What’s in a power processing unit?

Umair Siddiqui
Oct 23 · 4 min read

Traditionally, spacecraft engineering is structured with propulsion engineers focusing on maximizing thruster performance while systems engineers are tasked with the challenge of fitting and integrating the propulsion system — which includes much more than just a thruster — into the spacecraft.

That’s at least in part because electric propulsion (EP) components have historically been sold piecemeal. Thrusters are a separate product from power processing units (PPUs), and fluid systems may be sold by an altogether different company.

What we’re building at Phase Four is fundamentally different from that traditional electric propulsion paradigm, so I’ll start with a quick vocabulary review to get us all on the same page. When we talk about electric propulsion systems, we mean the complete package: thruster, PPU, system software, propellant tank, and fluid management. For example, Maxwell, Phase Four’s turn-key propulsion system for small satellites, is laid out below.

Phase Four’s Maxwell propulsion system

Today, I want to dig into Maxwell’s PPU (which sits just next to the thruster head in the diagram above) for two reasons: it’s a commonly misunderstood part, and it’s the component most responsible for Maxwell’s compact, lightweight form factor.

What does a PPU do?

The Phase Four PPU takes DC battery power from your spacecraft and delivers RF energy to the thruster head, which efficiently ionizes the xenon propellant and accelerates that plasma out of your spacecraft.

How is your PPU so tiny?

It comes down to basic arithmetic. RF plasma propulsion technologies have 1 power input and 1 power output, whereas Hall thruster-based systems must manage 1–2 power inputs, and up to 6 power outputs. At a minimum, power outputs include: 1) a heater for the cathode, 2) an ignition circuit, 3) electromagnet generation, 4) an anode, and 5) a cathode.

Maxwell’s PPU was designed and developed in-house at Phase Four. It’s made up of three core elements:

1. The Voltage Regulator. The voltage regulators you most commonly use sit in your cell phones and laptops. Maxwell’s voltage regulator works to make sure the flow of power from your spacecraft battery, which can arrive in fits and spurts, is even.

2. The RF Inverter. The inverter takes the steady DC power from the regulator and “inverts” it into an AC radio frequency signal at low MHz frequencies. MHz inverter circuits can be very small and efficient, unlike GHz frequency systems commonly used in other RF industries. An example of this is in your cell phone’s fancy new wireless charging mat. Wireless charging mats use low frequency RF power inverters to generate the signal that charges your phone. Because they’re low frequency and efficient, they can fit into a tiny charging mat on your desk. We leverage these design architectures, and even some of the same RF components, to deliver RF power to our thrusters in a size- and power-efficient manner. You can’t physically achieve this at higher frequencies, and we couldn’t have achieved this without the advancement of the wireless charging industry pushing state-of-the-art component developers.

3. The Matching Network. The final piece of the PPU puzzle is Maxwell’s matching network, which ensures that the RF gets absorbed by the xenon plasma propellant. By combining our RF plasma physics knowledge with the latest in RF electrical engineering (two fields that don’t really communicate with each other normally), we’ve been able to miniaturize the matching network to a few passive coin-sized components. Compare that to the matching networks I used in grad school, which were a bit larger than an xBox.

Representative PPU volume in a simulated spacecraft bus, Maxwell vs. traditional EP

Maxwell’s PPU is roughly the size of a couple ice cream sandwiches. Traditional electric propulsion PPUs need two gallons of volume, weighing 5x Maxwell’s PPU. Drink up!

What does this mean for satellite manufacturers and operators?

Firstly, the small PPU allows Maxwell as a whole to fit in a toaster. This brings plasma propulsion to a class of small satellite that couldn’t otherwise fit any meaningful plasma thruster on board. Also, Maxwell’s lightweight form factor translates to real cost savings on launch, because spacecraft mass is directly proportional to launch cost.

Dry mass of Maxwell vs. legacy EP systems in the same power class

Each kilogram of mass represents about $10,000 in launch cost. Relative to traditional EP systems in its power class, Maxwell saves you $150,000 per spacecraft. If you’re launching 7 spacecraft, that translates to over $1M of cost savings on Maxwell’s mass alone. Nice.

To learn more about Maxwell, visit phasefour.io/maxwell. To learn more about RF plasma propulsion and our performance results, visit phasefour.io/rf-plasma-propulsion.

Plasma Matters

The Phase Four Blog

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