I'm asking this because I don't know (and hope to learn something)

With all the issues surrounding the foam used on the shuttle's external tank why not either

1. ..... spray the foam on the inside of the tank or

2. ..... only spray the foam where it can not impact the shuttle or

3. ..... place deflectors or guards in front of the critical areas or

4. ..... just fly the shuttle without the foam. It did for several years.

I'm asking this because I don't know (and hope to learn something)

With all the issues surrounding the foam used on the shuttle's external tank why not either

1. ..... spray the foam on the inside of the tank or

2. ..... only spray the foam where it can not impact the shuttle or

3. ..... place deflectors or guards in front of the critical areas or

4. ..... just fly the shuttle without the foam. It did for several years.
I was wondering about just spraying a sealer over the foam to keep it in place and stable. Of course there may not be a sealer that can withstand the heat of re-entry with out burning off.:2cents:

The foam is on the external fuel tank.

See: http://en.wikipedia.org/wiki/Space_Shuttle_external_tank

The fuel inside the tank is liquid hydrogen and liquid oxygen, so the hydrogen temperature is about 20 K (-423 F).

Weight is the other major concern. If you add something stronger to cover the foam, then you add weight. More weight means you need more thrust, which requires larger rockets, which have more weight, etc.

NASA has tried different forumlas for the foam. Typically things that are good thermal insulators are very low density and have very poor strength.

Wes R.

The foam is on the external fuel tank.

See: http://en.wikipedia.org/wiki/Space_Shuttle_external_tank

The fuel inside the tank is liquid hydrogen and liquid oxygen, so the hydrogen temperature is about 20 K (-423 F).

Weight is the other major concern. If you add something stronger to cover the foam, then you add weight. More weight means you need more thrust, which requires larger rockets, which have more weight, etc.

NASA has tried different forumlas for the foam. Typically things that are good thermal insulators are very low density and have very poor strength.

Wes R.The temperature in space is about 3 deg K, so this should not be an issue. If the foam were insde it would reduce the volume of the tanks unless they were redesigned.

The temperature in space is about 3 deg K, so this should not be an issue. If the foam were insde it would reduce the volume of the tanks unless they were redesigned.

Describing the temperature in space is kind of tricky. There is basically no air, so a normal thermometer won't work well. The sunny side of spacecraft can get rather warm on the surface -- 1000 W/m^2 of heat from sunlight and no air to cool it off. The shady side can get quite cool.

Besides, the liquid fuel in the boosters is used up before it gets into space The insulation is for when it is sitting on the launch pad or on the way up, not for when it is in orbit.

Tim F

The temperature in space is about 3 deg K, so this should not be an issue. If the foam were insde it would reduce the volume of the tanks unless they were redesigned.

Hi Al,

The foam insulation does two things: 1. slows the rate of heating from the outside of the liquid hydrogen and oxgen tanks, 2. helps prevent air from changing to a liquid as a result of cooling from the tanks.

Air liquefaction occurs because the boiling point of nitrogen (78 % of air by volume) is -196 C (77 K) is much higher than the temperature of the liquid hydrogen, and the boiling point of oxygen (22 % of air by volume) is -182 C (90 K), which is also much higher than the hydrogen. Note there are many other elements in air, but nitrogen and oxygen are the two primary components, outside water vapor, which would also freeze.

Ice from water vapor has been a problem on prior shuttle launches.

As Tim pointed out, the issue is what the foam does on the ground as far as insulation goes regarding thermal properties.

Wes R.

I prefer NASA's actual plan, to do away with the shuttle entirely and just go back to single use rockets. Less Total Cost Per KG of Payload. (So I've been told by NASA geeks)

1st...the tank it is jettisoned after boost. Approximately 8.5 minutes into the flight, with its propellant used, the tank is jettisoned.

2nd...re-entry isn't a problem, it isn't reused, so who cares if it is burnt up


The tank is the "backbone" of the Shuttle during the launch, providing structural support for attachment with the solid rocket boosters and orbiter.

The skin of the External Tank is covered with a thermal protection system that is a 2.5-centimeter (1-inch) thick coating of spray-on polyisocyanurate foam. The purpose of the thermal protection system is to maintain the propellants at an acceptable temperature, to protect the skin surface from aerodynamic heat and to minimize ice formation.


Possible solution from a mechanic.
A lightweight material similar to the mesh used for a bag of oranges, made out of kevlar (very light, very strong) wrapped around the tank could be used, but the lame, non-visionary engineers involved will never think of something that simple (and most likely inexpensive). DUH!

Right, but isn't the problem that as the foam breaks free it hits the ceramic tiles making up the heat shield on the underside of the shuttle? I may not have been following the last shuttle explosion as closely as some, but that was what I was lead to believe was the issue.

If I was mistaken, ignore me.

snip...


Possible solution from a mechanic.
A lightweight material similar to the mesh used for a bag of oranges, made out of kevlar (very light, very strong) wrapped around the tank could be used, but the lame, non-visionary engineers involved will never think of something that simple (and most likely inexpensive). DUH!

I had heard/read about this possible solution somewhere (Discovery channel maybe). The concern was that with a mesh over (in) the foam, it could cause the foam to fall off in larger chunks, which would be more dangerous. There were many possible solutions, but most require major redesigns and may not create that much additional safety - all on a project that is being phased out.

Kevlar mesh applied in a laminated style isn't going to break because of the dynamics encountered at liftoff. Kevlar is essentially an artifical steel manufactured from chemicals (of course it isn't really steel it's a chemical based fiber). I know a bit about the strength of kevlar having been shot in my vest a couple of times in years past.

From NASA: http://www.nasa.gov/returntoflight/main/email_responses.html

Is there a problem with the foam adhering to the aluminum surface of the External Tank?

Our primary failure mode is cohesive failure of the foam; we have experienced little, if any, adhesive failure of the foam not sticking, per se, to the tank. Cohesive failure of the foam is different from the adhesive failure mode in that the foam itself fails to stay together and portions of the foam breaks free. Cohesive failure can be caused on a small scale by build up in pressure of the closed-cell foam that we use as the foam is heated during ascent. We refer to this phenomenon as "pop-corning." Voids that form within the foam during application to the tank can also lead to cohesive failure. Air entrapped within the voids can expand with the heating experienced during ascent, increasing the pressure, and ultimately cohesively failing the foam between the void and the foam surface. Note that the delta pressure (change in pressure) across the foam between the void and the surface is not only influenced by ascent heating, but also by the ever-decreasing ambient pressure until the vacuum of space is realized. Where voids form near or at the interface between the foam and the tank structure, entrapped air will be liquefied in the presence of the Liquid Hydrogen or Liquid Oxygen temperatures at the tank's aluminum surface, the potential for cohesive failure is exacerbated.

Why doesn't NASA apply paint, a cover, or net over the tank?

One might remember that we painted the first couple of External Tanks with white paint in the early 1980's. In both cases, we had a significant amount of foam loss during ascent. Although at face value applying a net or some other foam entrapping method to the External Tank sounds easy, it is not without concern. After careful examination of this approach, NASA's conclusion is that portions of the net could become in itself an undesirable debris source. Depending on the material used (Kevlar, aluminum, etc.), the density of the netting material would present a more critical debris source than foam to the Orbiter Thermal Protection System. Through a rigid certification process, we would also have to understand if and when the netting material could come off and in what quantities or mass that the netting material could present. Our assessment is that the process of certifying a netting material for flight would take several years and would not be available until late in the Space Shuttle Program life. NASA's goal remains to eliminate the potential for critical debris from all sources, including the External Tank foam.

Good find Tom. Of course seeing that we are near the end life of the program the mesh issue could have been in the works.

As to the expanding gas issue, with some basic engineering and building modifications I wonder if spraying the foam in a low/negative pressure environment could have eliminated some of the bubbles? The tank is big, no doubt about that, but as with many other complex problems there always seems to be a simple soultion that gets overlooked because they are just that, simple. "Take some air out of the tires"

Something else I read (don't have a link) also indicated the problem got worse when they switched the type of foam due to elimination of CFCs. So many variables in so many hostile environments can't be a fun problem to solve.