SP-466 The Star Splitters
 

5

MISSION PLANNING

 

[29] Once the barrier for mission approval and funding has been surmounted, three more hurdles must be cleared before a mission can be considered a success: the instruments must be built, they must be put in orbit, and they must collect and transmit useful scientific data back to Earth. These objectives cannot be achieved in isolation. Rather, they are all intertwined, and plans in one area must be carefully orchestrated with those in the others.

Once the overall budget for the program has been set, the type of launch vehicle to use is more or less determined. It is the smallest one that can safely put the payload in orbit. For HEAO, the launch vehicle chosen was the Atlas-Centaur. This, in turn, limits the satellite volume and weight. The Atlas-Centaur rockets are built by General Dynamics Convair Aerospace Division under the direction of NASA's Lewis Research Center in Cleveland, Ohio. The Atlas is powered by three engines-two Rocketdyne engines providing 1 646 500 newtons (370 000 pounds) of thrust and one Rocketdyne engine with 267 000 newtons (60 000 pounds) of thrust. All three engines operate on liquid oxygen and RP-1 propellants. The Centaur upper stage is powered by two Pratt and Whitney engines with a total thrust of 133 500 newtons (30000 pounds). These engines operate on liquid oxygen and liquid hydrogen. The total height of the HEAO Atlas-Centaur space vehicle ready for launch is 39.9 m (131 feet) with a total launch weight of 165 tons, of which the spacecraft plus experiments consist of three and a half tons, including one and a half tons for the experiments.

The spacecraft that carried the HEAO experiments was built by TRW Systems of Redondo Beach, California. Dick Whilden directed the TRW effort from the planning phase through the launch of HEAO-I. After the launch of HEAO-I, Whilden left the program to become Vice President and General Manager of the Test and Field Operations Division. Marshall Novick became the HEAO Program Manager for TRW and, in the words of Halpern, "played a key role in seeing the program through to a successful conclusion." TRW's role was to design and develop the HEAO spacecraft, to integrate the missions, to support launch operations at the Kennedy Space Center, and to perform flight operations of the in-orbit observatories in the mission control center at Goddard Space Flight Center. By all accounts, the performance of TRW was nothing short of outstanding. Patrick Henry of the Smithsonian Observatory spent eight months at TRW coordinating the integration of the HEAO-2 scientific instruments with the....

 


[
30]

Richard D. C. Whilden

M. W. Novick

.

Richard D. C. Whilden

M. W. Novick

 

...spacecraft. His impression was typical of many of the HEAO scientists "They were very good. I was amazed that a large company could do things so well." On March 10, 1978, the National Aerospace Club in Washington. D.C., honored TRW with the prestigious Nelson P. Jackson Aerospace Award for its work on the HEAO program.

The basic subsystems design of the spacecraft was the same for all three HEAO missions, with minor exceptions to accommodate unique experiments or mission requirements. The spacecraft subsystems were de signed so as to take advantage of existing hardware designs developed in other spacecraft programs. About 80 percent of the HEAO spacecraft hard ware is "off the shelf."

 


Schematic of Atlas-Centaur D-IA, the launch vehicle for HEAO.

Schematic of Atlas-Centaur D-IA, the launch vehicle for HEAO.


[
31]

Engineers check out the first HEAO.

Engineers check out the first HEAO. The large wing-like structures on the top and the right side are solar panels that provided about 400 watts of power to run the observatory. (TRW photo)


[
32]

HEAO 2 is prepared for flight.

HEAO 2 is prepared for flight.

 

The satellite consisted of two main parts, an experiment module and a spacecraft equipment module. The latter was an octagon-shaped prism about 1 m tall and 2.5 m in diameter, roughly the size of a large circular table. This module remained essentially the same on all three observatories, supplying the support systems needed to operate the spacecraft and its scientific experiments. In the center of this structure was a rigid cylinder that contained the reaction control system propellant tank assembly. This gas was used to control the motion of the spacecraft by means of gas jets. A closed honeycomb structure, to which electronic components are attached, surrounded the central cylinder. These components were readily accessible, so they could be checked during preflight testing and removed if necessary. They were distributed so as to maintain thermal equilibrium and mass balance. The spacecraft equipment module was attached to the bottom of the experiment module. The overall length of the observatories, that is, the spacecraft equipment and experiment modules, was about 19 feet, roughly the size of a small truck. Solar panels produced about 400 watts of power to run the equipment on the spacecraft.

The observatories were controlled in orbit from the HEAO Operations Control Center at Goddard Space Flight Center. Data were collected by the observatories and stored in onboard tape recorders. These were then transmitted to tracking sites and from there back to Operations Control Center. The two tape recorders on the spacecraft provided sufficient storage capacity to accumulate data for more than four orbits without dumping if the ground stations were not available when the spacecraft passed over. Control of the observatories in orbit was directed by the Marshall Space Flight Center through flight control engineers assigned to Goddard. Flight control operations were executed by TRW personnel under the direction of [33] the Marshall flight director supported by scientists associated with each mission.

An onboard computer, common to all three observatories, allowed the orientation of the spacecraft to be controlled automatically. In the event of a critical power loss, a low-voltage sensor automatically commanded the observatory into a contingency mode in which the orientation could be controlled based on Sun sensor data only. This ensured the survival of the observatory until the source of difficulty could be determined by ground control.

 


HEAO 3 undergoes preflight checks.

HEAO 3 undergoes preflight checks.

 

[34] The spacecraft were placed in low circular orbits, about 300 miles above Earth. HEAO A and HEAO C were scanning missions, designed to perform all-sky surveys. They rotated slowly end over end, with one revolution about every 30 minutes, spinning in such a way that the solar arrays always pointed at the Sun, and the entire sky was scanned in six months. HEAO B, with its X-ray telescope, needed precise and highly accurate pointing capability. This was achieved by a set of rotating precision wheels that provided the desired orientation by minute changes in their rate of rotation. Star trackers were used to find known reference stars and thereby determine points of reference in the sky.

The most serious restriction in the revised program was the imposition of a fixed life on all three HEAOs. This policy was adopted in an effort to cut down on program scope and budget requirements. In practice, a fixed lifetime was ensured by the limited amount of propellant for the gas jets that could be carried by the spacecraft. Once the gas was used up, it would be impossible to control the orientation of the spacecraft in the face of destabilizing gravitational and magnetic forces, and its useful lifetime would come to an end. Other satellites had used magnetic torquing systems to orient the spacecraft; in these systems magnetic fields are applied to bars...

 


Orbital orientation for the HEAOs.

Orbital orientation for the HEAOs. (a) HEAO I and 3 slowly tumbled end over end as they scanned the sky. A full celestial scan was completed in six months. (b) HEAO 2 was a directable mission, so it would point at specific sources for as long as a day at a time. All observatories were oriented to keep their solar panels directed toward the Sun for as much time as possible.

 

....in the spacecraft, causing it to turn. These were rejected because they are not fail-safe. If the solar panels were not pointed toward the Sun every five hours, the batteries on the spacecraft would run down. It was feared that the magnetic torquing system would be too slow, and a failure would result. A backup system of gas jets would be needed; this would increase the cost; thus, the magnetic torquing system was rejected. Later efforts were made to include a low-cost magnetic torquing system for HEAO B. It was argued that this mission, with its focusing X-ray telescope, might be worth prolonging for several years. The request was not granted, so another tactic was used to prolong the lifetime of the observatory. HEAO scientists Ethan Schreier and Harvey Tananbaum of the Smithsonian Astrophysical Observatory,, together with Marshall Space Flight Center engineers Tom Recio and Tom Guffin, conceived and implemented an ingenious momentum management program. This plan minimized the use of gas by selecting optimum times to observe specific targets and deleting targets in unfavorable positions. As a result, the lifetime of HEAO B was extended to more than two and a half years from the originally planned one year.

Spacecraft such as the HEAOs have thousands of components, any one of which could fail in orbit; some of them undoubtedly will fail in orbit. The key to a successful mission is to design the spacecraft and the experiments such that a single failure cannot cause the loss of a substantial portion of the mission's scientific objectives. This is the principle of "soft failure." In order that the inevitable failures be "soft" rather than "hard,"...

 


Control Center at the NASA-Goddard Space Flight Center.

Control Center at the NASA-Goddard Space Flight Center. (Photo by Dane Penland; Smithsonian institution Photo No. 80-18165).

 

[36] ...all critical components should be redundant. During the early design phase of HEAO, single failure point analyses were performed to identify all mission failure points. When the analysis indicated that mission failure could be avoided by redundancy, one or more extra components was added. Most instruments had a high degree of modularity, which was built-in insurance against single failure points. For example, an X-ray detector might have several modules, and the failure of any one of the modules would not affect the performance of the others. By the same token, the individual instruments were designed to be independent of one another as much as possible, so that a failure of one instrument would not jeopardize the performance of the others. Extra gyroscopes, tape recorders, star trackers, etc. were added to the spacecraft. Once the redundant components were added they were checked time and again until launch to make sure that redundancy still existed on launch day.

 

previousimdexnext