- 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.
January 2014 