When I say “SWaP,” you might be thinking: A swap for what? What’s in the trade?
SWaP, in fact, is a common term in the defense community, and it stands for size, weight, and power—the holy grail of technical performance for defense systems. Size determines what can be loaded into the weapon system volume, which is especially important when rocket propulsion is involved—whether within the atmosphere (i.e., missiles) or in space. Weight is important regardless of the type of system we’re talking about, be it missiles or artillery or rocket launchers. Why? Lighter weapons require less propellant (or a smaller engine) to be mobile on the battlefield. Finally, power refers to the energy needed for continuous operation of the weapon; this is a major consideration for battery-operated systems, such as communication or computation modules.
How do you compare designs to optimize SWaP? That is a more disciplined science than you might think. Perhaps the most refined standard for SWaP is for “CubeSats1.” The CubeSat specifications were first proposed in 1999 by professors Jordi Puig-Suari and Bob Twiggs, of California Polytechnic State University and Standford University, respectively, and though this concept is now over 20 years old, it’s continued to gain traction—especially since 2013 (Figure 1):
Figure 1: Data collected by the Nanosats Database shows a marked increase in the number of nanosatellites launched since 20132.
What is the size of a single unit CubeSat? These are “10 cm × 10 cm × 11.35 cm (3.94" × 3.94" × 4.47") units designed to provide 10 cm × 10 cm × 10 cm (3.9" × 3.9" × 3.9") or 1 liter (0.22 imp gal; 0.26 US gal) of useful volume, with each unit weighing no more than 2 kg (4.4 lb).” Standard CubeSats are generally comprised of three such units (3U), a specification that accounts for over 40% of all nanosatellites launched to date, though form factors can range from a single unit (1U) to as many as 12U, which is achieved by stacking 3Us side by side1.
Think of a 1U CubeSat as about the size of a quart of liquid but weighing twice as much as a quart of water.
Now, consider the size of circuit boards. Can you think of any IPC specification that refers to a standard size? I can’t, but many others have, and they’ve formed their own consortia to publish standards on their products’ interchangeability. The PC/104 Consortium3 is one that adopted a 3.550 in × 3.775 in format for boards. Say, that sounds like it would fit nicely in the CubeSat space of 3.94 in × 3.94 in, doesn’t it3? And with so many CubeSats being launched, Cal Poly engineers needed to develop a missile format to launch these “same size” satellites; this system, called the Poly-Pico Satellite Orbital Deployer (P-POD), allows all CubeSats regardless of length to be launched and deployed using a common deployment system4.
Enough about the size of the electronics in our analysis—what about weight and power? Since the folks who work on CubeSats have specified maximum weight values, weight is less of a concern than it otherwise would be. The CubeSat standard sets the maximum mass of a 1U CubeSat at 1.33 kg, with larger masses selectively considered5.
Power, though, is more subjective. Power is based on the use rate, or the ability of a CubeSat to re-charge its solar batteries and the weight of battery that can be budgeted. As CubeSats are in low Earth orbit, they circle the earth at a minimum altitude of 93 miles every two hours or less, meaning that our satellite can only charge its batteries half the time. Additional factors, such as deployable solar surface, efficiency of solar cells, battery storage efficiency, etc., must also be considered. In our example SWaP design, our CubeSat’s peak power is 34 watts—about as powerful as a pretty dim lightbulb. Of course, designers do not intend a CubeSat to operate at peak power right from launch, as both the solar cells and the battery will degrade over time. Still, 18 charge/discharge cycles per day equates to 6,500 cycles per year, in sharp contrast to the 365 diurnal cycles for batteries and solar cells on earth. Perhaps the only saving grace is that the CubeSat under discussion only has an expected lifespan of three years—but that is nowhere near the longevity required by most defense applications.
To finish off our CubeSat analysis: How much does it cost to launch a 1U CubeSat? Prices vary, but a high ballpark estimate comes in at around $100,000, though $75,000 per kg is a more likely figure.
Of course, many defense-related SWaP applications are earth-based. “Man-portable” electronics equipment, which weighs in at 14 kg or 31 lbs.6, is an area of emerging opportunity for such applications. Some sources carry that concept further with man-portable air-defense systems (MANPADS) or even man-portable anti-tank systems (MANPATS). As their names imply, MANPADS are weapons systems intended to bring down low-flying aircraft, while MANPATS are designed to destroy battle tanks. The SWaP here is principally the launcher and the weapon, as well as any additional electronics necessary for target acquisition.
Another SWaP situation is making warfighters that are capable of analyzing the immediate battlespace, communicating with their units, and transmitting essential targeting information to weapons systems that they cannot personally operate. I think we have all become familiar with the importance of 5G communication through our own smartphones. Can we bring that same technology to the battlefield? Cellphones may be small, but the 5G base stations required to link phones together are not. Could a single designated infantryman, then, become a “cell tower”? It’s something to think about.
Finally, most of us have heard that “quantum computing” is the key to future electronics. What if that bulky quantum computing environment could be shrunk down to become man-portable? DARPA is funding the Quantum-Inspired Classical Computing (QuICC) contract, and who knows—man-portable quantum computing systems (MANQUCS) may be just around the corner.
- “CubeSat,” Wikipedia.com, Nov. 28, 2022.
- “Total nanosatellites and CubeSats launched,” nanosats.eu, Aug. 01, 2022.
- “PC/104 Consortium,” Wikipedia.com, Sept. 27, 2022.
- “CubeSat Concept: P-POD (Poly-Picosatellite Orbital Deployer),” eoPortal.org, May 30, 2012.
- “Section 3.2.10-126.96.36.199,” by The CubeSat Program at Cal Poly SLO, CubeSat Design Specification Rev. 13, Feb. 20, 2014.
- “Man Portable,” TheFreeDictionary.com.
Dennis Fritz was a 20-year direct employee of MacDermid Inc. and is retired after 12 years as a senior engineer at (SAIC) supporting the Naval Surface Warfare Center in Crane, Indiana. He was elected to the IPC Hall of Fame in 2012.