External Tanks in Orbit

Insulation

Problem: The External Tank is covered with orange Spray-On Foam Insulation (SOFI) to insulate the ET, and the supercold liquid H2 and O2 while on the launch pad and during the heat of ascent. There is some concern that this material could erode once in orbit, causing annoying and potentially dangerous debris.

Possible Solutions:

Develop a version of SOFI which does not erode on orbit.

Cover the SOFI before launch with a protective layer to prevent on-orbit erosion.

Cover the SOFI after the tank reaches orbit with a protective layer.

Scrape the SOFI off after the tank reaches orbit, perhaps using an automated removal system.

Leave the External Tank in a 160 mile high orbit for about a month to allow all the SOFI to oxidize off of it.

Develop a way to shed the SOFI on the way up to orbit.

Develop a version of the SOFI which is made to be taken off once the tank reaches orbit.

Develop a version of SOFI which is made to sublime off before reaching orbit.

Develop a foam cloak to insulate the tank that could be jettisoned moments before launch.

Observations:

On-orbit operations are expensive and potentially dangerous.

Any new formulations of the SOFI or covering layers must be very weight conscious.

Ice which forms on the external tank before and during launch has the potential to damage the shuttle's protective tiles when shaken loose.

Heating during launch and after arrival in orbit could make the residual LH2 and LO2 more dangerous.

SOFI must not produce debris hazardous to the shuttle on ascent.

Particles which come off at higher orbit could become dangerous orbital debris.

Critical Issues:

When during the flight profile does the tank really need to be insulated?

When does removed SOFI become hazardous debris and when can it be harmlessly shed?

How specialized is the material properties of the SOFI, and what material alternatives exist?

Background: (quotes from elsewhere)

To prevent the super-cold fuels from forming ice on the outside surfaces of the ET, a multi-layered thermal protection coating approximately one inch thick is applied. The insulation allows the ET to withstand the extreme internal and external temperatures generated during prelaunch, launch and flight.

These tanks are designed and insulated for supercold, cryogenic temperatures: minus 298 degrees F. for liquid oxygen and minus 423 degrees F. for liquid hydrogen. An outside, sprayed-on foam insulation and premolded ablator materials serves as a thermal protection system during liftoff and also gives the tank its famous red coloring.

The ET thermal protection system consists of sprayed-on foam insulation and premolded ablator materials. The system also includes the use of phenolic thermal insulators to preclude air liquefaction. Thermal isolators are required for liquid hydrogen tank attachments to preclude the liquefaction of air-exposed metallic attachments and to reduce heat flow into the liquid hydrogen.

Three distinct layers make up the External Tank SOFI.

1.MA-25

MA-25 is a medium density ablator/insulator that has been used extensively as thermal protection on aircraft and space launch vehicles. On the Space Shuttle, short duration exposures to temperatures exceeding 1,200 degrees F may be encountered. In aircraft engine applications, continuous use temperatures range from 300-500 degrees F, with brief excursions well above 500 degrees F. The silicone based elastomeric MA-25 can be applied by spraying, troweling or bonding. Sprayable MA-25 can be applied with any standard spray equipment and is available in one and five gallon quantities for primary application. The troweling/molding mix can be used for repair of damaged areas or as primary application for small areas. MA-25 is also available as cured sheet stock that can be bonded in place with a high temperature silicone based adhesive for damage repairs or primary application.

2.MI-15

When weight is critical, low density MI-15 is ready for use in protecting aluminum or composite parts from high operating temperatures and to serve as a flame safety barrier. Available in sprayable or trowelable form, MI-15 provides improved thermal characteristics as well as a low density formulation for use in the most demanding of environments . MI-15 comes in a three part kit consisting of: A) master batch, B) curing agent and C) coupling agent. Sprayable MI-15 can be applied using standard commercial spray equipment and comes in one and five gallon kits. Component shelf life is six months at ambient temperature .

3.MI-15 Topcoat

MI-15 is a tough, fiber-filled material that extends the life of thermal materials. Topcoat is abrasion resistant and provides outstanding protection against hot hydraulic and other aircraft fluids.

Topcoat is highly cost effective compared to other protective materials, proving its value on commercial jet airliner fleets every day. Topcoat is applied by spraying, with a typical application thickness of twenty thousandths of an inch.

Thermal Protection System

"The entire outer surface of the external tank is insulated with a half inch thick cork/epoxy layer covered with 1 to 2 inches of spray-on foam" (Damon, 1995, p. 134). "The system also includes the use of phenolic thermal insulators to preclude air liquefaction. Thermal isolators are required for liquid hydrogen tank attachments to preclude the liquefaction of air-exposed metallic attachments and to reduce heat flow into the liquid hydrogen. The thermal protection system weighs 4,823 pounds" (Dumoulin, 1988, p. 4) The two reasons protection is essential are because both propellants are very cold and they boil at very low temperatures.

The following are problems that could happen if there was no insulation (Damon, 1995):

This poses two problems: excessive loss of hydrogen and oxygen through vent valves and buildup of excessive pressure in the tanks. Controlled boiling is necessary on the launch platform to keep the tanks pressurized for structural strength and also to assist the pumps in moving the propellants out of the engines. During flight, the tanks are pressurized by gases from the engines. In addition, because of the cold temperatures, if the tank were not insulated, water vapor in the air would readily condense as ice on the sides. At liftoff, the ice would break loose and damage the Shuttle (p. 134).

Latest Discussion

Rob Price 1996: The insulation tends to flake off on orbit, leaving a cloud surrounding the tank, and wrapping the tank to contain debris after orbit insertion would be virtually impossible.

Rob Price 2000: I have heard the flaking problem is not true, but was never able to get a sample tested in a thermovac chamber. Easy to get samples. Easy test. Just need budget.

Cris Fitch: Given the amount of money spent over the years on small ET studies, I wonder why no one has checked out the insulation flaking problem. My information is that the SOFI has stayed relatively consistent over the years. Vacuum and differential heating/cooling can't be that hard to test, you'd think.

Rob Price 2000: The insulation flaking issue would be easy to test. Just get a sample, say a foot square, put it in a small thermovac chamber, say 3 or 4 foot diameter and length, with the proper radiators (light & heat) and a hookup to a refrigerator (I think liquid nitrogen would not be necessary, I think the temperatures can be met with conventional mechanical refrigerants) draw out the air, and temperature cycle it every 90 minutes or so, and wait to see what happens. As I said, easy to do.

Alex Gimarc: Part of the Pathfinder mission proposed out of MSFC was to inspect, photograph, remove a bit of SOFI. There was a proposal to imbed wires beneath both layers & pull them, slicing off chunks like a cheese slicer. Joe Carroll noted once that you could simply fly the ET in a low orbit for a while and molecular oxygen would polish the outside nicely over time. It's pretty light stuff and ought to deorbit quickly.