Modular Spacecraft

Unlike the history of aircraft development, the development of the rocket - from the simple artillery rockets of past centuries to the complex vehicles of today - has been characterized by several factors: (1) unpiloted designs, (2) throwaway pieces, and (3) modularized components. The relationship of these factors to one another is obvious, since an unpiloted projectile lobbed at the enemy would of course be thrown away, especially if it blew up at its destination. Modularized components also lent themselves to being discarded, especially if they improved the overall performance of the projectile. This process began with the addition of extra stages, as tubular missile sections were stacked one atop another. The technique was extremely successful in boosting a practical payload into low Earth orbit, especially if that payload was a satellite that had to be boosted only once. It therefore made sense to throw away the booster stages as soon as they had fulfilled their purpose.

This modularized approach was quickly extended to the spacecraft itself, when engineers realized that it was possible to discard certain components at certain points in the mission, to achieve overall success. Thus the retro-rocket pack affixed to the blunt end of the Mercury spacecraft was jettisoned just after burning its pro-pellant, and just before atmospheric entry. The Gemini and Apollo spacecraft each had a crew cabin or Command Module (CM) and an unmanned section or Service Module (SM). The Russian Soyuz spacecraft has three sections: an Orbital Module, a bell-shaped Descent Module, and an unmanned Instrumentation and Service Module.

The Apollo Moon missions between 1968 and 1972 depended on modularized components. As each stage or module completed its assigned mission, it was left behind or cast off, and the crew carried on in the remaining spacecraft. A typical Lunar landing mission started with eight spacecraft pieces stacked and packed into the Saturn V, including the launch escape tower, and came back with only one, the conical CM. The Apollo-Soyuz Test Project in 1975 used a Docking Module, which allowed the Apollo Command and Service Modules to dock with the triply modularized Soyuz spacecraft (Fig. 4.4). Of the six modules used during the historic link-up, only two returned to Earth with their crews following the mission.

Most space stations (Fig. 4.5) consist of a large number of carefully designed modules, which plug into the station at various points. To build large structures in space, it has been necessary to build them piece by piece on Earth, and then loft the individual components into orbit one module at a time. The single exception to this rule was Skylab, a large three-man orbital workshop launched in 1973. It was not built up out of small modules at all, but was constructed from a converted S-IVB rocket stage. Skylab was probably the single largest piece of space hardware ever launched. It was, however, one huge module, boosted into orbit by the powerful Saturn V launch vehicle (Fig. 4.6).

The convenience and popularity of the modular technique in spaceflight can be explained by several factors. These include the all-important mass ratio, payload capacity, governmental largess, and practical design considerations. Let us take a

Apollo-Soyuz Rendezvous and Docking Test project

Fig. 4.4 Technical drawing of the Apollo-Soyuz Test Project vehicles, clearly showing the modularized components used during the historic American-Russian link-up in July 1975 (courtesy NASA)

Apollo-Soyuz Rendezvous and Docking Test project

Fig. 4.4 Technical drawing of the Apollo-Soyuz Test Project vehicles, clearly showing the modularized components used during the historic American-Russian link-up in July 1975 (courtesy NASA)

quick look at the old Apollo Lunar mission architecture, which incidentally will be virtually replayed in a slightly modified, beefed-up, and updated version when NASA returns to the Moon around 2020.

The first three stages of the Saturn V launch vehicle were used to insert the spacecraft into low Earth orbit. There were five F-1 engines in the first stage burning kerosene and liquid oxygen, followed by five J-2 engines in the second stage and a single J-2 engine in the third stage, each burning liquid hydrogen and LOX. Upon low Earth orbital insertion, the third stage was not jettisoned, because it still had enough propellant remaining to insert the spacecraft into a trans-Lunar trajectory. By now the launch escape system (Fig. 4.8) had long been jettisoned. At the proper time and after appropriate checks of all systems, the third stage engine was restarted and boosted the remaining stack toward the Moon. At this point, the transposition and docking maneuver took place, in which the Command and Service Modules (CSM) separated from the third stage docking adapter, turned around, docked with the stowed Lunar Module (LM), and extracted it from its berth (Fig. 4.7). All this happened while the assembly coasted at 25,000 mph away from Earth. Ridding itself of the empty third stage, the Moon-bound spacecraft now consisted of four modules: the conical CM housing the three-man crew, the

unmanned cylindrical SM, the two-man LM ascent stage, and the unmanned LM descent stage with its four gangly landing legs. Each of these modules, or stages, had its own rocket engine, with the single exception of the CM. The CM had small thrusters only. Upon reaching the Moon, the high area-ratio Service Propulsion System engine was fired to decelerate the spacecraft into Lunar orbit. This was followed by undocking of the LM from the CSM, with two astronauts at the controls. Descent was made to the Lunar surface using the lone engine in the LM's descent stage, which would later serve as a launchpad for the ascent stage (Fig. 4.9). After

Fig. 4.6 A fictional example of a nonmodular space station, from the movie 2001: A Space Odyssey. The station is apparently under conventional construction, rather than relying on modular architecture (courtesy NASA)

Lunar explorations were complete, the ascent stage engine was fired, leaving the descent stage behind. The two Moon explorers now rendezvoused with the orbiting CSM, and they transferred themselves and their Moon rocks into the CM. At this point the ascent stage was cast off, and the combined CSM returned to Earth, again using the Service Propulsion System engine. Just before impacting the Earth's atmosphere at more than 36,000 ft/s, the cylindrical SM was set adrift, so that the conical CM with its three-man crew and lunar samples could make a safe and free return to Earth.

Fig. 4.7 Cutaway drawing showing the modular components of the Apollo spacecraft and stowed Lunar Module (courtesy NASA)

APOLLO LAUNCH CONFIGURATION FOR LUNAR LANDING MISSION

Fig. 4.7 Cutaway drawing showing the modular components of the Apollo spacecraft and stowed Lunar Module (courtesy NASA)

Fig. 4.8 Technical drawing of the Apollo Command and Service Modules, together with the launch escape system, which was jettisoned prior to reaching orbit (courtesy NASA)

What is the relationship of missiles to modules? Launch vehicles derived from ballistic missiles depend on modular construction to work at all. Each stage is, in effect, a module. Likewise, each module is, in fact, a stage. This design methodology makes spaceflight much easier than would otherwise be the case, but it also boosts operational costs to the point that a vicious circle results. These costs - and the expectation of continued high costs - dictate the planning of many fewer missions while demanding much higher reliability. This drives up the complexity and cost of all components, which in turn leads to low launch frequency, thereby continuing this malicious cycle.3

Rockets vs. Spaceplanes

The best thing going for rockets today is the fact that we have them, we understand them, and we know how to fly them with reasonable proficiency. With modular designs and stacked stages, it is a fairly straightforward procedure to attain the energies

Fig. 4.9 Detailed drawing of the two-piece Apollo Lunar landing vehicle, consisting of descent and ascent modules, each with its own rocket engine (courtesy NASA)

required for spaceflight. Conventional launch vehicles enjoy simple construction in which most flight loads are longitudinal. From a purely engineering standpoint, the designs are simple, robust, and lightweight. They work.

The major factor arguing against the ballistic booster is the waste and the inherent cost associated with every launch. Not only is the launch vehicle, along with its complex rocket engines, sacrificed on every launch, but Earth's atmosphere is ignored. It is not used for its free lift, its propulsive potential, or its oxygen content. It is seen only as a barrier to be crossed. Rockets throw away their gas tanks and engines every time they are launched. Can you imagine doing that with the family car?

The Achilles' heel of all conventional vertical launch vehicles is their one-time usability. This single factor leads immediately to extremely high costs and lower reliability. The space launch industry is typified by a 2% failure rate in its launch vehicles. Such failure rates in the airline industry would never be tolerated. Millions of passengers fly every day, and only occasionally does an airliner crash. With a 2% failure rate, a private pilot with 300 h total time would be dead six times over. The low reliability of ballistic launch vehicles is due to their high cost and consequent inability to be tested like reusable aircraft.

Another problem with ballistic launch vehicles is the market. The only customer for vertical launchers has been the occasional satellite, interplanetary probe, or manned mission. Space tourists have been very few, and so far have had no impact on flight frequency or improved launch architecture. All of this is about to change, with the advent of the spaceplane.

Spaceplanes will be far safer than vertical launch vehicles, precisely because of their reusability. Their wings and wheels (Fig. 4.10) will allow a development and flight test program to proceed the same as for any other aircraft. Thousands of hours will be spent putting the vehicle through its paces, gradually expanding the flight envelope, and identifying inherent design problems that can then be corrected. The greatest challenge will be in upgrading the performance and reliability of the rocket engines, but even this task will be attainable, in principle. Reusability and the resulting increased flight frequencies are fundamental in developing good, reliable, relatively inexpensive space vehicles.4

Spaceplanes will someday completely replace missiles and modules. It may take a long time before this vision is completely realized. For the next several decades, rockets will continue to be the mainstay of space access, especially where heavy lift is required. But for the short term, spaceplanes will blaze the way for ordinary people to enter space as tourists. The first space tourists will pay in the neighborhood of $200,000 each for the privilege of experiencing a few minutes of weightlessness at altitudes above 100 km. These brave souls will be the first private suborbital spacefarers.

We already have suborbital spaceplanes. SpaceShipOne flew in 2004, twice within 14 days, winning Scaled Composites the $10 million Ansari X-prize. SpaceShipTwo, an enlarged version of SS1, is being built even now under a veil of extreme secrecy, and will soon make its first test flights. Other companies are working on spaceplanes of their own. Some companies envision using carrier aircraft to launch spaceplanes from altitude. Others plan a simple runway takeoff without the

Fig. 4.10 Wings and wheels are the key to repeated flight test of new spaceplanes. This is the HL-20 lifting body parked on the ramp at Langley Research Center in 1992 (courtesy NASA)

expense of operating a separate large mothership. Some companies like hybrid rocket engines; others prefer only liquid propellants. Some companies are building their own engines, much as the Wright brothers did over a century ago. When will these suborbital vehicles appear in large numbers? They could appear any day now. By the time you read these words, they could already be flying on a regular basis.

The greatest strength of the spaceplane is the fact that it has wings and wheels, which of course translates immediately to reusability. Reusability leads to more flights, better testing, lower costs, and greater reliability. Reliability, in turn, leads to more confidence and greater flight frequency, which in turn leads to more flight experience, more design improvements, better vehicles, and even more flights at lesser cost. This self-perpetuating circle is just the opposite of the vicious circle of rocketry described earlier, in which an expectation of higher costs leads to fewer flights with a demand for greater reliability, which in turn leads to higher costs and even fewer flights. Striving for improved reliability without increased flight frequency is clearly not the way ahead.

Superior aerospace vehicles, as spaceplanes will be, demand superior design, better materials, and optimal flight profiles. Apparent weaknesses will be turned into strengths, turning for example a vehicle whose propellant tanks are too large into a space tanker.

In traditional rocket science, dead weight is regarded the same way one might regard a 100-lb tumor clinging, leachlike, to one's body. Yet we have already seen that the wings and wheels of a mature spaceplane are critical in ensuring that the most vital aspect of the spaceplane - its reusability - is preserved. So from the perspective of the spaceplane, wings and wheels are not dead weight at all, but essential parts of the ship. And this should be true regardless of how far into the ocean of space that ship sails.

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