Redesigning the Space Shuttle's External Tank

Posted by Nicole Sharp on 30 August 2005 at 19:42

Senior year is already shaping up to be crazy. I have a feeling that I'll be having a hard time keeping up with everything. Recounting all of that, however, is not the purpose of today's entry. I plan to use this entry to keep track of resources, both for myself and my classmates, that I've found for a particular class project.

Our first project in EMAE 360, formerly Engineering Design (now heaven only knows what), involves redesigning the external fuel tank of the space shuttle to prevent insulating foam from detaching and damaging the shuttle during launch. The project itself is an individual project, but I imagine that it will require the combined efforts of many students in order to track down much of the information that we are expected to have. My goal is to use this entry as a repository of relevent information for the project. If anyone has anything to contribute, please feel free to comment or send me an e-mail. I'll update this entry with whatever I get.

The list of requirements that our designs must meet include, but are not limited to:


  • Static and time-varying loads imposed on the fuel tank

  • Vibration factors

  • Thermal insulation requirements

  • Weight limitations

  • Size constraints

  • Materials compatibility

  • Overall launch compatibility requirements

  • Implementation time constraints

Information Gathered

    From NASA - Return to Flight - External Tank
  • WEIGHT:
    • Empty: 78,100 lbs
    • Propellant: 1,585,379 lbs
    • Gross: 1,667,677 lbs
  • Propellent Weight:
    • Liquid oxygen: 1,359,142 lbs
    • Liquid hydrogen: 226,237 lbs
    • Gross: 1,585,379 lbs
  • Propellent Volume:
    • Liquid oxygen tank: 143,060 gallons
    • Liquid hydrogen tank: 383,066 gallons
    • Gross: 526,126 gallons
  • The external tank provides "structural support for attachment with the solid rocket boosters and orbiter."
  • The tank is jettisoned after approximately 8.5 minutes and is not reused.
  • "At liftoff, the External Tank absorbs the total (7.8 million pounds) thrust loads of the three main engines and the two solid rocket motors."
  • The solid rocket boosters separate around 45 kilometers above the Earth, while the tank separates around 113 km. (This is potentially useful for calculating thermal conditions.)
  • Labeled graphic of tank sections
  • The foam is a 1-inch thick layer of spray-on polyisocyanurate foam meant to maintain appropriate propellent temperatures, protect the skin from aerodynamic heat, and minimize ice formation.
    Information from NASA on the Liquid Oxygen Feedline Bellows on the External Tank
  • "During tanking and in flight, the ET expands and contracts by small amounts."
    Information from ET Press Release dated Dec. 31, 2004
  • The skin of the tank is made of aluminum.
  • The press release reports propellent volume as 535,000 gallons, as opposed to 526,126 gallons.
  • The ET is 27.6 ft. wide and 154 ft. tall.
  • The aluminum skin is only about 1/8th in. thick.
  • "The Space Shuttle Propulsion Office at Marshall manages the tank project. Lockheed Martin Space Systems Co., in New Orleans, is the primary contractor."
    Information from NASA's Thermal Protection System (TPS) Fact Sheet (PDF)
  • Thermal protection is provided by "low-density, closed-cell foam" and an ablator made of silicone resins and cork.
  • "The closed-cell foam used on the tank acreage is a spray-On-Foam-Insulation often referred to by its acronym as SOFI (pronounced so -FEE). The composite material is Super Lightweight Ablator, known as SLA (pronounced slaw)."
  • Ablators are used on areas prone to extreme heat: the aft dome near the engine exhaust and on aerodynamic protuberances, for example.
  • The foam "keeps the Shuttle's liquid hydrogen fuel at minus 423 degrees Fahrenheit and the liquid oxygen tank at minus 297 degrees Fahrenheit."
  • "The foam insulation must also be durable enough to endure a 180-day stay at the launch pad, withstand temperatures up to 115 degrees Fahrenheit, humidity as high as 100 percent, and resist sand, salt, fog, rain, solar radiation and even fungus."
  • During launch the foam must withstand temperatures of up to 1200 F.
  • The foam also serves to hold the tank together so that it disintegrates over the right remote ocean region after breaking away.
  • "Though the foam insulation on the majority of the tank is only 1-inch thick, it adds 4,823 pounds to the tank's weight. Insulation on the liquid hydrogen tank is somewhat thicker -- between 1.5 to 2 inches thick."
  • Foam density is between 2.27 and 2.4 lbs/ft^3.
  • "The tank's foam is polyurethane-type foam composed of five primary ingredients: polymeric isocyanate, a flame retardant, a surfactant, a blowing agent, and a catalyst."
  • Check the fact sheet for details on types of foam used on different sections of the tank.
  • Maybe or maybe not of interest or use: The foam on the shuttle tank apparently had to be exempted from a Clean Air Act passed by Congress and the EPA.
    Information from the Protuberance Air Load (PAL) Ramp Fact Sheet (PDF)
  • The ramps exist to prevent unsteady aerodynamic flow and consist primarily of thick sections of foam.
  • NASA is looking at minimizing or eliminating this from future designs.
    Information from Intertank Flange Fact Sheet (PDF)
  • Foam loss from the hydrogen tank to intertank flange area is not to exceed 0.04 lbs. Tests have typically shown losses of less than 0.1 lbs.
  • The explanation of cryo-ingestion and its contribution to foam loss in the article is well-worth reading.
    NASA's Answers to E-mail Questions MUST READ
  • 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 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 exasperated.
  • 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.
    Notes on Foam Loss in STS-114 Mission
  • The largest piece of debris lost appears to have come from the PAL ramp according to the caption belonging to this high-res photo. The light area under the liquid oxygen feedline is the location of the foam loss.
  • Images from an early mission briefing show debris locations.

Further Resources

Considerations to Take Into Account

  • The President has ordered the replacement of the space shuttle by 2010. This means that large modifications and redesigns of the current shuttle are unlikely to be implemented.

3 comments 1 trackbacks permalink

Trackbacks
  • Google and NASA Posted in Nicole Sharp's Blog on 29 September 2005 at 07:23

    I should really be working on the vestiges of my homework (as always) right now, but it's my birthday, so...

  • Polyurethane Foam Insulation Posted in Polyurethane Foam Insulation on 22 February 2006 at 17:34

    . Monolithic Dome "R" number. It is absolutely. ridiculous. Yet we often will prevent more thermal barrier. Over the fiber insu...

  • Polyurethane Foam Insulation Posted in Polyurethane Foam Insulation on 22 February 2006 at 18:37

    . Monolithic Dome "R" number. It is absolutely. ridiculous. Yet we often will prevent more thermal ba...

Comments
  • 1
    Got Gravatars?

    Mike Lorrey said

    On 09 October 2005 at 19:56

    Actually, the current Super Lightweight ET design is only about 58,000 lbs, not 79,000 lbs of the original tank design, nor the 65,500 of the Lightweight Tank design that arose after the Challenger accident.
    As for the falling foam issue, I have good intelligence from a friend who is an engineer at Lockheed Martin (aka Lockmart) who was in charge of finding suitable substitutes for all uses of CFC freon after the CFC ban treaty went into effect. Turns out freon was used to clean the surface of the ET prior to application of the foam. The best substitute cleaner they found was not as good, and this was when the problem of falling foam started, so if you want to deal with the falling foam issue, you ought to first look at why its not bonding as well to the aluminum-lithium surface of the SLWET.

  • 2
    Got Gravatars?

    Dan L said

    On 05 December 2005 at 19:36

    Found a brainographical error:
    exasperated should be exacerbated

  • 3
    Got Gravatars?

    Rainer Gerhards said

    On 19 November 2007 at 03:57

    Hi, thanks for this excellent information. Its also generally useful, not just as a think tank. I am writing about shuttle launches and have quoted some of the information on the external tank on my blog:

    http://spacelaunch.gerhards.net/2007/11/why-is-external-tank-not-painted.html

    I hope the quote is OK with you - if not, please let me know.

    Many thanks,
    Rainer

Post a Comment