Tom Mueller closes $500M Series D for Impulse Space, betting on in-space mobility as Starship drives down launch costs

Impulse Space closes $500M Series D, betting on in-space mobility as launch gets cheap

Tom Mueller, who spent the last six years at SpaceX working on Starship before founding Impulse Space five years ago, has just closed a $500M Series D led by 137 Ventures. The company now has 500 employees, a 200-person open headcount, and is moving into a 240,000 sq ft facility in El Segundo — the same block as its previous 60,000 sq ft space, a long way from the 7,000 sq ft garage where it started.

The core thesis

Mueller's bet is that launch is largely solved. Starship will drive down the cost per kilogram to orbit dramatically, but a rocket that gets cargo to low Earth orbit still needs something to move that cargo where it actually needs to go. Impulse builds the next leg of that journey.

The company's MIRA spacecraft handles precision maneuvering in LEO — hosting, deploying, and rendezvousing with payloads. Three are flying now, more are in build. The bigger product is Helios, which Mueller describes as a rocket on a rocket: a third stage bolted to a Falcon 9, carrying 12 tons of liquid oxygen and liquid methane, that gets a payload from LEO to GEO in a single day. The alternative for commercial operators is spending months using electric propulsion to crawl up to geostationary orbit. Helios also delivers roughly 10x the payload to the moon and 5x to Mars compared to flying a Falcon 9 alone, according to Mueller.

“Our Helios product is what we call a rocket on a rocket. We add basically a third stage to a Falcon Nine — 12 tons of liquid oxygen and liquid methane, a very high performance pump stage combustion engine — and it can get you from LEO to GEO in a day. It basically helps Falcon Nine do what Falcon Heavy does for tens of millions of dollars less. We've got 500 employees right now and 200 open job requisitions.”

Why GEO still matters

Geostationary orbit has been somewhat overshadowed by the Starlink-era LEO build-out, but Mueller argues it remains strategically critical. You can cover the entire Earth with three satellites at GEO. The Space Force is increasingly active there. And adversaries — Mueller names China and Russia explicitly — are already maneuvering around U.S. satellites at that altitude, which creates demand for American in-space mobility capabilities.

Where Starship fits

Starship is an accelerant, not a competitor. Mueller says Starship will be the definitive LEO cargo vehicle because it can reach orbit fully reusable without refueling, dramatically lowering cost per ton. But sending Starship itself to GEO or the moon means hauling 120 tons of stainless steel up and back — which may not pencil out when a smaller expendable upper stage like Helios can do the job far more cheaply. For truly high-energy missions like crewed lunar or Mars flights, Starship with on-orbit refueling is the right call. For most commercial and government payloads, Mueller thinks Helios will be more economical.

Manufacturing and vertical integration

The $500M is going into production capacity. Mueller is explicit that the hiring mix has shifted from development engineers to manufacturing engineers and technicians. The playbook mirrors SpaceX's — bring everything in house, control cost, schedule, and quality through vertical integration. Raw materials go in, finished spacecraft come out. The 240,000 sq ft El Segundo expansion is the physical infrastructure for that ramp.

The longer arc

Mueller is more bullish on the moon than Mars in the near term. He argues the moon has everything needed to build megastructures in space — metals, water, oxygen — without the burden of launching all that mass from Earth. He also points to near-Earth asteroids as underappreciated targets: some have gravity wells measured in meters per second rather than the moon's 2.4 km/s, making the propellant economics dramatically better even if they take longer to reach.

The SpaceX IPO announcement, in Mueller's view, has supercharged the broader space industry. Elon Musk's public talk of orbital megastructures and millions of AI servers in space aligns directly with what Impulse is building toward — the infrastructure to move things once they're off the ground.

With 200 open roles spanning propulsion, avionics, structures, software, guidance navigation and control, and orbital mechanics, Impulse is hiring across essentially every discipline a spacecraft company needs.