Is it possible that the problem with falling foam damaging the space shuttle heat shield tiles can be remedied by coating the tiles in an impact resistant substance?
Such a substance may be a couple of inches thick and could be applied to the tiles to protect them from falling pieces of foam during lift off. Then, on re-entry the coating would simply burn off, leaving the tiles to do their job.
Is this at all feasible?
You are going to have to be more specific about this “impact resistant substance” of which you write. What is it made of, how is it attached or adhered, and how does it get applied? How much dead weight does it add to the STS at launch, and what is the impact to payload capacity and ability to achieve specified orbits.
Stranger
Could it be made to work? Maybe, but doubtful. Would NASA consider it? No, even if it would work.
The extra layer will add to the mass taken to orbit, which is a very precious commodity.
Whilst burning it off you would need to be able to ensure that the aerodynamic profile remained smooth and well understood. One of the big risks is that an imperfection in the skin disturbs the hypersonic flow further downstream, and results in a hot spot that can damage the craft. So a cold area that burns off slower may result on a hot spot in a critical area that breaches the tiles.
The big thing that means NASA would never consider such a plan is the need to validate the change. Since the craft has changed its aerodynamic shape for launch, all the launch data would be invalid. This especially affects the abort profiles, where the shuttle needs to fly back to earth in a hurry. Then validating and ensuring the safety of the craft is safe at all other parts of the mission, including emergencies.
Finally, any such coating would clearly be some sort of applied foam - not unlike the external tank’s foam. It would be subject to popcorning like the ET’s foam. At least it isn’t subject to the cryogenic temperature issues of the ET, but it would be subject to two weeks in space, so hard vacuum, and full bore solar radiation - which includes lots of UV radiation. Something most plastics are not fond of. I can imagine it degrading and popping like crazy. NASA would need to test and validate all aspects of this.
NASA have considered and rejected other changes to the thermal protection system. The tiles are far from a happy solution. But the cost, and time needed to change has never proven worthwhile. (Of course had they bitten the bullet 20 years ago - say after the Challenger accident) it would have been money well spent by now.) But it would probably costs many hundreds of millions and take two or more years to perform even a simple change to the shuttle. NASA have made some changes the the TPS, in less critial areas. The thermal blankets that cover the top of the craft have been modified over time.
Finally, remember the damage that doomed Columbia was not to a tile, but to the carbon-carbon leading edge of a wing. It is very unlikey that any layer of cushioning material would have helped here. After all it was a piece of lightweight foam that did the damage in the first place. Just putting a layer of foam on the wing would not spread the force, nor absorb much of the energy. The falling foam piece was totally shredded by the impact on the wing, but still imparted enough force to crack the RCC material.
The right answer would be to reimplement the TPS end to end with newer technology materials that don’t have the difficulties and maintenance headaches the current design has. But that isn’t going to happen.
Irrespective of the added weight, additional cost and aerodynamic issues already mentioned, you are missing one significant point. The Space Shuttle is a dead-end program and scheduled for decommissioning and abandonment next year.
They stopped painting the main fuel tank to gain payload which is only about 50,000 lbs. The reason the tiles are used in the first place is weight.
The problem with falling ice is the speed the shuttle is traveling.
Don’t you think that if NASA had this magic, imaginary substance they would already be using this magic, imaginary subsance?
(It’s not because govenment regulations forbid the use of magic, imaginary substances. 
)
Or not. There is talk of extending it.
Right now I would be prepared to put money on the shuttle being extended. Not a lot of money perhaps, but I will bet that it is. There is the smell that the current funding and broad mission definition of NASA may actually get sorted out, at least a little. But it will cost real money to do, and that is always hard. NASA has endured three decades of doing things badly and with very poor cost effectivness due to penny pinching. The appalling cost of the shuttle (half a billion per launch) could have been avoided had funding in the 70’s been kept up during its design and construction. That funding would have been paid back many times over. But that isn’t how governments work.
I work in the world of high temperature ceramic research. As mentioned before the problem with the OPs idea is the added weight. Ultimately if NASA was going to perform an overhaul, this type of coating would be unnecessary. We have materials that have higher mechanical strength, better high temperature oxidation resistance, increased durability, and are lighter than the currently used tiles (although they are more expensive). Even with some of the upgrades they have done through the years, pretty much everything on the shuttle is old technology now. As has been mentioned before, such a major change to the outside of the shuttle is just not going to happen so late into the program. To certify materials for flight is a massive PITA that is both time consuming and incredibly expensive.
The sort of material I imagined was the soft, plastic like substance that is used to hold panes of glass in their frames.
Stranger On A Train and Exapno Mapcase, I picked up a hint of ridicule in each your answers. I never really thought I had come up with any great discovery that NASA’s engineers had overlooked. It’s just something I was wondering about one night when I couldn’t get to sleep.
Thank you Francis Vaughan for you excellent answer
The fundamental design flaw was not insufficient protection from debris impacts. The flaw was having anything at all on the rocket assembly that hung above the orbiter/lander during launch. I’m pretty sure they’ll never make that mistake again. Assuming we’ve learned anything at all from the Columbia accident, all future manned spacecraft designs will place the crew compartment firmly above everything else in the launch stack.
Glazing compound? You can cut that stuff with your thumbnail. The amount of protection it could possibly provide would be minimal, and the weight would take off several thousand pounds of payload capacity, assuming that it has to carry it up into orbit. You’d also have to worry about it peeling off during ascent flight and possibly fouling the mass and inertial properties of the vehicle, and possibly creating a debris field during on-orbit operations, plus all of the out-gassing that might affect Shuttle systems and instruments on-orbit.
Well, the issue is a little more nuanced than that. For one, the STS is more vulnerable to impact from flying debris; only only crap that falls off of it, but other things that it may encounter, like hail and birds. Both Apollo and the proposed Orion space capsules have large polycarbonate or aluminum honeycomb cover that provides protection against impact and that is pulled off by the Launch Escape System jettison motor after it reaches that abort mode. The STS doesn’t have this and is therefore vulnerable to bird strikes and other debris regardless of falling ice. With the Shuttle, it isn’t the “crew compartment” that is vulnerable, nor the tiles, but the reinforced carbon-carbon (RCC) leading wing edges. No amount of applied coating is going to protect those relatively fragile parts from impact.
With regard to the “fundamental flaw” of parallel staging (i.e. having strap-on boosters and tankage offset from the Orbiter) this is a trade-off in that parallel staging allows for a smaller reaction structure to transfer loads and a more adaptable form factor to add high thrust boost without causing the vehicle to have excessive length. Very long, slender booster stacks have their own very significant problems with bending modes and wind shear, and placing something the size of the Shuttle Orbiter on top of a stack would simply not be structurally feasible. Virtually all modern heavy lift vehicles use some form of parallel staging, either with strap-on solid or liquid rocket boosters (Delta II, Titan III/IV, Vulkan) or multiple core stages (Delta IV Heavy, Atlas V Heavy). Given the required configuration of the STS Orbiter (cargo bay capable of carrying reconnaissance satellite payloads for NRO) this was the only realistic configuration, and all proposed Shuttle designs used some form of parallel staging similar to the operational STS, as did the Soviet Buran shuttle. The only proposal that deviated from this was the novel and innovative Chrysler SERV proposal, which had a detachable personnel spaceplane mounted atop the vehicle like the X-20 Dyna-Soar and the main unmanned SSTO with a large cargo bay that returned via blunt-arsed heat shield (like a very large Apollo capsule) and soft-landed using air breathing jets.
Stranger
Heh, that reminds of a quote from HL Mencken: For every complex problem there is an answer that is clear, simple, and wrong.
I think the problem is that a layman cant just think up “Whoa, those eggheads at NASA are on the wrong track, what they need is silicon caulk and lots of it!” I think the issue is that you need a few years of material science under your belt or some similar experience before coming up with a big grand but simple solution to a serious problem.
I would re-read Stranger’s comments. He’s a font of knowledge on this subject and you could learn something.
You may also be interesting in posting your idea here:
I plead ignorance on the following;
First there have been a couple references here to ice falling - but isn’t the main problem foam detaching from the booster?
Secondly, if detached foam is a problem, wouldn’t they have been looking at how to keep the foam on better, rather than (as this thread seems to follow) making the shuttle more immune to a foam-strike? I would have imagined some sort of coarse netting made of some miracle-tough-fiber as an outer layer of the booster. Again I realize weigth and flow-dynamics are an issue.
Even if they extend it, it wouldn’t be for too long. The next-gen is in its final testing stages.
I remember a TV news report in the early days of the shuttle. The reporter showed one of the thermal tiles that had been in an oven for hours, and was even glowing a little bit. But he picked it up and held it loosely by the edges. I always thought the point of having the tiles was that they were very resistant to conducting heat, expecially from the air around them. It takes them a long time to get hot, and a long time to cool down.
So in addition to the weight and aerodynamics questions, would a coating defeat the whole purpose of the tiles? It seems to me that a coating (especially one that was designed to absorb heat and burn off) would likely transmit more heat to the tiles than air friction transmits to an uncoated tile.
There are at least two people in this thread who are infintely more qualified to answer this question than I am. I’m curious if my intuitive sense is even remotely close to being correct.
There is both ice and foam. Because the ET contains cryogenic fuels it forms ice on its surface, despite the foam insulation. Ice tends to fall off pretty early in flight, so poses less of a danger, but it is still a danger. Yes, NASA have been doing a lot of work on foam adhesion. There are a number of problems. It isn’t just that the adhesion might fail (and when you consider that the adhering layer is subjected to quite severe thermal stresses, it isn’t a trivial thing to get right.) If the foam has any included voids, which might be due to inhomogenaities in the appliaed foam, or due to trapped layers of air as the foam is sprayed on. These voids contain air at atmospheric pressure. As the shuttle climbs the air pressure drops and the void can exert enough force to pop the layer of foam above it off. NASA spent a huge amount of effort working though the foam application process after the loss of the Columbia. They redesigned some parts, and requalified other parts of the process. And they perform lots of tests, including lots of imaging of the foam with various techniques. I don’t think anyone thinks there is going to be a Colombia sized strike again, but they keep a very tight vigil on the foam, and it gets reported in the press because everyone remembers Columbia.
The replacement for the shuttle is actually a very long way off. The first test flight of a sort of mockup of Ares-1 is due in a month. However there is actually almost no flight hardware in common with the real thing. The booster is a modified shuttle booster with an inert fifth segment - it isn’t a proper booster. The proper five segment booster has undergone one static test firing. The second stage of the test flight is inert. Just a metal can of the right weight. It will be about five years before there is a human rated flight. Ares-V is little more than a paper design. First flight is slated for 2018.
The Ares project is so little progressed that there is a very serious risk that it will be cancelled. Personally I doubt it. Doing so would tantamount to the US declaring that it can’t afford to do leading edge space travel anymore, and it is leaving things to the Russians and the Europeans.
What you describe is an ablative heat shield. And apart from the shuttle, pretty well all re-entering spacecraft use these. The trick is that the mass being burnt off is removing heat as it leaves. So long as you have enough ablative mass to last you though the re-entry phase you are fine.
No, it isn’t. In fact, every single part if it is massively behind schedule.
Indeed, the heat shields on the Gemini and Apollo capsules are so robust that the could be reused again. (The Gemini XIII capsule actually was reused for an unmanned test flight to support the Air Force ‘Blue Gemini’ program to support the Manned Orbiting Laboratory before it was cancelled.) Aside from passive high temperature heat tiles and heat resistant carbon-carbon nosetips/leading edges there are heat sinks (heavy pieces of dense metal capable of absorbing a lot of heat energy) and active cooling (thin metal shields backed by intricate plumbing that uses a cryogenic coolant–typically the fuel itself–to provide heat transfer away from the heat affected area). Heat sinks were used in early ICBM reentry vehicles, but add a lot of weight and are limited to the metal melting temperature, while active cooling systems have thin margins and complex design. The blunt-arsed capsule design used in Gemini, Apollo, and Soyuz has proven to be a robust and reliable design, albeit one that doesn’t offer the cross range required for single orbit ascent-descent profiles (hence why the Shuttle, constrained by a requirement to deploy polar orbiting surveillance satellites from SLC-6, has to have such a broad wingspan).
The tiles aren’t really of issue, again; it is the carbon-carbon leading edges that are most at risk. The Orbiter typically loses a few tiles on each mission, which are subsequently replaces without great difficulty, and in fact many of the original tiles have been replaced with thermal blankets. The leading edges, however, are critical to the structural integrity of the Orbiter upon reentry.
Stranger