Solid Motor Idea


We’ve spent decades developing technologies for space exploration. Some great innovations have come from government programs–yes, the famous spinoffs that lead to new jobs, new products, and a better life for us all. How would we take those indispensable selfies without the JPL-developed CMOS cameras in our smart phones?

But what about the original inventions? Those all-but-forgotten heat shields, optics, avionics, or solid rocket motors that someone spent a lot of effort designing? Despite that spacecraft program managers seek “heritage” components for the systems they’re designing, i.e. historical precedent and provenance for the technologies they use, every new spacecraft project still seems to be designed to demand many custom solutions. It’s as if no single entity is really in control of both the mission objectives and the engineering that delivers them. Too often the former drives the latter, no matter what time and money could be saved by adopting a holistic view in which what’s available informs the project’s goals.

A quick example. Honeywell builds reaction wheels, spinning discs that torque a spacecraft, making it able to rotate in space without propellant and point its instruments precisely. These devices are robust and well-engineered. As electromechanical systems, they are fairly straightforward. Here’s the thing. These wheels can come with any of eight different electrical and data interfaces, a complex situation that is not straightforward at all. Honeywell has done a great job of accommodating its many customers but, instead, if all spacecraft were designed to accept a single standard interface, this expensive proliferation of nearly identical hardware would vanish like the extinct branches of an evolutionary tree.  In the extreme, imagine a USB-style plug-and-play satellite. Government and commercial customers would save time and money.  Honeywell would have a more straightforward product line to manage. Achieving these goals requires leadership from the government or an industry consortium, like the JPEG working group. We’re not there yet.

Spacecraft often incorporate exquisitely engineered, uniquely sized bits of technology to solve problems. Let’s turn that tendency on its head. Let’s ask what we could do by repurposing what has come before. Instead of seeking a redesign of familiar components each time, let’s build on what we’ve already got. Doing so demands that we find common ground between what we really want to achieve (the space science) and how we achieve it (the space technology). As an example, how about we strap a small satellite to a famous old rocket motor and see how far it will go?  Specifically, let’s try to launch nanosatellites from the surface of Mars into orbit with hardware that has been around for decades: solid rocket motors.

Spend a few minutes browsing ATK’s fantastic catalog of rocket motors, and you’ll have plenty of Spacecraft a Week ideas of your own. It’s nothing less than a history of space exploration from the perspective of the chemical propulsion technology that made that history possible. You’ll find a rocket motor there for any application, mostly because of–or maybe despite–our nation’s abortive and often uninformed attempts to establish spaceflight standards, and its tendency to associate grand visions with specific people and abandon them when those people move on.

Take the Star 4G, for example.  What a great idea: a nanosatellite-scale solid motor that takes advantages of the benefits of small size to provide a compact, mass-efficient propulsion system for a CubeSat. Well, almost. It’s just a hair too big to fit in a CubeSat. That’s a real missed opportunity. From the look of it, ATK executed this project brilliantly, even taking this design from concept to test in a matter of months. It’s a pity that the requirements did not originate from a perspective of what it takes to infuse technology into how we explore space.

One of the attractive features of these STAR motors is their exceptional propellant mass fraction. That’s the fraction of the engine’s mass devoted to propellant. (Remember that a solid rocket motor comes with a slug of solid propellant; you don’t fill or, generally, refill it.) If the rocket motor has no tanks, combustion chamber or nozzle, only propellant, the mass fraction would be 100%. So, it’s remarkable that some of ATK’s motors approach 95%. The STAR 13 motor has a mass fraction of 87%. If one were to spin-stabilize and light up this motor with nothing attached to it, the resulting impulse would impart over 5,700 m/s of velocity (we call that increase delta-V). You could send this motor by itself from low Earth orbit all the way to the moon. What for? I don’t know, but I’ll bet a lot of ATK engineers have wanted to give that a try.

The STAR 13 motor weighs about 35 kg, the mass of an elementary school kid. You could pick it up. So could an astronaut. Imagine a 9 kg spacecraft attached to this roughly 42 cm diameter motor. That’s nine 1U CubeSats or roughly a single 6U CubeSat. Make it five 1U CubeSats along with some hardware to eject them from the motor, which would be necessary to avoid the heat from so-called thermal soakback after the propellant has burned away. That combination could launch from the International Space Station and acquire 3200 m/s delta-V. It would escape Earth’s gravity. It could head out on an interplanetary trajectory. And these motors are not expensive. Some cost less than $1M. Think of the opportunities to explore the Cosmos that are right there in front of us. And we’ve had these parts for years.

I expect that the ISS Program Office would hesitate to put a fully fueled solid rocket motor on board the ISS for simple reasons of safety. That risk posture probably makes sense, although I would point out that rocket motors were launched as part of satellites deployed by the Space Shuttle.

Finally, how about that unmanned Mars application? I would propose a launch pad on Mars that spins up the stack of 3 1U CubeSats and a Star 13 motor to about 120 RPM. These motors have seen that sort of speed before, according to the catalog. The spin stabilizes the attitude—like a Frisbee or a football. Distributing these CubeSats around the perimeter adds inertia to the spin axis, which aids stability. The spin also prevents small center-of-mass misalignments from causing the spacecraft to flip over as the motor thrusts. So, fire the motor. The delta-V is about 4,100 m/s, enough to overcome the drag of the thin Martian atmosphere, overcome gravity, and insert these CubeSats into a low Mars orbit. Maybe they can carry samples of Mars soil back to Earth.

I have immense respect for the spacecraft engineering of the ‘60s and ‘70s.  It was innovative, robust, and usually elegant in its simplicity. And even the fonts in their drawings were cool. The good news is that these achievements are still with us. They haven’t gone anywhere. They are ready to fulfill the promise of the Apollo era.  Let’s bring them back to help us finish what we started.


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