Acta Astronautica VoL 7, pp. 1239--1258 Pergamon Press Ltd., 1980. Printed in Great Britain

Use of pallet-type structures in Shuttle-attached and free-flying modest

A. McGRATH European Space Research and Technology Centre (ESTEC) Zwarteweg 62, Noordwijk, The

Netherlands

AND

G. PETERS European Space Agency, 10 ave Mario Nikis, Paris, France

(Received 30 November 1979; in revised form 8 May 1980)

Abstract--Modular pallet-type structures, designed for optimal usage of the Space Shuttle as a launch vehicle, can be used in three mission modes--Shuttle-attached, attached to an orbiting Power Module, or as fully autonomous free flyers. The requirements for such structures are defined and three structural solutions--the Spacelab pallet, a cubic framework structure and a honeycomb panel structure--are described. System and subsystem concepts of varying complexity and autonomy are indicated and their applications to several typical payload examples are ~own.

Introduction THE ADVENT of the Space Shuttle with its large (4.5 m diameter and 18 m long) cargo bay will allow the transportation into space of many types of payload ranging from large items such as Spacelab and Long Duration Exposure Facility down to small experiments of mass i.n the order of a few kilograms. The large payloads- -wi th diameters comparable to the Orbiter cargo bay itself--will interface directly with the Orbiter a t tachment points. For smaller payloads and, in particular, for combinations of small payloads, a support structure in the form of a pallet which attaches to the cargo bay sill and keel mounting points, and which makes optimal use of the cargo bay shape, will be needed. Such a pallet-type structure (PTS) will provide convenient at tachment interfaces for the individual payload items and will also carry any necessary subsystem equipment required for the supply of resources and services to the payload. Modularity of structure length will allow maximum flight opportunities on "mixed cargo" Shuttle flights and allow effective use of the NASA .Space Transportat ion System (STS) charging policy. This paper describes some of the pallet-type structures which are current ly under development or under study in Europe. The utilisation of these s t ructures for both Shuttle-attached and free flying modes of operat ion is described and the applications of the different modes are discussed for some typical potential payloads.

?Paper presented at the XXXth Congress of the International Astronautical Federation, Munich, F.R.G., 1%22 September 1979.

1239

1240 A. McGrath and G. Peters

Utilisation potential of different mission modes Three alternative mission modes can be identified for pallet-type structures:

Mode l w P T S attached to Shuttle Mode 1 is the Shuttle-attached, or sortie mode, where the PTS remains firmly

attached inside the Orbiter cargo bay during the mission and is then returned to ground with the Orbiter. This mode has the advantage that it can utilise the Orbiter supplied resources of power, heat rejection, data handling and attitude control. The main disadvantages are the limited time in orbit (normally less than 30 days) and the impact of the Orbiter on the payload environment--in particular for aspects such as contamination, EMI and disturbances (from crew motion and thruster firings) of pointing instruments. Mode 1 is best suited for missions with one or more of the following characteristics:

--frequent flight opportunities wuseful results obtainable with a short time in orbit ---quick return to ground of samples, film, etc. --direct involvement of Orbiter crew members - -Use of liquid cooling, purge gas, etc. ---qualification of hardware for later (and longer) flights in other mission

modes

Mode 2 - - P T S attached to power module Mode 2 is a mode in which the PTS is deployed from the Orbiter and is then

attached to a system such as the Power Module, which has been previously placed in orbit by an earlier Shuttle flight. The Power Module is a NASA concept for an orbiting system providing power, heat rejection, data handling and attitude control services for a variety of payloads attached to the several docking ports provided. Solar arrays are the primary power source for the Power Module and so---unlike the fuel cell powered Orbiter, which has an orbital stay time limited by the consumables needed for power generation-- continuous periods of orbital operation in the order of years are possible and higher power levels are available for payloads. Contamination levels will be lower than for Mode 1 since the Power Module does not have contaminant sources such as the Orbiter's thrusters, flash evaporator, water dumps, etc. Also, the stability of fine pointing instruments will not be disturbed by crew motion and thruster firings (the Power Module will use control moment gyros for attitude control).

Whereas the Power Module can remain in orbit for long periods, the individually attached PTS can be serviced during Orbiter revisits, removed for return to ground, or replaced as required. The main disadvantages are that the orbit and attitude of the Power Module will not be adjustable to suit the requirements of each individual user. Also, the large appendages of the Power Module--the solar arrays and radiators---may impose some constraints on experiment viewing.

In principle, Mode 2 is applicable to most scientific/applications disciplines requiring low Earth orbits but is best suited for missions with one or more of the

Use of pallet-type structures in Shuttle-attached and free-/lying modes 1241

following characteristics: - -need for more than 30 days in orbit mhigh (relative to conventional satellites) power requirements muse of liquid cooling - -use of Orbiter for servicing and retrieval ---compatibility with the Power Module standard orbit(s) - -use of hardware previously qualified on Mode 1-type missions

Mode 3 - - P T S as a fully autonomous free flyer Mode 3 is a mode where the PTS is deployed from the Orbiter and then

operates as a fully autonomous free flyer in the same way as a conventional satellite. Thus the PTS has to be equipped with a full set of subsystems to provide the functions of power supply, attitude control, communications, etc. This autonomy allows, of course, much greater freedom to select orbits and attitudes. The in-orbit environment for the payload will be better than for the other two modes since the contamination and disturbances associated with the Orbiter and the payload viewing constraints of the Power Module are avoided. Mission durations can be as long as required and retrieval is possible using the Orbiter. Power levels available to the payload may be lower than for the other two modes. Other disadvantages resulting from autonomy are higher subsystem costs and increased development and mission preparation time.

In principle this mode is applicable to users requiring orbits ranging from low Earth to geosynchronous and is particularly suited for missions with one or more of the following characteristics:

-- long periods in orbit --atti tude changes during mission - - low levels of contamination - - low revel of disturbance forces --usage of hardware previously qualified during Mode 1 or Mode 2 missions.

Requirements for structure The Orbiter provides standard attachment fittings at sill and keel level at

numerous locations along the length of the cargo bay so that considerable flexibility exists in the positioning of the PTS.

A PTS will generally be only a part of the Orbiter's total cargo and will, therefore, have to share the cargo bay with a wide range of other payloads of varying length and mass. Thus the PTS needs modularity of length and, in particular, adaptability to the very short spaces which may be available on some Shuttle missions which have large primary payloads. Optimum utilisation of the STS charging policy, whereby the user is charged according to cargo mass or length (whichever is dominant), leads to an optimum mass/length ratio for the PTS plus payload of about 1600 kg/m (based on a Shuttle launch capability of a 29,500 Lg cargo into an orbit of 28.5 ° inclination). The shortest cargo length for charging purposes is approx. 0.9 m and so the length of the shortest PTS module should be somewhat less than this.

Adaptability to experiments of different shapes also dictates flexibility of

1242 A. McGrath and G. Peters

configuration in directions perpendicular to the cargo bay main axis. Low structure mass and cost, adequate load carrying capability and readily accessible mounting interfaces for payloads and subsystems are needed. Other important concerns are alignment, provision of a benign mechanical and thermal environ- ment to the user, handling on ground and in-orbit, etc. Commonality of structure and (as far as possible) subsystem hardware between Modes 1-3 is a key requirement for low total program cost.

Requirements specific to Mode 1 PTS length requirements will range from short (less than 1 m) elements to

long (many meters) trains. Obstruction of experiment viewing by the cargo bay requires that the PTS should be able to provide a high level platform for the smaller experiments. Special configurations, such as a bridge over the Spacelab module access tunnel, may be required. The Shuttle-attached mode requires the ability to interface with the Orbiter and other attached payloads (e.g. Spacelab) for the supply of power, heat rejection, data handling, etc. resources.

Requirements specific to Mode 2 In Mode 2 the PTS requires interfaces for Shuttle deployment and retrieval

using the Orbiter Remote Manipulator System (RMS) and for docking (and undocking) with the Power Module. Optimum utilisation of a Power Module docking port may require a relatively long PTS to maximise user accom- modation. Interfaces with the Power Module resources of power, heat rejection, data handling, etc. will have to be provided.

Requirements specific to Mode 3 For Mode 3 the Shuttle deployment/retrieval interfaces will be similar to

those of Mode 2. Full autonomy of operation will require the support of extra items such as solar arrays and attitude and orbital control systems (AOCS). Maximum length of the PTS may be limited by the attitude cootrol capabilities of available low cost conventional AOCS hardware because of the effect of disturbance torques.

Structural solutions Spacelab pallet and derivatives

The pallet developed by British Aerospace as part of the Spacelab Program is an aluminium U-shaped aeronautic shell structure (see Fig. 1) designed to provide a maximum available volume for payloads within the constraints of the Orbiter cargo bay envelope. Large payloads can be attached to hard points located at the main structural members (frames and iongerons) of the pallet. Small payloads can also be attached to inserts mounted in the aluminium honeycomb panels forming the pallet skin. A platform facility is under development and will provide a modular secondary structure mounted to the hardpoints. This will allow small and medium sized experiments to be located at higher positions above the pallet floor, in particular to optimize field of view. The

Use of pallet-type structures in Shuttle-attached and free-flying modes 1243

~ O R B I T E R SILL FITTINGS S l L L ~ ~ t~_

IER SKIN PANEL (TYPICAL)

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Fig. I. Spacelab pallet structure.

basic pallet segment is 3 m long and individual segments can be rigidly connected to form continuous pallet trains of 6 and 9 m length.

The basic pallet structure is capable of modifications to increase the modu- larity of pallet length. Studies have been completed on the modification of the basic pallet into a half pallet (1.5 m long) and a quarter pallet (0.75 m long). These short pallets can either be used as independent elements, separately attached to the Orbiter cargo bay, or can be combined with each other and with the basic pallet segments to form an additional range of pallet train lengths. The platform facility will adapt to fit all pallet lengths (see Fig. 2).

The basic pallet structure design is applicable to all three mission modes. For Mode 2 an RMS grapple fixture is added to allow Shuttle deployment/retrieval and a docking adapter is added to one end of the pallet train to provide an interface with the Power Module. Mode 3 also requires an RMS grapple fixture and some modifications to the honeycomb skin panels to allow the accom- modation of subsystem equipment in the space between the inner and outer skins of the pallet.

1244 A, McGrath and G. Peters

D I A ' I r ' ~ D M C A ~ ' T I T T V

Fig. 2. Spacelab pallet and derivatives together with platform facility.

Cubic framework Modular Payload Support Structure Presently under development at MBB is a Modular Payload Support Struc-

ture (MPSS) consisting of basic carbon fibre strut and titanium node elements from which a variety of configurations based on a cubic framework can be assembled. Payloads can be mounted either directly to hardpoints at the node elements or to inserts in aluminium support panels which can be mounted on the node elements (see Fig. 3).

The length of the MPSS is 0.7 m in its shortest configuration and additional structure lengths can be assembled in 0.7 m increments. The basic design provides a bridge at sill level in the cargo bay to provide unobstructed viewing capabilities for small payloads. Larger payloads can be attached to the side of the bridge structure or underneath it. Payload equipment can also be mounted inside the framework structure. Apart from the bridge configuration other configurations, such as U-shapes and cylindrical shapes can be assembled to suit particular payload requirements (see Fig. 4). In another application the MPSS can be configured as a secondary structure located on the Spacelab pallet.

The MPSS structure is applicable to both attached and free flying modes. For Modes 2 and 3 RMS grapple fixtures and Power Module docking adaptors are added as required. In Mode 3 the MPSS is the structural carrier for the MBB Shuttle Pallet Satellite (SPAS).

Honeycomb panel structure A PTS based on triangular and rectangular aluminium honey comb panels has

been studied by Dornier. These panels are edge mounted to each other to form

Use of pallet-type structures in Shuttle-attached and free-flying modes 1245

\ \

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.UMINIUrl EQUIPMENT SUPPORT PANELS

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Fig. 3. Cubic framework modular payload support structure.

an enclosed structure and payload equipment can be mounted directly to the panels (see Fig. 5). With 4 different sizes of rectangular panel and 2 different sizes of triangular panel numerous different configurations can be assembled such as bridges at Orbiter cargo bay sill level, bridges above the Spacelab access tunnel, U-shaped structures, etc. No node elements are necessary but, for heavy payloads, hard points can be added at the panel intersections.

The shortest possible structure length is 0.7m and additional structure lengths are possible in 0.7 m increments. The panel structure is readily adaptable to both attached and free flying modes in the same way as the two previously described structure types.

Overall characteristics of the three types of structure The modularity and configuration flexibility of the three different types of

structure allow some adjustment in the performance characteristics to suit the requirements of various payload applications and the exact performance charac- teristics of all possible configurations are not yet defined. Each structure type

1246 A. McGrath and G. Peters

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has certain advantages or disadvantages for different applications. Table 1 summarizes the main performance characteristics of interest to the user.

System/subsystem aspects The PTS structural capabilities are shown in Table 1. Volumetric capability

depends on PTS configuration. For example U-shaped configurations allow a payload diameter up to 3.6 m and a length ( with overhang)--limited ultimately by the Orbiter cargo bay---of up to 18 m. Larger diameter payloads could be end-mounted to a PTS but are generally more suited to a direct interface with the Orbiter.

For the Shuttle-attached Mode 1 a PTS without subsystem equipment can be used to support self contained payloads which have their own power sources (usually batteries) and data recording equipment. With more complex payloads and, for use on the free flying Modes 2 and 3, the basic structure must be complemented by subsystems which can provide the necessary supporting ser- vices.

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Table 1. Main structure performance characteristics

CHARACTERISTIC SPACELAB PALLET CUBIC FRAMEWORK PANEL STRUCTURE

Basic configura- U-shape Flat bridge Flat bridge tion-Cross sec- U-shape tion of primary Structure

Alternative cross section configuration

Secondary structure

None

Horizontal platform

Length of 3m (baseline smallest ele- pallet) ment 1.5m (half

pallet) 0.75m (quarter

pallet)

Length of lon- 9 m gest continuous train

Approx. mass of primary struc- ture for 0.75 m element 260 kg 1.5 m element 395 kg 3.0 m element 628 kg 9.0 m element 1748 kg

Approx. load carrying cap. for 0.75 m element 2000 kg 1.5 m element 3000 kg 3.0 m element 3500 kg 9.0 m element 5500 kg

U-shape Disc-shape Disc-shape

Horizontal & vertical panels

0.7 m

None required

0.7 m

not analysed not analysed

(Flat bridge configuration)

180 kg 155 kg 262 kg not analysed not analysed not analysed not analysed not analysed

1620 kg 1500 kg 2140 kg not analysed not analysed not analysed not analysed not analysed

N.B. Structure lengths quoted are nominal values only.

For Mode 1 the Orbiter resources of electrical power, heat rejection and data transmission can be used to the maximum extent possible or the PTS system can provide varying degrees of autonomy from the Orbiter services. In the former case power levels of 7 kW average and 12 kW peak are available (with compar- able heat rejection). Higher power levels may be available later when sup- plementary power sources such as the Power Extension Package or the Power Module are coupled to the Orbiter power system. Data transmission rates via the Orbiter and the Tracking and Data Relay Satellite System (TDRSS) are 50 Mbit/sec for digital data and 4.5 MHz for analog and TV signals. Basic Orbiter attitude control capabilities provide a pointing accuracy of around 0.5 °. In- dependent pointing systems attached directly to the PTS can improve accuracy to arc seconds or better.

Autonomy from Orbiter services has advantages resulting from the reduced time and facilities required for integration into the Orbiter. The simpler inter- faces with the Orbiter also facilitate the "pallet-of-opportunity" concept whereby the PTS system can take advantage---at short notice,--of any space remaining available on Shuttle flights.

Use of pallet-type structures in Shuttle-attached and free-flying modes 1249

An example of maximum usage of Orbiter resources is the baseline Spacelab pallet plus "igloo" shown in Fig. 6. The igloo provides a pressurized environ- ment for data handling and electrical power control equipment while space hardened items such as inverters are mounted on the pallet end frame. All electrical power comes from an Orbiter fuel cell and heat generated by payload and subsystem equipment is rejected (via cold plates) into a freon loop interfac- ing with the Orbiter heat rejection system. This arrangement, where the sub- system equipment in the igloo occupies a significant part of the cargo bay length, is best suited to missions where the pallet is the sole cargo. For "flights of opportunity", where the available cargo bay length may be short, it is ad- vantageous to mount subsystem equipment within the length of the PTS--an arrangement possible with all three types of structure. Figure 7 shows such an arrangement for a semi-autonomous pallet with subsystems (including batteries). Heat rejection autonomy can be provided by direct radiation to space with, if necessary, a payload dedicated radiator and freon loop or heat pipe system. More complex autonomous PTS systems, including a dedicated fuel cell to power material processing equipment, have been proposed. Communications autonomy can also be achieved via PTS mounted antennas.

In Mode 2 the PTS must be fitted with a docking adaptor for berthing with the orbiting Power Module and a grapple fixture for interfacing with the Orbiter RMS. The RMS is then used to remove the PTS from the cargo bay and to dock it with the Power Module, where it can remain in orbit for periods of months or years. Figure 8 shows such a configuration. Power and heat rejection (fluid loop) interfaces between the PTS and Power Module are similar to those between the PTS and Orbiter for Mode 1 missions and PTS subsystems equipment such as cold plates, electrical harnesses, power distribution boxes and data handling hardware can be similar.

Communication with the ground is via the TDRSS using a commun- ications kit mounted either on the Power Module or the PTS. The total resources of the Power Module may be shared among several docked payloads including the PTS. Maximum power capability is 25 kW but heat rejection capability could be less so the PTS may need its own radiator system. The Power Module has a fixed orientation with a basic pointing accuracy of 0.5 ° and supplementary PTS-mounted pointing systems can be used for pointing in any direction with pointing accuracies of arc seconds or better.

At the end of the mission the Orbiter will rendezvous with the Power Module and the PTS is then undocked, placed in the Orbiter cargo bay, and returned to ground. Earlier Orbiter revisits are possible for servicing or replacing individual experiments and subsystems on the PTS.

Mode 3 differs primarily from the other 2 modes in that the PTS needs all the subsystems required for fully autonomous free flying operation following deployment from the Orbiter. An AOCS provides 3-axis attitude stability (with pointing accuracy of a few arc seconds) and allows orbit acquisition and maintenance. The AOCS typically consists of star trackers for primary attitude measurement (earth sensors for Earth pointing missions), gyros for inertial reference, reaction wheels for actuation and thrusters for momentum dumping,

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Use of pallet-type structures in Shuttle-attached and f ree-flying modes 1253

attitude acquisition and orbit control. A solar array (retractable to allow PTS retrieval) provides payload power of the order of 1 kW and batteries are used for eclipse periods. Thermal control is primarily passive, although heaters and louvres can be used. For payloads with high dissipation elements a fluid loop/radiator system could be added. The command and data management subsystem uses an onboard computer and databus. Communication with the ground is via the TDRSS using either the S-band or, for high data rates, the Ku-band link. Most of the subsystem hardware-apart from arrays and anten- nasmis located inside the PTS structure to avoid infringing payload space.

For all three modes emphasis is on standardization, modularity and the use or adaptation, wherever possible, of already developed items.

In Modes 1 and 2, for example, standard Spacelab power distribution boxes and cold plates can be configured as required for each payload, the Spacelab Instrument Pointing Subsystem (IPS) can be configured as required for each payload, the Spacelab Instrument Pointing Subsystem (IPS) can be used, com- puter (e.g. space hardened Spacelab type) memory size can be adjusted in modular steps, etc. In Mode 3 the solar array could be based on the Space Telescope array and the power level can be adjusted by flying either one or two arrays, by changing the number of panels and by adding batteries in parallel as required.

Orbit control variability is achieved by selecting the number of thruster gas storage tanks. The central computer/data bus system---e.g, the ESA On-board Data Handling System--allows the addition of remote interface units as required.

Typical mission applications A few examples are shown of payloads accommodated by a PTS used in the

various mission modes. Figure 9 shows the cubic framework PTS configured as a 1.5 m long bridge and used to support an Earth observation payload for Mode 1.

Figure 10 shows, also for Mode 1, the panel structure PTS in a 0.75 m long configuration designed to fit around the Spacelab access tunnel and to support a solar physics payload. In Mode 2, where a longer PTS is cost effective, Fig. 8 shows an astronomy payload, which includes instruments attached to pointing systems allowing individual telescopes to point independently of the Power Module orientation. An example of a relatively simple autonomous free flyer is shown in Fig. 11, where the cubic framework PTS, in 0.75 m bridge configura- tion, is equipped with batteries, nitrogen attitude control thrusters and com- munication antennas. This system, including its payload, will act as a deploy- ment and retrieval test article on an early Shuttle flight. Another more complex example of the PTS configured for Mode 3 is shown in Fig. 12, where a half pallet (1.5 m long) is equipped with an X-ray astronomy payload.

Conclusions Modular pallet-type structures designed for optimal usage of the Space

Shuttle Orbiter can be adapted for use in various attached and free flying mission modes. Together with a range of modular and standardized subsystem