Heat shielding

Even though heat shielding isn't too much of a problem in X-Prize class ships now, it will undoubtedly be so in the future. Would ceramic cloth offer a solution? It's supposed to withstand a peak temperature of over 3000F/1648C, which is more than shuttle's re-entry surface temperature. In 1998, that stuff did cost ~$200 per square yard though...

An example of ceramic cloth

I checked a few websites that I searched in google, and it seems 3000F is for a shuttle sized ship or smaller accept able. (if my information is correct).

For the shuttle "entry temperatures reach nearly 3,000 F" so it's really close to the peak temperature limit, but I think the second versions of the x prize ships who'll need to re-enter orbit will be smaller as the shuttle, so lower temperatures.
so I would say, yes it can be a good solution.

My only question would be : How fast does ceramic dissipate heat? All I know is that the reinforced carbon-carbon that NASA uses on the shuttles dissipate heat very efficently. They can heat it up with a blow torch and literally an instant later it is safe to touch.

I thought it was the silica tiles you can pick up red-hot with bare hands...

Anyway, the mass and thickness of the heat shielding has to be determined in a way that by the time the aerodynamic heating has stopped, the dissipating heat will not rise high enough to damage the mating points of the fuselage and the heat shield. Thermal properties of the material would have to be tested, and the shielding must be designed accordingly.

Just a layer of fabric surely wouldn't offer enough shielding, but a few layers bonded in some ceramic matrix probably would... perhaps even high quality clay would be enough.

There are some extraordinary ceramics out there. The CRC Handbook quotes one source putting the melting point of Tantalum Carbide (TaC) at 4880 Celsius. But nothing's perfect . . . all ceramics are brittle. I've seen ceramic molds used in iron casting, but you have to heat the mold red-hot before you pour the iron in. . . otherwise the temperature change will crack it. And for the reasons already discussed, the layer has to be thin. I do think, though, that with the right processing, materials like TaC can be used effectively.

The brittleness is indeed a serious problem indeed, and that's where the ceramic fibers come into the picture. Despite being ceramic, the fibers are flexible; layered within ceramic matrix, the strength and toughness of the composite would be greatly enchanced. The ceramic cloth described in the link should have structural properties close to those of regular fiberglass.

The heat expansion /shrinkage properties should be matched though, to prevent the composite from disintegrating in flight, also the thermal conductivity should be low enough to provide a required insulation at reasonable mass. I wonder if mixing microballoons into the mix would improve the thermal properties, or would it just screw up the manufacturing process.

As for heat dissipation after aerodynamic heating stops...
It's really not a problem; Jettison the heat shield.
They did this on some of the older Soviet re-entry capsules to prevent it from getting toasty inside.

That's not really a re-usable solution, I'd say. I think in the long run (and within the X-Prize contest) this is not a good solution.

Jettisoning the heat shield may be an excellent solution. Remember that the it was the overriding goal of reusability on the Shuttle that made costs higher than expendable rockets and was a major contributing factor to the catastrophic nature of the Challenger fatalities (e.g. placing the crew cabin parallel to instead of on top of the rockets.)

In my opinion, this is another case of political goals, which kill people and why private companies which take the risk and responsibility are nearly always preferrable to government bureaucrats.

In any case if the result of this design, is lower cost with improved safety, it is a good idea. Also the reusability requirement for the XPRIZE is only 90% of the non-fuel mass. That is to mean that you can replace up to 10% of the non-fuel mass.

i no expert on heat shielding so forgive me if this is a stupid question, but: as i understand it, much of the heat comes from the friction caused by going through the atmosphere. so, hypothetically speaking, if a way was found to get through slowly would the heat shielding be necessary?

Short: yes.

Coming through slowly is easy in the X Prize as your descent starts at a velocity of zero. Coming in from orbit however your velocity is already mach 25. If you carried a huge payload of fuel you could slow yourself down a lot before you hit the atmosphere.

There are heat shield alternatives... Firing a rocket engine during decent creates a plume that would protect the craft from the atmosphere in a shield of exhaust.

If you are not interested in maximum payload a more traditional option would be to put the near invincible RCC in a thin skin right aroung the craft.

I have also seen a lot of discussion about exaporative water or LH2 cooling of the shield.

Replacable or disposable shields have never been shown to be workable on a reusable craft yet. Which is probably why they are the right way to go.

i imagine that a good replaceable shield may be the way to go for a while, at least until we need shielding for more than just reentry (combat, flying near to sun, etc)

Yup...

Gotta move to Deflector Shields eventually.

Remember that this is the absolute maximum. For example, if part of their life-support system goes bad, and they've already lost 10% as the heat shield, they'll go over the limit and disqualify themsleves. Another problem is getting the thing to land safely: once disconnected from the craft, the heat shield is uncontrollable, and could land just about anywhere. One of you is going to say "well, just ditch it over the ocean like we did with the capsules". And just how much ocean surface do you think the U.S. Navy had to secure and search to recover each capsule?

electrical based armor is the way to go ultimately (assuming we won't find something better than electricity), after all, how much more efficient weight wise can you get?