From storage reservoir(s), controlled amounts of water are pumped to the areas of the Shuttle that bear the brunt of re-entry. When the area is heated by friction, the water boils and is released through perforations in the skin. The phase-change from water to steam removes heat, the ejection of the steam removes the heat permanently, and the ablated steam is also further heated by some air that would be hitting the skin, thus removing even more (potential) heating.

Benefits to the space program: payload flight weights can be normalized and c.g. balanced by auxiliary water-tanks (or lack of same); excess water can be reservoired at or near the Space Station and then used if a heavily laden Shuttle comes up without water.

For safety reasons, the new Shuttle should be designed to withstand one "dry" reentry in case of catastrophic failure of the system; the extra cost/weight of the more physically robust design is easily offset by the reduced maintenance. Of course the compartments have a safety pressure-valve and the hull compartments and pipes are dry prior to re-entry (preventing ice blockage).
-- FlyingToaster, Dec 10 2007

oh ye of little faith... http://www.theregis..._launch_april_2012/
[FlyingToaster, Mar 29 2012]

US7281688 http://www.google.c...&dq=US7281688&hl=en
[xaviergisz, Mar 30 2012]

//the new Shuttle should be designed to withstand one or two re-entries "dry" in case of catastrophic failure// So, a full conventional heat-shield, then? And why "two"?

Have you made a calculation of the mass of water that would need to be vaporised?
-- MaxwellBuchanan, Dec 10 2007


original post changed to be more understandable.

And yes, a lightweight emergency heat shielding or more thermodynamically robust intrinsic design that would *not* have to be maintained or replaced unless there's a failure of the main system on the previous flight.

While true a simplistic calculation of kinetic energy vs. latent heat of evaporation yields a ridiculously large figure, I doubt that that's even close to the same order as the amount actually required.
-- FlyingToaster, Dec 10 2007


Probably cheaper to carry the weight as a second layer of tiles rather than water. After all, if there's water left over I'd think you'd want to leave it in space, given how much effort you put in to getting it there to begin with. Lastly, it seems likely that skidding on a layer of water vapor would change the dynamics of reentry.
-- phoenix, Dec 10 2007


//You're gonna need a lot of water//

I did (a simplified version of) that calculation too, but there's also some other major factors; the kinetic energy transference is not simply water evaporating from 100C liquid to 100C gaseous.

- heating of the mass of the shuttle's skin absorbs alot of joules (which is then slowly dissipated later on)
- heating of the atmosphere directly from the skin (in low pressure areas)
- heating of atmosphere directly from the ejected superheated steam
- further heating of the already ejected superheated steam from the atmosphere.

Anybody got a url or cite for how many joules the current tiles ablate ?
-- FlyingToaster, Dec 10 2007


This is kind of baked, but they use a solid to phase change instead of water. The old space capsule used this system. I'm sure the material used a lot more energy to phase change than water, used less weight.
-- MisterQED, Dec 10 2007


//Anybody got a url or cite for how many joules the current tiles ablate ?//
The tiles don't ablate, they insulate.
-- ldischler, Dec 10 2007


the advantage of liquid over solid is the same reason as liquid-fueled rockets over solids : greater control and less susceptibility to physical defects... of course the best would be a non-viscous liquid with a very high latent heat index on evaporation, but water has the advantage of being multi-purpose, non-reactive with the atmosphere and environmentally friendly.
-- FlyingToaster, Dec 10 2007


A shuttle that sweats? Nice. Too bad we can't use that heat for something.
-- RayfordSteele, Dec 13 2007


As a thought, for a crew vehicle, using spare stored gas and liquid makes some sense (esp CO2 or similar that there's limited advantage to leaving in orbit). In practical terms, the lightest possible heat shield is going to be the best, and that's not going to be water.
-- MechE, Mar 29 2012


This (new) shuttle design doesn't incorporate replaceable ablative tiles. There may be a coating, enough to withstand one dry reentry in case of a catastrophic failure of the steam system, but that's just for safety.

In stead, water is pumped underneath the skin of the craft, heats up from the skin and exits as steam through perforations. Not only does this take a very large amount of heat with it but the exiting steam acts as a buffer between skin and atmosphere.

Water as a liquid has 8x the heat capacity of solid steel, as steam 4x, and the amount concerned to get it from liquid to steam is a whopping 4,500 times.

anyways <happydance> <link> except they're using cryogenic Nitrogen; the only advantage I can see is that "liquid nitrogen" sounds more impressive than "water".
-- FlyingToaster, Mar 29 2012


One small point: I understand that the heating of meteorites comes not from friction but simply from the extreme compression of air in front of them. If the compression is high enough, your water may not boil at a useful temperature.
-- MaxwellBuchanan, Mar 29 2012


[FlyingToaster], have you read Larry Niven's "Hammerfall" ?
-- normzone, Mar 29 2012


^ "Lucifer's Hammer": "hammerfall" is the event that takes place therein (yay WP, I can't remember the book offhand though I know I've read it... and where do alien elephants come in ?)

[Max] That sounds more like a bonus than a problem: the longer the water stays liquid, the more heat it carries away when it eventually slides across the tile to a lower-pressure area and evaporates.

Other possible reasons:
- the passageways in the tile are too small for H2O molecules compared to N2(?) or there's a viscosity factor.
- the tile might be oxidised (though the fact that 20% of the air hitting the tile at Mach20 is O2 sortof negates that)
- ice expansion (though again that doesn't seem to be a real problem: just load the water when the tile is warm and flush it before they get cold, or electrically heat it)
-- FlyingToaster, Mar 29 2012


[FT] Different book "Footfall" is what you get when you add alien elephants to "Lucifer's Hammer". (kinetic strikes rather than random impact)
-- MechE, Mar 29 2012


Thank you, [MechE], that's what I was aiming for..."Footfall".

I do believe that something similar to this idea took place in that book - water was stored as ice / insulation from laser attack, and steam vented as directional control.

Which might make a portion of this idea preheated.
-- normzone, Mar 29 2012


//laser attack// I guess you work with what you have but sodium would probably do better in that role: reflective, conductive; I think ice would just ablate in chunks, not being all that transparent to IR and all.
-- FlyingToaster, Mar 29 2012


Ah, then you have not read the book and are not familiar with Michael's construction (the archangel Michael cast Satan out of heaven). A recommended read.
-- normzone, Mar 30 2012


Maybe the LN2 is supposed to be harvested from the atmosphere on the way up ? eventually... that's the only way the calcs I haven't done might make sense.

But I digress: it's been ages since I've read L's Hammer: obviously Niven's always been on my shortlist, so probably when it was first published.
-- FlyingToaster, Mar 30 2012


Why don't they just wrap the big tank thingy in chicken wire, so the ice that falls off and damages the tiles is trapped and doesn't fall off and damage the tiles?
Then, instead of wastefully jettisoning the big tank thingy, they take it to orbit, harvest the ice, and use it to provide water for reentry.
NASA can contact me with a contract and big cheque at my usual address.

Edit: The big tank thingy can be filled with cheap space, and de-orbited to provide household cleaning appliance manufacturers with a cheap source of vacuum. Everyone wins.
-- AbsintheWithoutLeave, Mar 30 2012


Haw! if you don't post that I will.
-- FlyingToaster, Mar 30 2012


//Why don't they just ...// Is it my imagination, or is there no more Shuttle?
-- MaxwellBuchanan, Mar 30 2012


A use-once reentry vehicle, just glue some tiles onto it and pick it up from the ocean or on-the-bounce inland.
-- FlyingToaster, Mar 30 2012



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