop the penetrator cold, it still drastically increases local heating of the portion of the armor that was struck. Those of us who shoot guns at steel targets are familiar with this effect. The spot on the steel plate where the bullet struck or penetrated is hot enough to burn you in some cases.

If the molecule of fluid (gas or liquid) was not permitted to rebound away from the solid structure that it collided with, then heat would accumulate in the solid structure until it melted or thermal equilibrium was established. The solid structure would also necessarily become heavier by absorbing the fluid it travelled through. The best known ways to inhibit heat transfer are either by using a material that has exceptionally poor thermal conductivity or by electrostatically "repelling" some of the fluid molecules (ionization of the fluid).

tahanson43206 · 2019-05-18 10:17:19

For kbd512 ...

Thank you for taking up this challenge!

For the sake of forum readers who may wish to think about this question, I am hoping you will be willing to invest a bit more time developing/expanding upon your response.

Your "short" answer could be backed up by a longer answer, such as:

No, to the best of my knowledge.
No, as I understand physics.

Your short answer runs the risk of being wrong, because using that format for your reply precluded the possibility you might learn something.

Please expand upon the quote below. What (for your readers) is your definition of "heat" ? You've already shown the way forward with your clarification that kinetic energy transitions to thermal energy.

(th)

kbd512 wrote:

tahanson43206,
Short answer is "no". The collision is what generates the heat. It's conversion of kinetic energy into thermal energy.

kbd512 · 2019-05-18 10:17:22

Josh,

In the future, there simply won't be as many uses for Aluminum alloys or steel as CNT and Graphene supersede legacy metal technologies in most applications. If you were going to use significant quantities of steel, then it'd make a lot more sense to mine it where you intend to use it. If there's no steel where you want to live, then that's a clue that it's time to consider alternatives. There's no shortage of Iron on Earth or Mars and there probably never will be.

Carbon-based foams and fabrics would become the heat shield materials of choice if cost, weight, and availability of raw material are important considerations. The ubiquity of Carbon makes fabrication of these reentry heat shields possible on the Earth, moon, Mars, Venus, and most of the asteroids we'd want to mine. The production equipment required is also very compact. The ultimate durability of the material is less important. If a Carbon ablator heat shield was embedded in the ground, then we'd just leave it there and make a new heat shield. We only care about the metal payload that the heat shield protected.

For sake of argument, let's say that we wanted to ship things that we simply don't have much of here on Earth, such as Nickel or Platinum group metals. We're going to form those materials into slugs, coat the exteriors with various Carbon ablators, shoot them on a return trajectory to Earth with a rail gun, and use small propulsion stages to control their reentry into the atmosphere of Earth or Mars. The use of propellant-based propulsion systems of any kind has to be absolutely minimized to control cost.

kbd512 · 2019-05-18 10:34:11

tahanson43206,

Since this is Josh's specialty, perhaps he can provide a better answer.

The impacts or transfers / re-direction of kinetic energy between molecules of matter, in the case between the molecules of heat shield material and the gas molecules of a planetary body's upper atmosphere, throws off / re-radiates photons with thermal / infrared energy spectra. Some of that photon energy is radiated in other directions and some of it is radiated towards the heat shield, which "heats up" the heat shield.

A quick read for the definition of infrared radiation:

Definition of Infrared Radiation

SpaceNut · 2019-05-18 10:42:38

Matter for every element is made up of solid mass even in gasseous forms, that is why the kenetic energy is allowed to obsorb and transfer as heat.

tahanson43206 · 2019-05-18 18:19:23

For SpaceNut and kbd512 ...

Thank you both for contributing to this initiative to try to see/understand the functionality of heat shields at a detail level.

I'd like to go for broke here, and toss out the idea (building on kbd512's vision of throwaway carbon shields) of a business serving the space going community, of rental heat shields. The rental company would provide extremely light weight heat shields to match trajectories of incoming vessels or shipments. The business would be similar to container shipping containers which have become so popular on Earth. Shippers planning delivery to Earth would plan ahead so that the vehicles or shipping pods would fit snugly into the rental heatshield vehicle.

After achieving deceleration through the atmosphere as specified by contract, the rental heat shield would release the payload and transport itself to a refurbishing facility.

I am thinking here of an advanced version of the glass foam which was successful in protecting the Space Shuttle. As I understand the technology, the glass foam used for the Shuttle was capable of holding gases it encountered during descent, and NOT allowing those gases to pass through to the metal hull of the Shuttle.

In several earlier posts, going as far back as 2013, JoshNH4h has offered woven stony material (basalt) as a candidate for use in heat shields.

I am wondering of such material might function in a manner similar to the glass foam used for the Shuttle, by collecting the molecules of atmosphere within the interior of the weave, and NOT allowing those molecules (or the kinetic energy of those molecules) to pass beyond the weave to the hull.

(th)

SpaceNut · 2019-05-18 19:21:38

The Pica and Pica x are replaceable heat shields made from Phenolic Impregnated Carbon Ablator for use to come back to earth and for mars.

Dragon’s PICA-X heat shield protected the spacecraft during reentry from temperatures reaching more than 3,000 degrees F.
The PICA-X heat-shield material subjected to temperatures of up to 1,850°C (3,360°F) at the Arc Jet Complex at NASA Ames Research Center, Moffett Field, California.

But for Orion use of PICA and PICA-X which would be lighter than Avcoat and also easier to manufacture. Nasa thought that PICA which is brittle and would require the use of tiles which leads to gaps in the heat shield. NASA was worried about these gaps so they went with Avcoat which can be applied in a single gapless piece.

The Space Shuttle thermal protection system (TPS) is the barrier that protected the Space Shuttle Orbiter during the searing 1,650 °C (3,000 °F) heat of atmospheric reentry.

The shuttle tiles while they were heavy did mean they were reuseable so long as they stayed in tact to the under layer fabric that held them from the ships surface. The thermal blanket material kept the tiles temperature isolated from the shuttles aluminum hull. There are several tiles in this system all having different properties as to where they can be placed on the ships underbelly.

Most of the tiles are made of silica fibers, which are produced from high-grade sand. Silica is an excellent insulator because it transports heat slowly. When the outer portion of a tile gets hot, the heat takes a long time to work its way down through the rest of the tile to the shuttle’s skin.

https://ntrs.nasa.gov/archive/nasa/casi … 016878.pdf
Thermal Protection Materials: Development, Characterization and Evaluation

https://www.nasa.gov/centers/johnson/pd … 82-199.pdf
Thermal Protection Systems - NASA

https://ntrs.nasa.gov/archive/nasa/casi … 022148.pdf
Ablative heat shield design for space shuttle - NASA

https://en.wikipedia.org/wiki/Space_Shu … ion_system

tahanson43206 · 2019-05-19 12:25:13

Today I'd like to add to the topic (ISRU Atmospheric Entry) a development of a possibility brought up in post #23 .... the possibility was for rental heat shields.

A logical extension of that idea is a business comparable to the tugboat industry, which (according to www.briannica.com) was invented in 1736 in England.

As vehicles move back and forth between orbit and the surfaces of planets, a business opportunity will exist for those who chose to specialize in the capability to bring a deep space going vessel safely to the surface.

The company (of which I am sure there will ultimately be many) will bid for the opportunity to attach a landing craft to the deep space vehicle, to take it through atmospheric deceleration and on to a landing site.

In order for this concept to work, it will make sense for dimensions of vehicles to land to become standardized, just as commercial containers are standardized today on Earth.

However, as discussed in multiple posts in other topics as well as this one, the BIG market for this service would (presumably) be delivery of arriving payloads from the asteroid belt (or other celestial sources) to be delivered safely to the surface of the destination planet.

In that case, the miner would add a radio tag to the payload and send it toward Earth, after signing a contract with a capture and land company.

The landing company would secure a percentage of the value of the payload, and (since it would make sense to combine functions) it would (most likely) attend to the details of securing the best price for the payload.

(th)

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