Modular Technology for Space,
or Why can't we build spaceships out of Lego blocks?

by Pat Bahn

1) Introduction
2) Definition of modular vs structural technology
3) Why structural technologies dominate early
4) Why modular technology dominates later
5) Why spacecraft have been structural?
6) The case for modular Spacecraft (Examples -TGV, Teledesic)
7) What will Modular spacecraft bring?
8) Conclusion

For decades people outside of aerospace have wondered why planes, rockets and spacecraft can't be built out of simple plug together elements. In essence, why can't flight hardware, be built the way computers and consumer goods are, that is like Lego blocks? Lego blocks are the children's toy blocks that snap together while the come in kits for building say a house, a car or a rocket ship Ninety percent of the pieces of each kit are totally interchangeable. Those that aren't are like the roof, floor tile and fence for the house, the engines and the fins for the rocket and the wheels for the car. If a child has all three sets they could build a moonbase using the fence as radiators, the roof sections as blast barriers, the tile as a landing pad and the wheels and rockets could be used to build a vehicle which would fly using the rockets then be rolled into the base on the wheels. Modular hardware allows this kind repurposing of parts Custom hardware has dominated over Modular hardware, because aerospace customers have demanded maximum performance and have not cared about product development costs or schedules. Now a new set of markets is arising that will push a transition to Modular hardware and create a whole new world of aerospace customers.

Before we go too far, it is probably worthwhile to discuss what is meant by structural Vs modular hardware. For this discussion, I have borrowed heavily from "The Innovators Dilemma" by Clayton Christensen of the Harvard Business school.

Structural technology is considered to be any solution to a market that cannot be easily disassembled into component pieces and that when disassembled provided minimal value to customers nor easily interoperate with the other solutions in the market space.

Modular technology is conversely solutions that not only easily disassemble, but also provide discernable value as individual elements and easily interoperate with other solutions in the market space. It is important to recognize that modular and structural technologies are not mere engineering exercises but operate in the context of a market with economic signals.

Examples of modular vs structural technologies can be found in many industries. Consider the home stereo. Originally cabinet hi-fi units were large heavy units comprising speakers, amps, tuners, record players into 1 large decorative cabinet. Over time, speakers became separated then turntables, tuners, amps and signal conditioners (Graphic equalizers) all fitting into modular racks that consumers mixed and matched at will. Structural technologies will usually exist at the high end of any market, while modular technologies will dominate low ends and growth markets. Sport and race cars are usually structural, while low end cars modular using mass market engines, drive trains, electrical components, etc.

Why structural technologies dominate early

What the graph above (Christensen graph), attempts to describe are the relative market demand trajectory, compared to the technical capability trajectory for any given market. Early in the development of a market, customers have average technical requirements that no producer can achieve. Producers thus work hard to margins from products to increase coupling between components and improve performance, i.e. if you make the bus faster, then you also try and improve the memory subsystem, and the I/O channel architecture which drives one to produce faster printers. Race cars provide another example. An improved weight tire will force an improvement in the suspension systems as the vehicle ride needs to be adjusted. This then causes the chassis designer to reduce weight as less stress is loaded onto the vehicle, which causes an improvement in the transmission as faster acceleration is now possible and the gearing is changed,,,...

As time passes, technical skills mature and producers are now meeting the average requirement of customers. In order to increase market shares and improve profitability, producers overshoot the average market requirement. This is why early VCR's started off without tuners and with 45-minute tape capacity and over time grew to have 8-hr record capacity, 20-event programming and multiple input source support. However this has also resulted in the phenomena that most VCRs in America have a blinking 12:00 on the front panel.

As structural technologies overshoot the average market requirement, modular technologies begin to enter the market. While a VCR may seem to be a single module, it's actually a system of components. Two companies produce the bulk of tape transport mechanisms, two companies produce the majority of head assemblies, a few companies dominate the video boards and the manufacturers integrate these components in a fashion similar to PC manufacturing.

Structural vendors meanwhile are pursuing the most demanding customers with the greatest technical demands and profit margins leaving the low-end markets open to modular vendors. Sony with the better Beta-max technology pursued commercial and industrial video customers, abandoning the larger consumer markets to Matsushita and the VHS technology suite. Beta still dominates broadcast systems, but Matsushita has controlled the multi-billion dollar consumer markets. Consider also the case of aircraft. If range is considered the value of interests early passenger transports had very poor capability to meet market demands. Ultimately aircraft improved and were able to fly non-stop to not just NY-Chicago, but NY-LA. However the longest range requirement that now exists is NY-Sydney, no practical use is seen for aircraft capable of flying more then way around the Earth. Meanwhile small jets manufactured by companies other then Boeing and Airbus serving short ranges are entering the bottom of the market. Boeing and Airbus dominate the long haul, large dollar market, but have surrendered the regional market to Embraer and Bombardier.

Modular technologies usually proposed by new market entrants begin by attacking the lowest price points and least demanding customers. Modular vendors offer low performance goods quickly and cheaply. Modular vendors compete on price point and speed to market for solutions. Modular vendors alter the market dynamic, changing value requirement to customers. PC's are a good example of this, while high-end computing was requiring large highly structured machines built by vertically integrated companies like Cray, CDC or IBM; the PC companies were building simple machines for ordinary people. Cray depended upon its army of engineers and technicians to build and design everything from cooling systems to processors. Dell, Compaq and Apple depended upon Intel and Motorola to turn over chips, while they used commercial fans, disk drives, monitors and memory chips holding their value in marketing and integration services (Christensen graph 2) . Modular technology progresses at the average rate of the subsystem provider's network, while structural technology advances at the rate of the slowest technology in the structural vendor's organization. Despite relative design inefficiency, modular vendors are able to grow at significantly faster rate then structural vendors.

Why spacecraft have been structural

Spacecraft of all sorts have been highly structural due to the high performance requirements of the market to date. Customers average requirements have outpaced the growth capabilities of the existing vendors, forcing severe measures to meet market needs and capture market share ().

The above chart can be attributed to s/c weight, power, transponder count, processing capability in any measure over time.

From the earliest comsats to the HS 706, communication satellites have gone from passive to active transponders, higher orbits, higher emitted power and increased transponder count and intelligent antenna design. Similar phenomena can be seen in planetary spacecraft. From crash landing Rangers to micro-rovers on Mars, requirements have increased over time. Launch vehicles have also pursued this technical trajectory. As spacecraft weight increased, continuous improvement had to be squeezed out of available designs. Consequently, launch vehicle designers have borne more in common with race car builders then PC manufacturers.

The case for modular spacecraft

Because of the increased demand for technical capacity, the conventional aerospace industry has looked upwards at improved systems and completely ignored the case for modular spacecraft. Modular craft will be aimed at niches ignored or under served, but the major players.

One such case is in the suborbital launch services market. While existing vendors have worked hard to increase altitude of payloads, a reasonable opportunity exists for low altitude missions launched by modular systems. Modular spacecraft offer a chance to significantly lower prices in this market and change the user characteristics.

The TGV Rockets Corporation has proposed a modular approach to the suborbital market. A small low cost vehicle can begin servicing the low requirement end of the market while changing the user characteristics. Currently the users of all rockets are "Rocket Scientists", working for government, or large institutions. They have the ability to marshal unique skills necessary to access space. (Technicians, telemetry analysts, payload designers) A low cost modular launcher will be operated by Teamsters, not rocket scientists, and fly cargo, not payloads. TGV intends for their customers to stop being rocket scientists but physicists, chemists, material scientists, fluid dynamists who view the environment of space as another laboratory variable, not as end in itself.

One should consider what the existing suborbital vehicles were designed for and why science is performed upon them, before making a case for modular launches in suborbital. First every suborbital rocket flying today has been built using the specifications of a military weapon, particularly ammunition.

1) Weapons are expendable: winning a battle counts, nothing else does.
2) Performance matters: increased speed or acceleration are critical values on the field of battle.
3) Size matters: Carrying larger payloads increases effectiveness, or smaller rounds means increased storage capacity or reduced protection issues. The larger the ICBM, the harder the silo design problem. The smaller the missile, the more you can fit onto the deck of a cruiser. The larger the warhead, the more hitting power.
4) Price shoppers lose; cost is the least important metric, nobody pinches pennies in war.
5) Reliability doesn't: 85% chance of a kill, is almost as good as 100%, if in doubt shoot again.

The result is we see a whole series of suborbital vehicles all built around solid fuels, narrow bodies, with fantastic acceleration and relatively poor reliability. Given these very tough challenges, why is science performed with these missiles? For several reasons:

1) They are there; scientists will use any opportunity to gather data.
2) It civilizes weapons; this provides cover for what would be unpolitic militarism
3) It's glamorous: even scientists like the idea of working where few go.

Given the criteria for military suborbital platforms, what would be the design criteria for a modular platform?

1) Cost: build economically, fly economically, your customers are poor, relatively speaking, compared to the Military.
2) Reliability: the vehicle and payload must perform and return.
3) Operable: science is done 24/7 worldwide without interference.

With these 3 basic goals in mind, once can design a highly effective suborbital launcher.

What will modular spacecraft bring?

The rise of modular spacecraft will match the expansion of space markets to the citizenry. Structural vendors have a small core of highly sophisticated customers who are closely linked with their vendors to constantly improve performance. Modular spacecraft vendors will be seeking new large markets amongst relatively less skilled individuals. This will be seen to democratize the technology and make available space to ordinary citizens. Just as pilot and diver replaced aeronaut and aquanaut, astronauts will be replaced by space-pilot. Modular spacecraft will open the space frontier for all humanity. In essense, Modular Spacecraft will do for space travel, what PC's did for computing.


For rational reasons, spacecraft designers have preferred structural performance oriented vehicle designs. Modular spacecraft however will provide cheap access to space. Modular spacecraft will allow customer services to be put together like a child building with Lego blocks. Modular spacecraft will shift market criteria from performance to value as ordinary people replace rocket scientists in the space business.