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Tuesday, January 14, 2014

Reading List: Microlaunchers

Pooley, Charles and Ed LeBouthillier. Microlaunchers. Seattle: CreateSpace, 2013. ISBN 978-1-4912-8111-6.
Many fields of engineering are subject to scaling laws: as you make something bigger or smaller various trade-offs occur, and the properties of materials, cost, or other design constraints set limits on the largest and smallest practical designs. Rockets for launching payloads into Earth orbit and beyond tend to scale well as you increase their size. Because of the cube-square law, the volume of propellant a tank holds increases as the cube of the size while the weight of the tank goes as the square (actually a bit faster since a larger tank will require more robust walls, but for a rough approximation calling it the square will do). Viable rockets can get very big indeed: the Sea Dragon, although never built, is considered a workable design. With a length of 150 metres and 23 metres in diameter, it would have more than ten times the first stage thrust of a Saturn V and place 550 metric tons into low Earth orbit.

What about the other end of the scale? How small could a space launcher be, what technologies might be used in it, and what would it cost? Would it be possible to scale a launcher down so that small groups of individuals, from hobbyists to college class projects, could launch their own spacecraft? These are the questions explored in this fascinating and technically thorough book. Little practical work has been done to explore these questions. The smallest launcher to place a satellite in orbit was the Japanese Lambda 4S with a mass of 9400 kg and length of 16.5 metres. The U.S. Vanguard rocket had a mass of 10,050 kg and length of 23 metres. These are, though small compared to the workhorse launchers of today, still big, heavy machines, far beyond the capabilities of small groups of people, and sufficiently dangerous if something goes wrong that they require launch sites in unpopulated regions.

The scale of launchers has traditionally been driven by the mass of the payload they carry to space. Early launchers carried satellites with crude 1950s electronics, while many of their successors were derived from ballistic missiles sized to deliver heavy nuclear warheads. But today, CubeSats have demonstrated that useful work can be done by spacecraft with a volume of one litre and mass of 1.33 kg or less, and the PhoneSat project holds out the hope of functional spacecraft comparable in weight to a mobile telephone. While to date these small satellites have flown as piggy-back payloads on other launches, the availability of dedicated launchers sized for them would increase the number of launch opportunities and provide access to trajectories unavailable in piggy-back opportunities.

Just because launchers have tended to grow over time doesn't mean that's the only way to go. In the 1950s and '60s many people expected computers to continue their trend of getting bigger and bigger to the point where there were a limited number of “computer utilities” with vast machines which customers accessed over the telecommunication network. But then came the minicomputer and microcomputer revolutions and today the computing power in personal computers and mobile devices dwarfs that of all supercomputers combined. What would it take technologically to spark a similar revolution in space launchers?

With the smallest successful launchers to date having a mass of around 10 tonnes, the authors choose two weight budgets: 1000 kg on the high end and 100 kg as the low. They divide these budgets into allocations for payload, tankage, engines, fuel, etc. based upon the experience of existing sounding rockets, then explore what technologies exist which might enable such a vehicle to achieve orbital or escape velocity. The 100 kg launcher is a huge technological leap from anything with which we have experience and probably could be built, if at all, only after having gained experience from earlier generations of light launchers. But then the current state of the art in microchip fabrication would have seemed like science fiction to researchers in the early days of integrated circuits and it took decades of experience and generation after generation of chips and many technological innovations to arrive where we are today. Consequently, most of the book focuses on a three stage launcher with the 1000 kg mass budget, capable of placing a payload of between 150 and 200 grams on an Earth escape trajectory.

The book does not spare the rigour. The reader is introduced to the rocket equation, formulæ for aerodynamic drag, the standard atmosphere, optimisation of mixture ratios, combustion chamber pressure and size, nozzle expansion ratios, and a multitude of other details which make the difference between success and failure. Scaling to the size envisioned here without expensive and exotic materials and technologies requires out of the box thinking, and there is plenty on display here, including using beverage cans for upper stage propellant tanks.

A 1000 kg space launcher appears to be entirely feasible. The question is whether it can be done without the budget of hundreds of millions of dollars and years of development it would certainly take were the problem assigned to an aerospace prime contractor. The authors hold out the hope that it can be done, and observe that hobbyists and small groups can begin working independently on components: engines, tank systems, guidance and navigation, and so on, and then share their work precisely as open source software developers do so successfully today.

This is a field where prizes may work very well to encourage development of the required technologies. A philanthropist might offer, say, a prize of a million dollars for launching a 150 gram communicating payload onto an Earth escape trajectory, and a series of smaller prizes for engines which met the requirements for the various stages, flight-weight tankage and stage structures, etc. That way teams with expertise in various areas could work toward the individual prizes without having to take on the all-up integration required for the complete vehicle.

This is a well-researched and hopeful look at a technological direction few have thought about. The book is well written and includes all of the equations and data an aspiring rocket engineer will need to get started. The text is marred by a number of typographical errors (I counted two dozen) but only one trivial factual error. Although other references are mentioned in the text, a bibliography of works for those interested in exploring further would be a valuable addition. There is no index.

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