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21ST CENTURY SATELLITE TECHNOLOGY

Inflatable Structures
Taking to Flight

MICHAEL A. DORNHEIM/PASADENA and TUSTIN, CALIF.

Inflatable space structures will have several chances to prove themselves in orbit over the next few years, and may well become an important part of both exotic and standard spacecraft design.

Present at the beginning of the space age in the form of the 1960-64 Echo 1 and 2 balloons, inflatables have since languished in the conceptual phase or played quiet roles in the form of missile decoys and targets. They offer large potential reductions in stowed volume, cost, and of ten weight, but have suffered the chicken-or-egg problem-program managers weren't choosing them because they weren't space-qualified, and they weren't space qualified because they weren't being used.

However, the May 1996 flight of the inflatable antenna experiment (IAE) on space shuttle Mission 77 cracked that cycle (see photos, right). Now there are conferences on space inflatables, a few more companies have entered the field, and flight tests are scheduled.

FOR ANTENNA REFLECTORS, the stowed volume of an inflatable device is about one tenth that of a mechanically-deployed reflector, according to
The Inflatable Antenna Experiment was a stunning sight in orbit (see bottom photo). But the first deployment of this new technology (top) did not match the preconceived notion. The antenna almost entangled itself and rocked the Spartan bus.
estimates made by NASA's Jet Propulsion Laboratory (JPL) (see graphs). This can result in a smaller launcher, which can yield savings of tens of millions to $ 100 million dollars, and JPL estimates the cost of the antenna itself may be an order of magnitude less. Giant space structures such as 1,000-ft. antennas or solar sails may not even be possible with mechanical deployment, but may be doable with inflatable design. The state-of-the-art in large antennas is probably radio frequency intelligence satellites. Their mechanically-deployed antennas are up to the order of 100 yards in size, highly-developed and expensive devices in which TRW is a leader. Surface accuracy may be as precise as 0.06 in. A number of roles are envisioned for inflatables, including:

  • Sunshades for space telescopes.
  • Deployment and support of solar arrays.
  • Planetary rovers.
  • Pressurized habitats in space or on planetary surfaces. The inflatable TransHab may be used in place of the habitation module on the International Space Station, tripling the volume (Awe~ST Dec. 8,1997, p.39). Spacehab is working with Vertigo Inc. onan inflatable tunnel to connect the shuttle cabin to a Spacehab module. Com pared to today's aluminum tunnel, it could save several thousand pounds and free volume for payload that would be deployed before the limp tunnel is pressurized, said John (Mike) Lounge, Spacehab vice president of flight systems development.
  • Extremely light weight solar sails, exploiting photon pressure.
  • Balloons to operate in planetary atmospheres. French balloon experiments gathered data at Venus as part of the 1985 Soviet Vega 1 and 2 missions (AW&ST June 24, 1985, p. 22).
  • Antenna reflectors.
  • Solar concentrators.
  • Precision booms.
  • Optical telescope mirrors. This is considered a more distant possibility due to the extreme precision required.


Complex inflatables, such as reflectors, usually have two types of components- structure, and the gossamer membrane it supports. The structure is thicker and contains higher pressure than the membrane. In IAE's case, the tripod legs and circumferential torus were 0.011-in. thick and inflated to 3-5 psia., while the reflector and transparent canopy were 0.00025-in. thick and were to be inflated to 0.0006 psia.

The IAE was also a precision inflatable. The reflector was measured on the ground to be within 7 mm. (0.28 in.) of the designed offset-paraboloid shape, or 1/15th of a wavelength at 3 GHz. S band-accurate enough for a good radiation pat tern. But not all uses need that precision.

L'Garde Inc. has specialized in precision inflatables since 1971 and flew the 46-ft. dia. IAE. The company has made about 150 inflatables that have been flown-all of them simulated warheads for target or decoy purposes, except for the IAE. The IAE and the Echo balloons are the only inflatables that have flown that are not tar gets or decoys, to the recollection of L'Garde executives and other officials.

Program managers have a number of concerns about inflatables, including:

  • Leaks from mechanical flaws or micrometeorite hits.
  • The materials may degrade from long exposure to space environment.
  • Improper inflation.
  • The deployment sequence may be haphazard and flail through a large volume, possibly entangling the spacecraft or the inflatable itself.
  • Reaction loads of haphazard deployment may exceed the attitude control limits of the spacecraft.

While the IAE fulfilled some of its creators' hopes, it also bore out some of the concerns (AW&ST May 27, 1996, p. 58). The sketch (below) shows how the antenna was supposed to be pushed out to an orderly pre-inflation shape by a spring platform on the Spartan 207 free-flying bus. In reality, when the container doors opened, residual air and the resilience of the packed material caused the antenna to drift away from the spring platform be fore it activated. Upon inflation, the antenna swung well wide of boresight and seemed to almost entangle itself with one of its legs (see bottom photo, p. 60). The reaction loads kicked the Spartan 207 into oscillations of roughly 10-20 deg.

The antenna deployed to the proper shape-a dramatic sight. But the lens shaped reflector/canopy failed to inflate, so there were no inflight measurements of surface accuracy. And after about 10 min., the antenna started a pitch tumble, accelerating to one revolution every 85 sec., prob ably caused by a leak.

Flying an improved antenna to get the important surface measurements and develop the technology would seem to be the logical next step. L'Garde believes it has fixes for the problems and could build a new antenna for $1.5 million, but it has not flown again. Reasons include the high cost of space flight and the lack of a well funded plan to develop inflatables, said Costas Cassapakis, president of L'Garde.

The IAE came about almost by chance. NASA's In Space Technology Experiments Program (Instep) offered to test technologies that needed to be proven in flight, and L'Garde responded with the IAE proposal. Funding of the $14 million project (not including the unaccountable cost of the shuttle flight) was not part of a sustained inflatable program but the result of competitions with proposals having nothing to do with inflatables. It is not surprising there was no refight.

Arthur B. Chmielewski heads JPL's Space Inflatables Program with a $2-million annual budget. Given that an IAE reflight would cost $5 million, "how will I do that?" he asked. "Who will pay for this?" Chmielewski estimated the total U.S. budget for inflatables at $12-million spread among Defense Dept. and NASA programs.

Paradoxically, the same budget crunch that makes it hard to fund testing also cuts the money for spacecraft programs, pushing managers to look at promising unproven technologies like inflatables. Coupled with growing requirements for large apertures, NASA and the Defense Dept. may be forced to space-qualify inflatables to conduct certain missions, particularly since the IAE is now on the minds of many.


Solar sail has inflatable booms holding an ultra-thin reflector. Deep Space 5 may test a solar sail, and NOAA's Geostorm is to use one to remain aloft at suborbital speeds.

Upcoming flight plans include:

  • An inflatable solar array. A prototype was built by L'Garde under a Defense Advanced Research Projects Agency contract and demonstrated 60 watts/kg. in a small 250-watt array. Chmielewski and Cassapakis believe this could easily be raised to 90 watts/kg. The mechanical state-of-the art is about 44 watts/kg. with the concentrator arrays on the Deep Space 1 space craft. NASA decided this month to fund L'Garde testing the array as part of an Air Force Research Laboratory experiment on the Goddard Spartan bus. The shuttle mission is to occur in 1999 or 2000.
  • Demonstration of a sun shield for the Next Generation Space Telescope (NGST), the successor to Hubble. NGST is looking at several technologies for a deployable 32 X 14-meter (105 X 46-ft.) sun shield. The inflatable device has the lowest volume and mass, Chmielewski said. Structure will be fabric tubes with heat-hardening resin to retain shape after inflation, and a one third-scale demonstrator is to fly on shuttle Mission 107 in June 2000. ILC Dover and L'Garde are building the device for the $ 1-2 million joint JPL-Goddard program and have demonstrated a prototype on the ground
  • A 14 X 3-meter (46 X 10-ft.) Inflatable Solar Array Experiment as a full-scale test of the design for a 12-kw. array for Deep Space 4/Champollion. The JPL/Defense Dept./lLC Dover/L'Garde project will also be rigidized with thermoset resin and will fly on the shuttle, probably in 2000. It is funded at $3.3 million.
  • A synthetic aperture radar antenna with radiating elements carried in a membrane with accurate planarity. The JPL proposal is looking at designs by L'Garde and ILC Dover and will fly on the space shuttle in 2000-01. It is funded at $3.5 million.
  • Solar Orbit Transfer Vehicle. A solar concentrator heats a graphite block containing passageways to expand hydrogen fuel for orbit transfer propulsion, and also produces thermionic power (AW&ST Mar. 30,1998, p.76). The Air Force/ Boeing Huntington Beach project uses an SRS Technologies inflatable concentrator as the baseline design. The $33 million experiment could fly in 2002.
  • Deep Space 5. The basic mission will be chosen in May, and one contender is to test an inflatable-boom solar sail of 40-70 meters size weighing 10-30 gm./sq. meter (0.002-0.006 Ib./sq.ft.). The DS5 budget is $28 million, and JPL would like to launch it in Fiscal 2003. The specifications align with the National Atmospheric and Oceanic Administration (NOAA) desire for a "Geostorm" mission to give 30-min. earlier warning of solar wind disturbances by hovering 2 million miles away near the Earth-Sun line. The solar sail would give lift to compensate for the suborbital speed of Geostorm. If JPL picks solar sail for DS5, then it may become the first step in NOAA's Geostorm plan.
  • Deep Space 4/Champollion comet lander mission, set for launch in April 2003. The JPL spacecraft has electric propulsion and will have already tested the solar array in the Inflatable Solar Array Experiment.
  • A 220 X 220-ft. solar sail for the NOAA Geostorm spacecraft. DS5 could be the first of a series of Geostorms. For the desired total loading of 30 gm./sq. meters, the sail itself needs to weigh about 16 gm./sq. meter, which is near the limit of current technology. The membrane would need to be thinner than the l/4-mil Mylar used on IAE, which is the thinnest made in large quantity, Cassapakis said. · The National Radio Astronomy Observatory/JPL Advanced Radio Interferometry between Space and Earth (Arise) mission, which would use a 25-meter (82-ft.) inflatable antenna to observe at 8-86 GHz. bands in a 40,000 km. (21,600 naut. mi.) elliptical orbit (see cover). Inflatable technology should allow Arise to fit on a Delta launcher, saving about $60 million com pared to an Atlas 2 launch. Arise could fly in 2008. The difficulty is that the state of-the-art inflatable accuracy of 1-2 mm. is about five times worse than the 0.3 mm. required for good viewing at 86 GHz., but it should be adequate for the lower bands A 1.6-meter deformable graphite composite secondary mirror will compensate for the primary reflector errors and is a more certain technique than trying to further improve inflatable accuracy, Chmielewski said. L'Garde's Cassapakis believes more accuracy is possible with material improvements and other work, but "serious money has to flow into it."


With luck, inflatables may follow in the footsteps of ion rocket engines. Both are "tenfold" technologies-ones that offer a cannot-be-ignored tenfold improvement in performance-but have languished for four decades. With recent investment, the ion rocket is demonstrating its superb performance in Hughes communications satellites and NASA's Deep Space 1. Inflatables may finally be receiving the investment that could pay off in one or two flights.

 

FLIGHTS USING INFLATABLES
 1996 Inflatable Antenna Experiment, on Spartan deployed by shuttle.
 10999-2000 Inflatable solar array test, small 300-watt class, on Spartan deployed by shuttle.
 2000 Next-Generation Space Telescope sunshield test. On shuttle Mission 107 in June 2000.
 2000 Inflatable Solar Array Experiment, large 1 2-kw. class, prototype for Deep Space 4/Champollion. Shuttle flight.
 2000-1 Synthetic aperture radar antenna experiment, shuttle flight.
 2002 Solar Orbit Transfer Vehicle experiment. Inflatable solar reflector~ is baseine.
 2002-3 Deep Space 5. Mission to be defined in May, one contender is inflatable solar sail.
 2003

Deep Space 4/Champollion. Comet lander mission to have inflatable solar arrays to power ion engine.

NOAA Geostorm. Uses solar sail to remain at suborbital speed between Earth and Sun, may build on Deep Space 5.

 2008 Advanced Radio Interferometry between Space and Earth (Arise). Has 25-meter inflatable antenna.

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