Starship is Go...

GW Johnson · 2019-02-17 10:40:08

Point 1: Landing is not the worst problem, refilled takeoff is the problem. Takeoff weights, figured at local gravity, are roughly 5 times higher than landing weights. Landing weight times 2 (to cover the dynamics of touchdown) is still the smaller applied bearing pressure, of the two conditions. That being said, 3 landing pads ~ 1 m diameter is too small even just to land safely across the bulk of Mars's surface, and by a factor near 8.

Point 2: large fixed-geometry landing pad surfaces, whether discarded on takeoff or not, will seriously interfere with the entry hypersonic aerodynamics, shedding shock waves that will destroy adjacent structures by shock-impingement heating, not to mention upsetting aerodynamic stability and control.

Point 3: even if you solve the entry aerodynamics and heating problems, you must add heat shields to these landing pad structures to survive entry. By the time you have done that, you have very likely added enough weight to have covered hydraulically-opened and folded landing pad surfaces.

These reusable folding landing pad surfaces would resemble landing gear doors, built into the trailing edges of fins that contact the surface all along their trailing edges, not just at the tips.

I've already been through all the calculations that demonstrate much larger landing pad surfaces (around 45-46 sq.m total) will inevitably be required. These are posted over at my "exrocketman" site. If you go look, that same article shows a sketch of what I am talking about (folding landing pad surfaces). Such surfaces are ~ factor 20 larger in area than anything Spacex has considered so far, if the bulk of Mars's surface will be feasible for landings and takeoffs. Same for the moon.

This folding-pad idea is really very little different from the fold-out landing legs Spacex uses on its Falcon cores, or originally proposed on its earlier versions of the BFS design. Except, that I rearranged the geometry to get very large pad surfaces in an easy-to-build design, which fold to provide no aerodynamics or heating problems, and which require no special heat shielding.

They (Spacex) didn't do that, which suggests to me they have not yet thought their way through the surface safe bearing capability problem.

It's something they will have to face, right here on Earth for off-site emergency landings, as well as landing on unprepared surfaces on the moon and Mars. They obviously have not done that yet.

GW

Last edited by GW Johnson (2019-02-17 10:46:25)

Oldfart1939 · 2019-02-17 20:59:43

SpaceX seems to have put themselves into something of a quandary with the size of the vehicle--especially the overall weight. Maybe "bigger isn't better?" This is one of the dangers in skipping intermediate steps of an evolutionary design program. By abandoning the red Dragon project, they may have shot themselves in the foot--or kneecap. GW has correctly pointed out the fallacy of not doing their homework thoroughly yet.

Last edited by Oldfart1939 (2019-02-17 21:00:53)

kbd512 · 2019-02-17 21:13:46

Starship probably isn't going to Mars, as-envisioned, but it could certainly launch all the pieces of a vehicle that does. Why not keep our big, shiny, and expensive rockets near Earth for immediate reuse and just build a purpose-built landing craft that only have to get the humans back into orbit?

GW spoke of a reusable landing craft that was purpose built for Mars. If we have reusable rockets with massive payloads, then why can't a specialist variant of the BFR technology set be used for that purpose?

GW, if the landing craft only had to survive reentry from Mars orbital velocity, how much lighter could the heat shield be? Could it be a piece of replaceable fabric, like HIAD or ADEPT, or maybe even a high temperature ceramic that could live through several ferry missions?

Someone needs to get Blue Origin, Boeing, LM, NG/OATK, and/or ULA to contribute to the resource pool. NG did the on-orbit satellite servicing on their own time and their own dime. Someone needs to remind these companies that you have to invest to get a ROI.

SpaceNut · 2019-02-18 11:19:31

All are investing in R&D for individual gain and where there are some buying the technology from Nasa to base the work on for developing the product that we want but they do not like to share.

Gw did not think the shield after multiple uses for mars would hold up but for one earth entry at best and trying to slow down for earth orbit requires lots more fuel in order to switch ships.

We also know trying to stage in mars orbit a return ship runs into a simular effect of requiring fuel we do not have to slow.

kbd512 · 2019-02-18 12:51:58

SpaceNut,

I'm talking about the heat shield holding up to multiple uses under a 3.4km/s orbital velocity, not interplanetary reentry velocity. If an electric propulsion system is used to achieve a stable low orbit around Mars, then the reentry heating is far less extreme, there's far less forward velocity to kill using aerobraking, and the vehicle that ferries astronauts and cargo to the surface should be capable of making multiple trips. I'm basically talking about a 1.5x diameter (13.5m) "short stack" version of BFS, like StarHopper, only wider and using conventional passive heat shield technology with ceramics and fabrics.

This vehicle will have a wider landing gear track and be significantly shorter to improve rough field stability and cargo offload operations. StarHopper would self-deploy to orbit with no payload, be loaded and refueled in LEO by a conventional Starship BFS-C and BFS-T (refer to kbd512's StarShip variants), and subsequently make the transit to Mars robotically using an electric propulsion module that subsequently returns to Earth. The humans would make the transit to Mars in an inflatable habitat module put in orbit by BFS-C. In LMO, the humans would transfer to StarHopper. This MOR architecture is a slightly more sophisticated variant of LOR that reusable systems enable.

The lack of extreme reentries should enable multiple missions by both StarHopper and an all-electric MTV. The MTV uses a combination of ArcJet (low gear) and X3 technology (high gear) from AeroJet-Rocketdyne. In LMO, the electric tug that took StarHopper to Mars will serve as a backup propulsion system and secondary source of propellant for the MTV. When StarHopper returns to orbit, it will carry a small payload of Argon, LOX/LN2, and H2O from the surface of Mars to replenish the MTV's propellant and consumables.

If it's not totally clear, StarHopper is going to Mars and never coming back. Repair components for StarHopper will be periodically shipped with the MTV. Only the MTV comes back to Earth. A fleet of 3 to 5 StarHoppers will be forward deployed to Mars. The first payloads are the LOX/LCH4 plant, a LOX/LCH4 transfer vehicle / mobile surface base, and enough food / medical supplies / spare parts to skip a launch window and still keep the Mars exploration crew alive. The spares are intended to keep the life support consumables and propellant production plant operational at all times.

It's just my opinion, but I think it's too costly to constantly deliver propellant everywhere. On-orbit refueling should definitely be used when required, but the cost of doing that must be considered. Some kind of hybrid chemical-electric propulsion solution is required to keep the costs manageable.

Void · 2019-02-18 13:48:01

Yes, I think the direction to take.

Done.

SpaceNut · 2019-02-18 19:13:59

The "electric propulsion system" does not work until you do AG for the duration to which Space x is not for crew transport...

GW Johnson · 2019-02-22 13:06:26

I ran some one-stage reusable Mars lander designs and posted the results at "exrocketman". These were landers operated for the two-way trip between low Mars orbit and the surface, on one fill-up. I think they would fit inside a cargo BFS/Starship, or at least it's pretty close, where one could adjust the design to fit.

Reentry from low Mars orbit is less extreme than from above-escape direct from the interplanetary trajectory, yes. Speed at interface is about 7.5 km/s direct, for an effective plasma driving temperature in the vicinity of 7500 K. From low orbit, speed at interface is closer to 3.5 km/s than 3.4, but that's crudely a 3500 K driving temperature. Less extreme than direct, but still quite extreme. You do this with a real heat shield.

Ablatives we have now, including ablatives that survive a single handful of entries. We have had them since the Gemini B test flight in 1969. We certainly have better ones today. And that includes PICA-X. The foldable fabrics and inflatables are working in still-highly-experimental tests. They are NOT YET READY for general application! Once they are ready, they'll be better. Meanwhile just go with what you know for sure already works. Simple. To the point.

As for a transportation system to Mars, you "dance with who brung you", and add the extras later. Nothing else makes any sense, unless you want to wait another generation or two to go. Just like for entry heat shields.

There are large chemical rocket engines ready to fly, and some existing whole stages currently flying. There are not any large electric propulsion items available yet. There are some small ones, but we are talking big vehicles here to go to Mars. Small electric items are useless for that. You MUST have big ones. When they become available, use them, they will be better. Until then, use what you have ready-to-fly right now.

Artificial spin gravity can be worked into most any design, especially those that coast long durations between impulsive burns. The only "trick" is using things you already must have to take anyway, to be your spin structure. If rigid, there's very little development to do, unlike cable-connected systems. Just get on with it.

Same goes for radiation shielding: just get on with it. Use things you already have to take anyway as your shielding. Such as propellant. And water. Maybe even frozen food.

We can argue all day about details. But the point is, what we can do to go "right now" is already known, and there exist technologies and equipment with which to do it. No new developments, or you'll never fly: that's the history lesson here.

There's a private company or two out there that seem willing to follow this approach and just get on with the job, using what is already available for general application. NASA does not seem willing to do this, and hasn't been willing for a long time now. They adamantly refuse to address the crew survival-critical issues of providing artificial spin gravity, and adequate radiation shielding for a large solar flare event. Until that changes, they will continue killing crews, whenever they do get around to flying outside LEO.

GW

Last edited by GW Johnson (2019-02-22 13:16:44)

louis · 2019-02-22 17:12:52

GW - Did you review this video (I posted a link to it a few posts back)?

https://www.youtube.com/watch?v=LogE40_wR9k

I thought it was an excellent exposition of how Space X intend to deal with entry heating and it all looked good to me. There's certainly more to cooling than ablative shields. Space X appear to be adopting a multi-factorial approach.

There is no suggestion Space X will opt for spin gravity and I think that decision is completely correct. Bone and muscle wastage in zero G has virtually been eliminated I recall from most recent discussions here. Yes, there are risks associated with Zero G but there are risks associated with artificial gravity as well. Good crew selection will further diminish the risks for Zero and Low G survival.

GW Johnson wrote:

I ran some one-stage reusable Mars lander designs and posted the results at "exrocketman". These were landers operated for the two-way trip between low Mars orbit and the surface, on one fill-up. I think they would fit inside a cargo BFS/Starship, or at least it's pretty close, where one could adjust the design to fit.
Reentry from low Mars orbit is less extreme than from above-escape direct from the interplanetary trajectory, yes. Speed at interface is about 7.5 km/s direct, for an effective plasma driving temperature in the vicinity of 7500 K. From low orbit, speed at interface is closer to 3.5 km/s than 3.4, but that's crudely a 3500 K driving temperature. Less extreme than direct, but still quite extreme. You do this with a real heat shield.
Ablatives we have now, including ablatives that survive a single handful of entries. We have had them since the Gemini B test flight in 1969. We certainly have better ones today. And that includes PICA-X. The foldable fabrics and inflatables are working in still-highly-experimental tests. They are NOT YET READY for general application! Once they are ready, they'll be better. Meanwhile just go with what you know for sure already works. Simple. To the point.
As for a transportation system to Mars, you "dance with who brung you", and add the extras later. Nothing else makes any sense, unless you want to wait another generation or two to go. Just like for entry heat shields.
There are large chemical rocket engines ready to fly, and some existing whole stages currently flying. There are not any large electric propulsion items available yet. There are some small ones, but we are talking big vehicles here to go to Mars. Small electric items are useless for that. You MUST have big ones. When they become available, use them, they will be better. Until then, use what you have ready-to-fly right now.
Artificial spin gravity can be worked into most any design, especially those that coast long durations between impulsive burns. The only "trick" is using things you already must have to take anyway, to be your spin structure. If rigid, there's very little development to do, unlike cable-connected systems. Just get on with it.
Same goes for radiation shielding: just get on with it. Use things you already have to take anyway as your shielding. Such as propellant. And water. Maybe even frozen food.
We can argue all day about details. But the point is, what we can do to go "right now" is already known, and there exist technologies and equipment with which to do it. No new developments, or you'll never fly: that's the history lesson here.
There's a private company or two out there that seem willing to follow this approach and just get on with the job, using what is already available for general application. NASA does not seem willing to do this, and hasn't been willing for a long time now. They adamantly refuse to address the crew survival-critical issues of providing artificial spin gravity, and adequate radiation shielding for a large solar flare event. Until that changes, they will continue killing crews, whenever they do get around to flying outside LEO.
GW

GW Johnson · 2019-02-22 21:45:50

Louis:

Yes, I looked at the video. He says some things that are right, and some that are just dead wrong. Because he is not an engineer who deals in heat protection (I have done this).

The "reflect the heat away" claim is just nonsense up to entry interface speeds at or above 10 km/sec. Mars direct entry is ~ 7.5 km/s, very comparable to Earth LEO entry at 8 km/s. Having a shiny surface reduces your ability to survive, because shiny is low emissivity, which drastically cuts your ability to radiate away heat to the environment.

The X-15 and the SR-71 were black, not shiny, and for a very good reason. They literally cooled by radiating away the friction heat. The dark surface of charred ablative heat shields performs the same function.

Cold-forming stainless or any other metal is not done at cryogenic temperatures. It is done at room temperature. That is different from hot-forming, which is done at some furnace temperature. Your everyday astronaut guy got that wrong, too.

SS310 is free-of-scaling for repeated high temperature exposures up to 1900 F. Its strength is pretty much gone at about the same temperature as any other stainless, including SS301: around 1000-1200 F. If you don't believe me, go look it up in Mil Handbook 5, or in the manufacturer's data sheets on line.

Titanium has lost its strength at ~ 750F. Aluminum is butter at about 300-350 F. Organic-matrix composites (graphite or other fiber) are all junk at about 200-250 F. Yeah, stainless is stronger hotter. So is Inconel. So is 17-7 PH. And so is Alloy 188. All but stainless are heavy and expensive (stainless is heavy, but cheap). Alloy 188 will not quite slump at 2000 F, the others are useless somewhat lower.

Besides, using methane or any other hydrocarbon as a liquid cooling agent requires far lower surface material temperatures during entry, just so as not to coke-out carbon crud all over your passage walls. That does tend to plug them up. You also have to have very high pressures to keep your liquid from boiling as it absorbs the surface heat. Gas phases are lousy coolants compared to liquids.

There is absolutely NOTHING on Spacex's official website about stainless anything for BFR/BFS/Starship. I find YouTube and Twitter less-than-reliable sources for factual information (demonstrably more liars than truthers out there). The new "grasshopper" style test vehicle in South Texas is stainless for convenience handling cryogens, which as yet says NOTHING about the design of BFR/BFS/Starship.

However, a stainless design with liquid cooling probably could be made to work as a reusable item. It would need a surface coating that is "black" (high emissivity = low reflectivity) in the shorter-wave infrared, in order to shed significant heat by radiation. That reduces greatly the heat that must be removed by the liquid coolant.

Whether transpiration (sweating) is really needed is another matter. That has been tried without real success many times over the decades. It requires a whale of a lot of expelled fluid to do any good. Usually too much to be really feasible, either technically, or economically.

We'll see what really transpires at Spacex over the coming months. The design has been evolving with radical changes on relatively short timescales, as has been very reliably reported on their official website. I have absolutely no reason yet to think that radical changes will not continue.

By the way, the old X-20 Dyna-Soar spaceplane that was cancelled in 1963 before it could fly was a metal airframe. Inconel skin, I think it was. Cooled by radiation, plus some liquid cooling. The nose cap and leading edges were metal wire-reinforced graphite ablative. This was a 1957-ish design. It was very black in color, too.

GW

Last edited by GW Johnson (2019-02-22 21:54:25)

RobertDyck · 2019-02-23 04:07:28

GW Johnson wrote:

there exist technologies and equipment with which to do it. No new developments, or you'll never fly: that's the history lesson here.
... NASA does not seem willing to do this, and hasn't been willing for a long time now. They adamantly refuse to address the crew survival-critical issues of providing artificial spin gravity, and adequate radiation shielding for a large solar flare event. Until that changes, they will continue killing crews, whenever they do get around to flying outside LEO.

That argument goes both ways. I have pointed out Gemini 8 docked with an Agena target vehicle, then developed a thruster fault that caused it to spin out of control. Gemini 11 used a tether attached to an Agena target vehicle, used rotation for stabilization. Gemini 11 was 1966. I have proposed on this forum several times a very simple experiment, the next logical step from Gemini 11. Attach a crew capsule in orbit with a tether to a cargo ship. Rotate enough to produce significant gravity, either Mars-level gravity or Earth-level. Gemini 11 didn't spin that much. While spinning, change orbit. It doesn't have to be a big change, just enough to simulate a mid-course correction enroute to Mars. Again, do that while rotating with the tether, don't stop the spin. You can do this with timing: apply thrust to "pull" on the tether when oriented in the direction you want to apply thrust. Never "push" a tether. And apply gentle thrusts, not enough to induce "bounce" in the tether. To make this very inexpensive, use a crew capsule that is returning from ISS anyway. And use a cargo ship that has off-loaded its cargo to ISS. Standard practice is to load an empty cargo ship with garbage, the cargo ship de-orbits, burns up in the atmosphere, crashes in the ocean. Cargo Dragon actually does a controlled re-entry, but other cargo ships don't: Cygnus, Japan's HTV (one flight per year), Europe's ATV (no longer flying), and Russia's Progress. And with any test, you have to prepare for success, but also have to prepare for the possibility of failure. If something happens that requires you to sacrifice one ship to save the other, if one has crew while the other has garbage, which to sacrifice is a no-brainer. Again, this is the next logical step after Gemini 11, which flew in 1966. But NASA somehow considers this too advanced. There are people in NASA who consider Gemini 11 to be beyond their ability.

Similarly, scientists working on unmanned probes for JPL don't want to test any new hardware. They want to focus on science only, not technology. They just refuse to test ISPP. And they refuse to test ADEPT or HIAD. ADEPT is the fabric heat shield that unfolds like an umbrella, it was developed by NASA in the 1980s, included in Mars Direct in 1989/'90. It won't be considered "mature" until it's been demonstrated, but it was ready for demonstration test in 1989. And there have been at least 3 projects for an unmanned Mars sample return mission. They all start with the realization that ISPP makes it cost no more than a major mission to Mars, less than Curiosity. Then someone on the project says "you aren't testing unproven technology on *MY* mission". ISPP gets deleted, that caused the cost to skyrocket, then politicians cancel it due to cost. This has happened 3 times that I'm aware of, probably more.

Last edited by RobertDyck (2019-02-23 13:05:18)

RobertDyck · 2019-02-23 04:35:05

GW Johnson wrote:

Cold-forming stainless or any other metal is not done at cryogenic temperatures. It is done at room temperature.

Thank you. I thought I missed something. Metals become brittle at cryogenic temperature, why would you form at that? It would break rather than ductile forming.

GW Johnson wrote:

Besides, using methane or any other hydrocarbon as a liquid cooling agent requires far lower surface material temperatures during entry, just so as not to coke-out carbon crud all over your passage walls. That does tend to plug them up. You also have to have very high pressures to keep your liquid from boiling as it absorbs the surface heat. Gas phases are lousy coolants compared to liquids.

I wondered about that. Why use LCH4? Using a fuel as coolant doesn't make sense. At atmospheric entry temperature, it would ignite in Earth's oxygen atmosphere. And we already have seen carbon crud on the outside of Falcon 9 boosters after landing. Their landing engine exhaust. If you want to "sweat" for cooling, use liquid nitrogen. It won't oxidize metal like LOX, and won't produce carbon crud like LCH4. LN2 boils in 1 atmosphere pressure at -195.8°C, liquid density at boiling 806.11kg/m³. LCH4 boils at -161.48°C, density 422.36kg/m³. LN2 would be easier to handle than liquid helium. LN2 temperature is comparable to densified LCH4, and density is actually greater.

GW Johnson wrote:

Whether transpiration (sweating) is really needed is another matter. That has been tried without real success many times over the decades. It requires a whale of a lot of expelled fluid to do any good. Usually too much to be really feasible, either technically, or economically.

That's why Shuttle used tiles. If atmospheric entry speed is reduced to be comparable to Shuttle, then you can use a similar heat shield.

Last edited by RobertDyck (2019-02-23 04:37:09)

GW Johnson · 2019-02-23 10:32:14

You'll notice that the original versions of BFS were externally heat-shielded with the same PICA-X that Dragon uses. That's because entry speeds at Mars range from 3.5 km/s from low orbit through 5.0 km/s at escape to 7.5 km/s from direct interplanetary entry. Entry from low Earth orbit is 8 km/s. Those are very comparable conditions.

PICA-X as installed on Dragon is rated for more intense reentry conditions. Coming back from the moon is just about Earth escape speed at entry interface, right at 11 km/s, which is enough to trigger the plasma radiation heating that varies with velocity ^ 6. That starts about 10 km/s. Direct entry from an interplanetary trajectory from Mars ranges from 12 to 17 km/s, depending upon the trajectory used.

Dragon/PICA-X was rated for 1 Mars free return up to something near 17 km/s. It was supposed to be good for 2 lunar returns. That means it should be good for 4+ low Earth orbit returns. It's pretty good stuff.

For use on a BFS sent to Mars and back, it must survive 1 Mars free entry at ~ 7.5 km/s (about like an LEO return), and then 1 free return at Earth from Mars, probably nearer 12 km/s than 17 km/s, because they don't have enough mass ratio to fly that fast a return trajectory. That should just about use it up, though.

As for liquid cooling jackets with high surface contact temperatures, one new development needing exploration would be to use a liquid metal that cannot boil at the surface contact temperature, but holds more heat than even water, in a closed loop accepting heat from hot structure, and dumping it in a heat sink somewhere, likely a propellant tank. It can't freeze at the contact temperature in the propellant tank. There has to be enough propellant in the tank to accept the entry heating (integrated over the time of the entry transient) without any significant boil-off. That would be your landing propellant. It's not a large quantity. Tough trade.

A part of this liquid metal cooling technique has a long history as sodium-cooled valves in automotive parts, plus one sodium-cooled reactor initially installed in the old USS Seawolf in about 1958 (not so successful in a submarine environment, but lots of potential). I'm thinking something like mercury or gallium or similar. Just an idea, no design analysis behind it. Yet.

GW

Last edited by GW Johnson (2019-02-23 10:37:20)

GW Johnson · 2019-02-23 13:23:24

Further:

In liquid rocket engines, the fuel has always been the regenerative coolant not the oxidizer. Historically, fuels have always been cheaper than oxidizer. Technically, for fuels the storage volumes on board are much lower than the oxidizer volumes (so that additions have less overall impact), and the fuels generally have the larger specific heat capacities, which makes them more effective coolants.

As to forming metals, all metals pretty much can be hot-formed. That starts with processing the initial ingots at the foundry after casting. Metals that respond to heat treatment (like alloy carbon steels) do not generally respond to cold-forming. Metals that respond to cold-forming (like 300-series stainless steels) do not generally respond to heat treatment. There are exceptions, such as the trace-copper aluminum alloys, for one. Those get harder under cold-forming, then get annealed to the soft condition (the T-rating of T0). Some can be heat-treated to higher strength (the nonzero T-rating as in 6061-T6), but it's trickier to do than steels.

GW

Last edited by GW Johnson (2019-02-23 13:27:04)

GW Johnson · 2019-02-23 13:38:23

Still further:

I do not know of a metallurgical treatment or coating that would turn stainless steel to a high thermal emissivity "black" condition. A metallurgist might. If there is one. There might not be.

I do know of a black ceramic furnace paint with a very high emissivity (97%+) and a high service temperature rating (2000 F). I do not know if it has the strength and adhesion to stay on a skin exposed to hypersonic wind shear forces. It is promising, but nowhere near ready to apply.

Up to at least 10 km/s entry interface speeds, high emissivity "black" is what you want out of surfaces exposed to hypersonic wind blast. You do NOT want shiny (reflective), because that is low emissivity, and will run a lot hotter, all other conditions equal. A whole lot hotter.

THAT, and no mention of it on the official Spacex website, is why I do NOT believe the new stainless steel "grasshopper" test vehicle in south Texas has anything to say about the BFS structure and entry heat protection. I do believe that BFS design is still in flux, meaning they do not yet have a final approach defined.

There is no reason that you could not glue PICA-X to a stainless tank shell just as easily as to a carbon-fiber composite tank shell. Both are cold on the inner face, with an enormous temperature contrast to the adjacent plasma stream on the outside face. If you can solve that bonding problem for one material, you can solve it for the other.

Pretty much the same problem as bonding ceramic tiles to an aluminum substructure on the space shuttle, except that cryogenic exposures on the inner face were not involved.

GW

Last edited by GW Johnson (2019-02-23 13:41:29)

kbd512 · 2019-02-24 05:02:56

GW,

This makes no sense from a mass-efficiency perspective since the PICA-X would come in somewhere around 2,000kg for the entire vehicle, but in order to reuse PICA-X you'd have to remove the charred material for it to ablate the way it's supposed to on subsequent flights.

How do you ensure that the vehicle will behave as expected after you start modifying the shape of the vehicle for subsequent flights?

What would happen on a subsequent flight if the material loss from ablation was uneven?

How do you remove the charred material on Mars? Some sort of robot with a belt sander, maybe?

Edit:

Cerakote C-7600 can withstand at least 2,000F. SureFire makes Inconel sound suppressors rated for continuous use with machine guns that use Cerakote. After you heat the suppressor up, you can immediately dunk it in water and the coating will be fine. C-7600 is $400+ per gallon, but I doubt SpaceX cares about a few tens of thousands of dollars worth of ceramic coating.

What good is the high temperature coating if the stainless steel turns to silly putty at the temperature that the coating can withstand?

Last edited by kbd512 (2019-02-24 07:17:33)

SpaceNut · 2019-02-24 10:49:54

Materials use for design one O one when building is to test under conditions of simulations that is expected to be seen. The pica for mars multiple use is not as bad since the second and followup temperatures are lower as these would be sub orbital hops. Its the return to earth that is a question without a test being done.
If we have a viable sample return that would be proof of concept testing of the design and if landed on earth gives the test article to measure to ensure its safe for men to use..

GW Johnson · 2019-02-24 11:01:41

Kbd512:

You don't need to remove any char before using the heat shield a second time. The hypersonic fluid shear does that job very quickly.

It's my understanding from Spacex's website that the BFS heat shield in its PICA-X incarnation was capable of maybe 4+ returns from LEO, maybe 2+ returns from the moon, and only 1+ round trip to Mars. A flight to LEO is low exposure on ascent, and exposure at about 8 km/s returning. A flight to the moon and back (supposedly with no refilling on the moon, but with a full refill on-orbit in order to depart from LEO) is one exposure at just about 11 km/s. There at least were posted some hints it could take 2 such trips before replacement.

Mars is a little different. Ascent is negligible, and you fully-refill on-orbit for departure. The direct entry at Mars is well above Mars escape (5 km/s) at about 7.5 km/s, very comparable to an Earth LEO return. You must fully refill on Mars in order to come home. The direct entry at Earth is at least 12 km/s, and in an extreme case 17 km/s, depending upon how fast a trajectory you fly. Despite a lot of fervent hopes on these forums, I don't see the mass ratio in the website-posted design to return as a fast trip. I think the exposure will fall in the 12-13 km/s class, from something very little (if at all) faster than a Hohmann min energy transfer ellipse.

Between that Earth return and the LEO-comparable Mars direct entry, that pretty much uses up the heat shield in one round trip. It being a round trip flight, all the replacement is done here on Earth. You power-wash the remnants off the airframe, prep the uncovered surface, prime it, and glue new panels of PICA-X on. Easy here, not very doable off Earth.

As for the high-temperature strength of stainless being "silly putty" (and it is), with an ablative over the structure, it is the char surface that is very hot, and also "black" for high emission. The ablative is far less conductive than a metal, so depending upon the thickness chosen for the ablative, its inner face (also the metal airframe) is much cooler. If you stay under about 1000-1200 F at that interface, a 300 series stainless still has a single handful of ksi strength, and can support air loads during entry, if configured correctly.

For an exposed metal surface, it has to run much hotter in order to radiate effectively (over 1600 F usually). Such simply cannot serve structurally as well: strength is under 1 ksi, it has turned to "silly putty". If you can effectively support weak shingles, you can use metal tiles this way. But it might be easier to just cool the surface with a liquid, and accept the weight and complexity.

There is more complexity to reentry-surface liquid cooling than most folks understand. If you use a rocket fuel for this, you have to do it at very high pressure to keep the liquid from boiling upon contact with hot metal. That is exactly how it is done in regenerative liquid rocket engines. Gas phases make lousy coolants: both specific heats and film coefficients are very low compared to liquids. Might be easier with a liquid metal, but that has never been attempted, to the best of my knowledge.

Transpiration cooling for 3-4 minutes of entry heating takes a very large coolant flow rate because the cooled surface area is so large. That also means considerable mass of sacrificial coolant is required. This has long been studied and proposed, but in demonstration tests, has proved impractical, precisely because so much sacrificial coolant is required, and because the porous skin (microporosity very evenly spread, a precision fabrication problem) is so expensive and difficult to make.

The exposed shiny metal problem is very serious. The more reflective a surface is at thermal wavelengths, the less emissive it is. Less emissive surfaces must run hotter to emit the same power. Lowering that temperature (even a little) can make or break a design. That is why the surface really needs to be "black" in the IR wavelengths.

The ceramic coating you mentioned is likely quite similar to the one from Aremco that I ran across. Literally a black ceramic paint with strong adhesion and high thermal shock resistance. Somehow differential thermal expansion of ceramic and metal has been minimized. This is furnace paint, or as you indicated, a coating on a hot firearms part. If it also has the surface shear strength at its bondline to resist hypersonic fluid shear forces, then it could also find application heat-protecting exposed metal surfaces in high-speed flight. That has yet to be tested and demonstrated, though.

GW

Last edited by GW Johnson (2019-02-24 11:28:05)

elderflower · 2019-02-25 03:20:50

Reviewing these recent posts I found something that needs to be corrected. I believe it was Robert who said that metals went brittle at cryogenic temperature. This is true of metals with cubic atomic grain structure, but not of Hexagonal close packed atomic grain structure which remains ductile. Main examples are copper, Aluminium and Austenitic steels, which include 3%,6% and 9% Nickel steels and 300 series stainless steels. 3% nickel is not so good when welded due to depleting the weld area of Nickel, but 6% is popular for things like LNG tanks when stainless is not selected. You need sufficient Nickel to maintain the hexagonal structure at very low temperatures despite depletion in welding. There are other alloying elements which also assist in austenitising steels but Nickel is the main one.

GW Johnson · 2019-02-25 09:31:58

I'm no metallurgist, but it looks like Elderflower might be. He's right about the austenitic steels (includes 300-series stainlesses) being good at cryogenic temperatures.

Weldable stainlesses generally have the suffix L added to their ID number, as in 304L being the slightly modified 304 which does not crack upon welding.

Elderflower, do you know of any metallurgical processes or coatings that blacken otherwise shiny 304L, 316L, or 347? Something that produces IR emissivity above 80%?

GW

JoshNH4H · 2019-02-25 10:07:35

Coat it with paint, rough it up with some sandpaper, spray it with teflon, allow it to rust or corrode...

Easy to make things absorb IR, harder (but not that hard) to make them reflect it

Void · 2019-02-25 13:26:43

This is not intended as an interruption of your discussions of heat shields. If heat shields are discussed, I try to learn. That is all for me and that.

Here is a recent interview with Elon Musk. It does mention stainless steel, but not heat shields.
Some of the factors that interest me in this interview are:
-He does seem to have problems expressing his ideas. Certainly not a lack of ability. I just think his visual mind travels much faster than his verbal mind. I have noticed that the people I most enjoy learning from often have this issue.
-He indicates that there is a good chance that a Moon base will happen before Mars.
-He indicates that his ship could make a good Moon base.
-He seems to favor underground hydroponic artificially lighted farming on Mars at first, but then as a romantic should he does speak of glass domes.

Here is the interview text:
https://www.popularmechanics.com/space/ … acex-mars/
Quote:

EM: Yeah yeah, you need to get there. That’s a big deal. I think Starship will also be good for creating a base on the moon. We’ll probably have a base on the moon before going to Mars.

Quote:

RD: Food. What’s the plan?
EM: I mean, the easy way to do the food would be just to do hydroponics. You essentially have solar power—unfoiled solar panels on the ground, feed that to underground hydroponics, either underground or shielded by wires, dirt. So then you can be sure that you don’t have to worry about excessive ultraviolet radiation or a solar storm or something like that. Really pretty straightforward. You could just use Earth hydroponics. Earth hydroponics will work fine.
Eventually if you terraform the planet, then you can walk around without a suit.
For having an outdoorsy, fun atmosphere, you’d probably want to have some faceted glass dome, with a park, so you can walk around without a suit. Eventually if you terraform the planet, then you can walk around without a suit. But for say, the next 100-plus years, you’ll have to have a giant pressurized glass dome.
RD: You seem unimpressed by the people who say you can’t terraform Mars.
EM: Of course you can terraform Mars. Why would they think you can’t? You totally can.

And that works for me. I am content to believe that the people who understand heat shields will understand heat shields and will do a doable plan

Thanks for that by the way.

Done.

Last edited by Void (2019-02-25 13:35:50)

elderflower · 2019-02-25 14:14:04

I'm not a metallurgist, but I've been an Engineer for a long time and had to study a lot of different things, including brittle fracture.
I'm not sure about high temperature coatings but I do recall that hot rolled stainless, the stuff supplied as bar for machining, has a black oxide coating (probably mainly FeO) on it from the mill and this adheres pretty well. I don't know whether you could produce this on sheet, or even whether you should try. It would cover any flaws and make inspection difficult. Maybe it would develop on re-entry into Earth's oxidising atmosphere, anyway.
The "L" in 304L or 316L indicates low carbon content. When welded, the Xmas versions (Noel-- ) suffer depletion of Nickel and chrome due to migration and to formation of carbides in the heat affected zone alongside the weld, even if you use high grade filler wire to prevent it from happening in the weld itself. This reduces strength and corrosion resistance.

GW Johnson · 2019-02-26 09:06:32

I've also been an engineer for decades, more in aerothermo and propulsion than materials and metallurgy. But I had to absorb a lot of that "by osmosis" to function. The more difficult the problem, the more it pays to have wide, interdisciplinary knowledge under your belt.

The metal surface heat protection problem is more severe the faster you fly. Temperatures get very large, thermal expansions get large, wind shear along the surface gets quite large. Plus, in some designs, shock waves impinge, a process that is fundamentally unsteady at small dimensions, involving very large forces applied very locally.

I don't know, but I believe, that the black surfaces on the SR-71's titanium skins, and the X-15's Inconel-X skins, were metallurgically-derived. That IR emissivity was fundamental to their survival in service, not being otherwise cooled. Titanium is not a high-temperature material, but Inconel-X is, and went to Mach 6+ on the X-15. The one flight with a white coating showed that color to be the wrong choice, an outcome overshadowed by the shock-impingement damage on that same flight.

Entry is far worse in terms of driving temperature than modest-hypersonic flight, but is far easier in terms of time: a brief transient you can heat-sink your way through, if you are knowledgeable and clever. Hypersonic flight is the fundamentally more severe problem, because you must deal with the heat balance as a steady-state problem, for the duration of your flight, which may be tens of minutes to over an hour.

GW

Last edited by GW Johnson (2019-02-26 09:07:36)

elderflower · 2019-02-26 14:18:24

Maybe a reentering vehicle could be protected by refractory metal scales, a bit like a fish. Transpiration then would occur from under the trailing edges, cooling the scale and venting as vapour to form a relatively cool skin film over the external surfaces. This was a shower thought!

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