The impact of launch vehicle constraints on U. S. space station design and operations

THE IMPACT OF LAUNCh VEHICLE CONSTRAINTS ON U. S. SPACE STATION DESIGN AND OPERATIONS

Judith H. Ambrus and Daniel H. Herman National Aeronautics and Space Administration

The Space Station, being planned as the next major U. S. Space Program, poses a unique design chal- lenge not only by i ts physical size, international aspects, and multiple functions, but also by its complex interfaces arising from the interdepen- dence of the Space Station with the Space Trans- portation System (STS) during both assembly and operations. The baseline design calls for a structure which eventually wi l l measure 45 by 105m with a transverse boom 145m long. Four modules (including one contributed by the European Space Agency (ESA) and one by Japan) with connecting nodes wi l l provide over 890 cubic meters of pressurized volume with habitation and laboratory space for up to 12 crew members•

Present plans allow for a "phased" approach to Space Station construction; the transverse boom with the four modules and all u t i l i t i es (including a 75 kW power system) would be completed between th~ end of 1994 and mid 1996. The rest of the capabilit ies, such as the service bay, the rest of the structure with additional payload attachment points, and 50 kW more power would be added later.

This structure wi l l be assembled and verified on- orbit from components delivered in discreet "launch packages" by the Shuttle (or other means of space transportation) and then maintained and resupplied over a long period of time on-orbit. I t is ob- vious, therefore, that the Space Station and the STS are very closely coupled, and that both the design and the operations planning of the Station wi l l be constrained by such factors as:

The up and down weight capability of the Shuttle;

The launch rate of the shuttle;

The capabilit ies of the Expendable Launch Vehicles (ELV's) that wi l l be part of the future U. S. space transportation f leet;

The requirements of the users of the Space Station;

The rate of logistics resupply needs of the crew;

The medical limitations on crew staytime on- orbit , and

The contingency planning for both the safety of the crew and their productivity should a logistics f l igh t be missed.

Since the Space Station wi l l be assembled over a period of several years, not only the f inal configuration, but the assembly sequence is also a crucial design parameter. The original baseline launch sequence for Phase 1 (Table 1) responds to the user's desire for early payload operations and on the need to minimize Shuttle based Extra- Vehicular Act iv i ty (EVA) time for assembly.

Accordingly, the f i r s t two launches wi l l result in a self-contained platform with guidance, naviga- tion and control capabil i t ies, 37.5 kW of power, and with attachment fixtures available for payloads. The f i f th assembly launch (MB-5) wi l l deliver the f i r s t laboratory, with outf i t t ing complete on the sixth launch.

BASELINE STS ASSEMBLY SCENARIO

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TabTe 1. Baseline Launch Sequence for Phase 1

At this time, laboratory operations wi l l be enabled using a vis i t ing crew for the length of the Shuttle's staytime, although no plans are being formulated at this time to halt the assembly at this stage for any length of time. Once permanent habitation is achieved, and the fu l l power capability (75 kW) is available, the nature of the interface with the transportation system shifts, as the remaining assembly of the Station and the logistics resupply needs of the crew and the payloads wi l l compete for transportation resources with the remainder of the assembly.

As noted above, this assembly sequence of the Space Station was developed with heavy emphasis on user and operational requirements; i t had been assumed that a robust Shuttle f leet was fu l ly available. The changing space transportation environment in the post-Challenger era places new constraints on the station/transportation interface which have to be accommodated.

During the ongoing redefinition of the STS, i t has become apparent that there wi l l be no more then five Shuttle f l ights per year available for Space Station. This presents a major problem, since with the presently baselined crew staytime in- orbit - 90 days - eight Shuttle f l ights are necessary just to change out the crew.

An additional problem is posed by the l i f t capability of the present Shuttle which necessi- tates the assembly of the f i r s t two launch packages at only 150 nm. Should the reboost system of those segments fa i l , reentry into the atmos- phere would occur in only 23 days, resulting in a loss of these launch packages and consequently cause major schedule and budget impacts. Another

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major r i sk is Fosed at launches MB-5 and -6, which are designed to put the Laboratory (LAB) Module in to space. While changing experiments and f i x t u r e s in the laboratory w i l l be rout ine during mature operat ions, f i r s t time o u t f i t t i n g on -o rb i t is considered to carry a large r i sk fac to r . With i t s present l i f t capab i l i t y , however, the Shutt le can only l i f t the laboratory with only about 50% of i t s necessary fu rn ish ings .

As a resu l t o f these considerat ions, ana ly t ica l studies are in progress to assess the mutual impact of the changing space t ranspor ta t ion environment and the Space Stat ion requirements. Spec i f i ca l l y , the fo l low ing groundrules were def ined fo r the i n i t i a l study:

1. The impact of crew staytime on-o rb i t on the Shutt le launch rate requirements were to be assessed, assuming that none of the ELV's ava i lab le or planned would be expected to be man-rated.

2. The t ranspor ta t ion a l t e rna t i ves f o r the Space Stat ion assembly were to include the shut t le (no more than f i v e f l i g h t s per year) and ELV's; however, the launch packages were to remain "dual compatible," i . e . , capable of being launched by e i the r the Shutt le or an ELV. This, o f course, immediately ruled out cer ta in ELV's f o r Space Stat ion assembly and defined con- s t ra in t s fo r any new design, such as a Heavy L i f t Launch Vehicle (HLLV).

3. Certain milestones f o r Space Stat ion assembly, such as that f o r Permanently Manned Capab i l i t y (PMC), were to be held constant as were the requirments of the users f o r ear l y payload placement and operat ional requirements, such as the r e s t r i c t i o n o f EVA hours and crew safety features of the design.

The resu l t s of the assessment of crew staytime versus Shutt le launch rate are shown in Table 2. The impact of extending the crew staytime is qu i te dramatic: an extension to 180 days would resu l t in a requirement f o r only four Shut t le f l i g h t s per year. This ra te appears to be a r e a l i s t i c f i gu re in l i g h t of the projected launch rate capab i l i t y of the Shutt le f l e e t in that time frame. The remainder o f the l og i s t i c s resupply can then be accomplished by any ELV in conjunction with an Orb i ta l Maneuvering Vehicle (OMV) to ass is t with prox imi ty operat ions.

CREW CONSIDERATIONS

Table 2.

• AVERAGE CREW DUTY TOUR REQUIREMENTS 4 Station crew I flight 4 STS crew changeout flights/year S STS f l i gh t s / year available for margin / flexabili~y

TOTAL NSTS FLIGHT~ PER YEAR STATION

CREW 4 5 6 7 8

4 9~ 73 61 S~ 4~

8 '182 146 122 104 g~

1,l 2?3 219 183 156 138

- Flight Interval g l 7J 61 $2 46

• CREW SIZE ANO TOUR HAVE MIN IMAL IMPACT UPON UP I DOWN ANNUAL MASS REQUIREMENTS

Assessmen~ of Crew Staytime versus Shut t le Launch Rate

In l i gh t o f the importance of these resu l ts , a l l medical data now ava i lab le have been ~eviewed, and the medical community concluded that an i n i t i a l basel ine fo r crew staytime of 120 days would be acceptable. An experimental approach is in place to v e r i f y th is number by ground tes t ing an~ to enable the extension to 180 days wi th in a year of placing the f i r s t crew on the Stat ion.

With a staggered approach to the replacement of the crew (an opera t iona l l y des i rab le fea tu re ) no modi f icat ions to the Shut t le are necessary. The modi f icat ions necessary to accommodate a l l e ight crew members f o r emergency evacuation purposes are under cons iderat ion.

Faced with the imperative o f minimizing the Shut t le launch ra te , but not changing the launch package design to f i t ex is t ing ELV's, the fo l low ing opt ions were examined:

i . Adjust ing the assembly sequence (o f f - l oad ing ) to minimize the r i sks associated with the above described problems wi th in present c a p a b i l i t i e s of the Shut t le .

2. Repeating that process f o r the Shutt le with improved up and down mass capab i l i t i e s now being planned.

3. Assessing the benef i ts oi an ELV (unmanned) cargo c a r r i e r which would be designed to accommodate the Shutt le-compat ib le launch packages.

Detai led examination o f the contents of the f i r s t two launch packages led to the reduct ion of the weight by 7,000 Ibs. , which would enable these packages to be assembled at 220 nm, where reent ry would not occur f o r 230 days, thus minimizing the r i sk of loss. Since, however, t h i s was accom- pl ished by taking o f f the two nodes containing the avionics, the pr ice of t h i s so lu t ion is the addi- t ion of a temporary avionics package, as well as an ext ra launch. The assembly sequence f o r th is case is given in Table 3.

INCREASED MARGIN SSP (B/L STS) ASSEMBLY SCENARIO

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Table 3. Assembly Sequence

Addi t ional advantages can be gained by some improvements to the Shut t le l i f t capab i l i t y , as summarized in Figure 1. I t can be seen that the development of an Advanced Sol id Rocket Motor (ASRM) would enhance the l i f t capab i l i t y of the Shut t le to 52,000 Ibs. This would give us the

advantage of l i f t ing the Habitation (HAB) Module, completely outf i t ted, and allow additional out- f i t t i ng to be added on the ground to the Labora- tory (LAB) Module. The launch of a 100% outf i t ted LAB Module cannot be l i f ted by the improved Shuttle, and for this purpose ELV's were examined.

STS PERFORMANCE AND ABORT DOWNWEIGHT

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Figure 1. Expected Performance Parameters of the Shuttle

Among the many potential ELV designs, the one chosen for further analysis is shown in Figure 2. I t is a Shuttle Derived Vehicle (SDV) ut i l iz ing already developed components, such as the solid rocket motor and Shuttle main engines. I t has a l i f t capability of 103,000 Ibs, to 220 nm. The advantage of using such a vehicle for assembly is primarily in i ts ab i l i t y to l i f t a fu l l y outf i t ted LAB Module. The reduction of that risk has to be balanced against the necessity of adding an Orbi- tal Maneuvering Vehicle (OMV) early in the program.

SHUTTLE-C REFERENCE CONFIGURATION

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Figure 2. Potential ELV Designs

The results from these in i t i a l studies have been summarized in Figure 3. The matrix shows the transportation options that have been considered and how the problems with the assembly sequence could be solved. The major conclusions that can be drawn are as follows:

• The extension of crew staytime on-orbit signif icantly reduces the Space Station's dependence on the Shuttle during the operational phase.

• The risk of loss of Launch Packages 1 and 2 can be reduced by assembling at 220 nm,

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instead of 150 nm, by using the present Shuttle and adding one more launch to the assembly sequence.

• Using the Shuttle with the new Advanced Solid Rocket Motor (ASRM) can reduce the risk of on-orbit out f i t t ing. Outfitting of the HAB Module could be accomplished completely on the ground, and that of the LAB Module could be increased to 72%. A completely outf i t ted LAB Module could, however, only be l i f ted by the Shuttle Derived Vehicle (SDV).

SPACE STATION TRANSPORTA~ON SUMMARY

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Space Station Transportation Summary

SPACE STATION TRANSPORTATION SUMMARY (Continued)

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Figure 3. Space Station Transportation Summary (Continued)

Further analyses are suggested by these in i t ia l results, and are to be accomplished before f inal decisions concerning the Space Station assembly, operations, and the STS to be used, are made. For example, should the items integrated into the LAB Module on the ground be judiciously chosen, the capability of l i f t i ng the module 72% outfi t ted might reduce the risk of on-orbit integration suff iciently to enable the assembly to be accomplished by using the enhanced Shuttle only. In that case, an SDV and OMV would only have to be added to the space transportation f leet during the operational phase of the Space Station.

Other studies wi l l address such concerns as the return capability of cargo from the Space Station, emergency and contingency planning, and other aspects of the Space Station/Space Transportation interface as they emerge.

Documents

The impact of launch vehicle constraints on U. S. space station design and operations