ISS Transhab Module

By: Tomoe Furuta

ISS Home Page Components and Devices Index
ISS abbr and Acronyms


Table of Contents

I. Introduction

II. TransHab Structure
    A. General Structural Configuration
    B. Level One
    C. Level Two
    D. Level Three
    E. Level Four
    F. TransHab’s Inflatable Shell

III. TransHab’s Material and Design Process
    A. Inner Liner
    B. Bladder
    C. Restraint Layer
    D. Atomic Oxygen Protection Layer
    E. Thermal Protection System
    F. Seal Interface
    G. Deployment System
    H. Micrometeoroid/ Orbital Debris Protection System

IV. TrahsHab Radiation Shield Water Tank

V. Testing
    A. Extensive Hypervelocity Impact Testing
    B. Hydrostatic Pressure Testing
    C. Full Scale Development Testing
    D. Future Test Plans

VI. References


Introduction

Transit Habitat (TransHab) is an inflatable space habitation module used primarily for human living quarters.  TransHab has a metallic center core with flexible outer shell.  Its interior core consists of four levels.  It was a proposal to replace the planned US habitation module, and is being developed and tested by NASA as the last component for the International Space Station (ISS).  At NASA’s Lyndon B Johnson Space Center (JSC) in Houston, Texas, TransHab originally was designed to support a crew of six on an extended space travel such as the six-month trip to and from Mars or as the module home on the Moon or Mars.  Many tests have been performed at NASA’s JSC to prove the technology requirement of inflatable habitation modules during the past two years.  TransHab would be placed on the ISS in the second half year the year 2004 if it is approved as an official ISS component.  TransHab would be launched during a single Shuttle mission and then inflated on orbit by using a prototype heat exchanger to heat the gas.

The inflatable habitation module is being considered and tested because it offers significant benefits over a traditional aluminum structure.  TransHab would be launched during only a single Shuttle mission because it can be completely outfitted which dramatically reduces launch costs.  TransHab’s many layers in the shell provide gas retention, thermal protection, radiation protection, micrometeoroid protection, and structural restraint.  It will be developed and inflated on orbit, and it provides more than three times the volume and twice the storage space than the aluminum structure.  The original aluminum habitation module, the US habitation module, includes a galley rack, a wardroom area, two Earth-viewing windows, four individual crew quarters, a piece exercise equipment (treadmill), three crew health care system racks, Full Body Cleaning Compartment, three refrigerator freezer racks, and an approximately 500 cubic feet of storage area (WWW-1).

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TransHab Sturucture

A. General Structural Configuration

The TransHab has a unique hybrid structure that combines a central hard structural core with an inflatable shell.  Figure 1 shows the ISS TransHab overview.  This unique hybrid structure incorporates the advantages of a load carrying hard structure and the packaging and mass efficiencies of an inflatable structure.  The central core is made from lightweight carbon fiber composite material.  It has radiation shield water tanks wrapping around the middle level’s crew quarters, two utility chases, integrated ductwork, and thirty six shelves in two different sizes: 30” x  84” and 50” x  84” (WWW-1).  During the launch, the central core stores all of the equipment that is used in TransHab.  Then the equipment is placed in the final location after TransHab is inflated.  The central tunnel provides access to the rest of TransHab.    The bottom tunnel is the unpressurized tunnel in which air tanks are placed for use in the initial inflation of TransHab.  TransHab’s inflatable shell consists of multiple layers which include an external thermal protection blanket, a meteoroid debris layer, the structural restraint layer, and the internal scuff barrier and pressure bladder.  TransHab is divided into four functional levels within its pressurized volume.  It is separated by two floors that are opened by a system of poles attached to an extendable structure.


 Figure 1.  TransHab Overview (WWW-1)

B.  Level One

The lowest level is the fully equipped galley, wardroom and stowage area.  It has three galley racks, a microwave oven, food preparation equipment, refrigerator-freezer racks, Earth-viewing windows, a soft stowage array, and a large wardroom table (see Figure 2).

The galley area consists of two rack-based refrigerator-freezers and a rack-based ISS Galley.  The Earth-viewing window is located near the wardroom table and is approximately 20 inches in diameter.  The window has four panels, which are 4 inches thick, and a hard frame around each window that attaches to all layers of the shell (WWW-1).  The soft stowage is comprised of the stowage array system and a hand wash.  The stowage stores clothing, spare parts, supplies, and other equipment.  The stowage array system on level 1 and 3 has a total capacity of approximately 880 cubic feet of stowage.  The wardroom table, which seats 12 crewmembers, is used for meals, conferences, socializing, meetings, daily planning, and public relations gatherings (see Figure 3).


Figure 2.  TransHab Level 1 Top view (WWW-1)


Figure 3.  TransHab Level 1 Galley/ Wardroom Area CAD image (WWW-3)

C.  Level Two

The second level of the TransHab is composed of a mechanical room and the six individual crew quarters.  The mechanical room uses only half of the floor space, the other half being used as a clearstory.  The mechanical room has a door on each side.  It provides return airflow from level 1 and level 3 through openings in the mechanical room.  It consists of power equipment, environmental control and life support systems, and avionics equipment.  During the launch, equipment is placed into the central core and then placed in the final location after TransHab is inflated.  Additional equipment may be added in later.
The crew quarters are located within the central core.  These are surrounded by a two-inch thick water tank and have a 42 inches central passageway.  Each of the crew quarters is approximately 81 cubic feet in volume and has a height of 84 inches.  Each of the crew quarters contains a sleeping bag, a computer for personal and entertainment use, personal stowage, and integrated air, light, and power (see Figure 4).  The wall panels will be designed for cleanability and change out to allow new crew members to bring their personalized panels.  An integrated soffit at the top of the crew quarters contains power cables, and the ductwork (WWW-1).


Figure 4.  TransHab Level 2 Top View (WWW- 1)


Figure 5.  TransHab Level 1 Galley/ Wardroom Area CAD image (WWW-3)

D.  Level Three

The third level is the soft stowage area and the crew health care area.  The stowage area is identical to level one stowage.  The crew health care area combines two ISS Crew Health Care System (CHeCS) racks, changing area, Full Body Cleaning Compartment, an Earth-viewing window, a partitionable area for private medical exams, and exercise equipment, which includes the treadmill and the ergometer (an exercise bicycle) (WWW-1).  The exercise equipment is placed near the Earth-viewing window, which is approximately 20 inches in diameter.  As exercise equipment mounting platforms and structural integration, launch shelves are placed on the floor struts.  Four movable partitions provide visual screening of crew members of private medical exams at the CHECS rack and pre and post full body cleaning activities (WWW-1).


 Figure 6.  TransHab Level 3 Top View  (WWW-6)


Figure 7.  Exercise Area (WWW-6)

E.  Level Four

The highest level is a pressurized tunnel area.  It includes two station standard hatches, power equipment, and avionic equipment.  Until TransHab is inflated in space, the packaged central core will vent during launch to a vacuum state.  The pressurized tunnel area provides a transition between the ISS node 3 and TransHab, houses equipment required during the inflation, and provides structural connection to the space station (WWW-1).

F.  TransHab’s Inflatable Shell

The inflatable shell, which insulates extreme temperature in the space environment that ranges from plus 250 degree Fahrenheit (plus 120 degree Celsius) in the Sun to minus 200 degree Fahrenheit (minus 90 degree Celsius) in the shade, is 16 inches thick with over 60 layers (see Figure 7).  The shell is composed of five major subassemblies:

  1. the inner liner, which is the innermost layer.
  2. the triply redundant bladder layers.
  3. a woven restraint layer, which withstands four atmospheres of internal pressure and supports the bladder.
  4. the restraint and bladder layers, which are protected from micrometeoroid impacts by the debris protection system.
  5. the outermost layer, which consists of atomic oxygen protective layers and multi-layer insulation.


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TransHab’s Material and Design Process

A.  Inner Liner

The inner liner is TransHab’s innermost layer, which provides a barrier that is flame and puncture resistant, durable and easy to clean, and has good acoustic absorption properties.  It functions as a protective layer for the redundant bladder layers.  The flame resistant surface is made on Nomex fabric that is easy to clean.  Kevlar and Nomex nonwoven felt provide puncture resistance and acoustic properties (WWW-7).

B.  Bladder

The bladder is a triple redundant system.  Several bladder materials were tested for characterization at various environmental conditions in space, flexibility, and permeability.  Table I shows the result of the bladder testing.  As a result of the tests, Combitherm was chosen as the initial baseline bladder material because it has a very low permeability and is flexible.  Combitherm consists of nylon, polyethylene, which is placed the outermost surface of the shell, and vinyl alcohol layers, which provide a barrier to gas lose (WWW-7).  Each bladder is surrounded by Kevlar felt bleeder cloth, which provides additional puncture resistance.  The water in the bladder will shield crewmembers against intense solar radiation.

Table I.  Bladder Testing Results (WWW-8)

Material
Perm
Tensile Strength(lb/in)
Tear Strength(lb)
Puncture
Toxicity
Flammability
Other result
Combitherm VPC 140
0.007
42.1
34.8
9.59
OK
In progress
 
Combitherm XX 70
0.09
18.1
9.03
6.91
OK
-
 
Urethane coated Nylon
16.52
150
22.76
-
-
-
 
Urethane/Mylar/Tedlar/ Polyester scim
25.7
170
88.42
-
-
-
One side sealable Non-Thremofomable ILC Proprietary
Armor Flex TM
0.32
16.1
12.46
-
Retest reqd.
-
ILC Proprietary
Combitherm-Kevlar Laminate-one sided
0.1
29
1.4
121.5
-
-
One side sealable Non-Thremofomable Experimental De-Lamination
 

Combitherm-Kevlar Laminate-two sided
0.002
300.2
127.5
-
-
-
Non-Thremofomable Experimental De-Lamination

C.  Restraint Layer

The restraint layer is required to be folded and inflated on orbit and last for over ten years.  It is primarily used to support the larger loads incurred from inflating a 25-foot diameter module to 14.7 psi (www-7).  Kevlar was chosen as the material because it has a low cost, is well known, and has a significant flight history.  Several other materials, which are phenylene benzobisoxde (PBO), Spectra, and Vectran were considered and tested.  PBO is almost as twice as strong Kevlar or Vectran (see Table 1).  However, it is very costly and has limited availability, so it was not chosen.  Spectra becomes brittle at low temperature and has adequate sensible strength (WWW-7).

Table II.  Comparative Evaluation of Candidate Fibers for the TansHab Restraint (WWW-8)

Fiber type
Tensile Strength (ksi)
Breaking Tenacity (gm/den.)
Tensile Modulus (gm/den.)
Elongation at break (%)
Density (gm/cm^3)
Resistance to Flex Cracking
Low Tem. Brittlense (F)
Flamm-ability (LO1)
Abrasion
Resistance
Kevlar
490
23
555
3.6
1.44
Poor
\-321
27-29
Poor
PBO
820
41
1300
3.5
1.5 
Poor
---
>=56
Poor
Spectra
1000
435
35
2000
2.7
0.97
Excellent
\-13
18-19
Excellent
Vectran
412
23
680-840
2.2-2.5
1.41
Good
\-256 
28
Good

D.  Atomic Oxygen Protective Layer

The atomic oxygen protective layer is TransHab’s outer most layer, which protects the shell layer from atomic oxygen.  A single-side aluminized Bataglass fabric was chosen as the material because of protection against atomic oxygen damage.  Vent covers and vent holes are placed on the atomic oxygen cover (WWW-7).

E.  Thermal Protection System

Thermal Protection System, the multi-layered insulation (MLI), is provided to protect TransHab from the extreme temperatures in space.  The MLI consists of multiple Nylon layers, which are reinforced, double aluminized Mylar, inserted between an inner and outer layer of double aluminized Polyamide film.  Kevlar cords attach the layer-to-layer and gore-to-gore layer.  The MLI would be put together in gore sections and then constructed onto the shell (WWW-7).

F.  Seal Interface

The interval between the central core and the inflatable shell is the most important area of TransHab, because this is where the restraint must react and allow the shell load into the core, and the bladder must maintain a leak tight seal.  The restraint interface is made by using individual clevis/roller assemblies to attach the restraint straps to the bulkhead.  The rollers help the load to share two adjacent straps by allowing the straps to self adjust.   The rollers are also used to preclude creasing of the straps.   At the bladder interface, the over sized bladder is attached to a metallic interface ring using a flexible flight certified adhesive sealant (WWW-7).

G.  Deployment System

The deployment system is used to hold the folded shell layers before it is deployed.  The system includes several deployment straps that span every third gore.  The third gore is pushed in towards the center core and folded over with contiguous gores.  The straps are attached to one deployment gore on the next gore and so on.  The straps are tied to deployment cords and put together in daisy chains.  The straps form multiple parts of rings that entirely hold the folded assembly.  The daisy chains can be cut and released by pencil cutters (WWW-7).

H.  Micrometeoroid/ Orbital Debris (M/OD) Protection System

M/OD protection system is used to protect TransHab from hypervelocity impact.  The shield includes ceramic fabric (Nextel) bumper layers, which are separated by low-density cored polyurethane foam.  The foam is compressed before launch to minimize volume.  On orbit, the foam regains its original thickness because of elasticity of the foam.  The stronger fabric (Kevlar) layer is located behind the Nextel layer.  When the hypervelocity particles impact each of the Nextel layer, they are destructed into smaller, slower particles over a larger area.  By the time they reach the Kevlar layer, they are so small they can be stopped (WWW-7).

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TransHab Radiation Shield Water Tank

As a part of NASA’s TransHab program, the Radiation Shield Water Tank (RSWT) is being developed and tested to provide radiation shielding and protect crewmembers from galactic cosmic rays and intense solar particle events.  It is also used for a potable water tank.  Another purpose of the RSWT is the use as the final stage of biological water processing.   It will installed around level two of the ventral core structure.  It will be a bladderless cylindrical annular tank of 11 feet (3.35 meters) in diameter and 7 feet (2.13 meters) in height.  The wall of the tank is a 2.26 inches (0.0574 meters) thick (WWW-8).

The RSWT composes several novel features, which control the air and water location within the tank.  Its wetting surface around the edges of the membrane allows micro gravity filling and draining.  The hydrophobic membrane is placed inside of the outer tank wall (see Figure 8).  Vent holes on the outer tank wall is a path for air inflow and outflow (see Figure 9).  Fill/drain ports, which are covered with pieces of hydrophilic membrane, are located in the corners of the RSWT.  The membranes help to prevent air ingestion into the port during the draining.

Composite materials are used for the external shell because these can minimize the system mass.  A PTEE membrane is used for the hydrophobic membrane on the interior of the venting wall because the size of the pore provides a minimum liquid breakthrough pressure of 60 psi.  The Nylon membrane is used for the hydrophilic membrane covering the liquid fill/drain port.


 Figure 8.  Outer Tank Wall (WWW-8)


Figure 9.  Tank Cross Section (WWW-8)

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Testing

Many TransHab tests have to be performed to be approved as an ISS component. In this chapter, some tests and the results are discussed.

A.  Extensive Hypervelocity Impact Testing

This test is performed to verify the micrometeoroid and orbital debris protection system by using various sizes of hypervelocity particles at speeds of 2.5 through 11 km/s.  As a result of the test, engineers found that the TransHab shield can stop a 1.7-centimeter diameter aluminum particle at speeds of seven km/s (WWW-8).  This means that the M/OD protection system meets the requirement for an ISS habitation module.

B.  Hydrostatic Pressure Testing

On September 12, 1998, a hydrostatic pressure test was performed on a 23-foot diameter inflatable TransHab Shell Development Unit (SDU) at NASA’s JSC Neutral Buoyancy Laboratory (WWW-8).  Engineers put the SDU into the 6.2 million gallon water tank to prove the structural integrity of the Kevlar restraint layer (WWW-8).  Figure 10 shows the scene of the test.  About ten percent of the structural integrity was reduced because of the effects of water.  However, the test was successfully performed, and the SDU was pressurized to four times ambient pressure.  The high-pressure condition was held for five minutes without degradation of the test article.


Figure 10.  TransHab Hydrostatic Pressure Testing (WWW-8)

C.  Full Scale Development Testing

In December 1998, this test was performed at NASA’s JSC to demonstrate the ability to assemble, fold, and deploy at vacuum.  The Full Scale Development Unit was folded by the 84 cables that were attached to the 21 gore interface to support the 10,000 pound shell weight.  Then, the unit was deflated and held by the overhead support structure (WWW-8). A minimal ground support equipment successfully folded the unit, which resulted in a final diameter that was small enough to be outfitted in the Shuttle cargo bay.  The unit was then inflated to 14.7 psi, which is more than sufficient to keep the TransHab inflated (WWW-8).  TransHab would hold its shape for an indefinite period even if some pressure were lost.

D.  Future Test Plans

Additional tests are planned to be performed over the next few years.  The tests include window design testing, bladder and M/OD system venting design testing, seal inter face design testing, adequate shell venting testing and thermal performance testing at vacuum (WWW-8).

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References

Davis, Steven M, Station Architecture, 2000, McGuire AFB, NJ

D. Cadogan, J.  Stein, M. Grahne,  Inflatable Composite Habitat Structures For Lunar
          and Mars Exploration, 1998, IAA 13204, NASA’s Johnson Space Center, TX

Eugene K. Ungar and Frederick D Smith, TransHab Radiation Shield Water Tank, 1999,
          SAE paper 1999-01-1936, NASA’s JSC, TX

Kennedy, J., Kriss, ISS TransHab: Architecture Description, 1999, SAE paper 1999-01-
          2143, NASA’s JSC, TX

WWW-1  NASA’s JSC, 1999, SAE Paper 1999-01-2143

WWW-2  Kim Dismukes, NASA Spacestation, 21 Sep., 2000
          Available:  http://spaceflight.nasa.gov./statio…emble
          Accessed:  13 Oct., 2000

WWW-3  NASA, 24- 26 August, 1999, Habitation Module Commercialization
          Conference
          Available:  http://technology.jsc.nasa.gov/habconference/overview_ transhab.html
          Accessed:  13 Oct., 2000

WWW-4  NASA’s JSC, 1999, SAE Paper 1999- 01- 2143

WWW-5  NASA, 1 Nov., 1999, NASA HSF Galley
          Available:  http://spaceflight.nasa.gov/galler…ation/transhab/htm;/s99_05360.html
          Accessed:  13 Oct., 2000

WWW-6  NASA HSF Galley
          Available:  http://spaceflight.nasa.gov/galler…station/transhab/html/img_02b.htm
          Accessed:  14 Oct., 2000

WWW-7  NASA’s JSC, 2000, AIAA 2000-1822

WWW-8  NASA’s JSC, 1999,  SEA Paper 199-01-1936

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