Y Combinator · Hard Tech
Hard Tech Startups Are Built One Milestone at a Time
Hart Aviation went from a 3D-printed demo to a United Airlines pre-order by treating every capital raise as proof of physical progress — a masterclass in milestone-driven hardware development.
Tap a timestamp pill below to jump the video to that moment.
The problem
Hard tech needs credibility before cash — but credibility requires building things first
Most hardware startups face a brutal chicken-and-egg problem: investors want proof that the technology works, but proving the technology works requires capital. Raising on a pitch deck alone is nearly impossible when the underlying physics, manufacturing, and supply chains are still unvalidated. Hart Aviation arrived at Y Combinator with something even thinner than a pitch deck — a small 3D-printed model of an aircraft.
What makes their story instructive is not that they bluffed their way to success. It’s that they recognized the problem clearly and solved it deliberately. Before they had real technology, they focused on generating early commercial interest: Letters of Intent from SAS, Braid, and the three Nordic airlines. The LOIs were real signals, even if the airplane was not.
“I came to YC demo day with a little 3D printed model”
“With YC, we were all focused on getting some like early interest from airlines. We got some LOIs from SAS and Braid and the three Nordic airlines. We really had no technology. Like I came to YC demo day with a little 3D printed model. So we went from having the LOIs to them building like the 400 kW motor which was like the size of a small jet engine.”
The gap between a 3D-printed model and a 400-kilowatt electric motor — roughly jet-engine scale — is enormous. Bridging that gap required a specific kind of discipline: every time Hart needed more capital, they had to show something physical and material, not just a slide or a forecast. That constraint shaped the entire company trajectory.
How they solve it
Chain physical milestones to capital raises, and let each unlock the next
Hart’s approach was to treat each funding round as a forcing function for a specific physical proof point. The LOIs from Nordic airlines attracted the capital to build the 400 kW motor. The motor — something you could stand next to and touch — attracted United Airlines as a pre-order customer. The United deal attracted more capital. The capital is now funding construction of the actual aircraft. Each step is legible and real.
“Every time you need more capital, it needs to be something material”
“If you’re a founder listening to this, the learning that you should have is every time you need more capital, it needs to be something material. Yeah. — Ideally, something physical you can touch. Yeah. That you can show that you made. Their first big airline customer actually came out of the spam folder. — United Airlines came through an inbound email through our info email. Clara was looking at it and she was like, I think this might be a thing. Like he was like cleaning out the spam and that led us like through the front door. We just brought them over, showed them the electric motor we built that was the size of the jet engines we tried to replace and they kind of immediately got it.”
The detail about United Airlines arriving through the spam folder is more than an amusing anecdote. It illustrates what the physical milestone unlocked: when you have a 400-kilowatt motor the size of a jet engine sitting in your shop, a tour is worth more than any sales deck. United “kind of immediately got it.” Credibility that took months to build in hardware can communicate in seconds once the artifact exists.
Hart also made a deliberate product design choice that reinforces the same philosophy. Where most electric aircraft startups design for spectacle — unusual configurations, futuristic aesthetics — Hart’s aircraft looks almost identical to the turboprops it aims to replace. The reasoning is pragmatic: airlines don’t want a sports car, they want a reliable workhorse they already understand operationally. The innovation is hidden, the interface is familiar.
“It looks very much like the turbo props we’re trying to replace”
“And the design itself, most electric aircraft startups try to look futuristic. Hart did the opposite on purpose. — It looks very much like the turbo props we’re trying to replace. We’re not trying to build a sports car. I’d rather build something that looks very conventional but is kind of hiding its Superman cape under the hood.”
Taken together, the milestone strategy and the conservative design philosophy are two sides of the same insight: hard tech succeeds when it minimises the number of new things a customer or investor has to believe in simultaneously. The motor is the only new thing. The airframe looks familiar. The sales call starts with a factory tour, not a vision deck. Each decision reduces the credibility debt that hardware startups routinely accumulate and then fail to pay off.
Takeaway
The quick version
- Start with letters of intent — even without technology — to establish that commercial demand exists before you build.
- Every capital raise must be anchored to something physical you made; a slide is not a milestone.
- Let artifacts do the selling: a factory tour of your working motor closes faster than a pitch deck.
- Reduce the number of new things customers must accept; hide the innovation inside a familiar package.
“I’d rather build something that looks very conventional but is kind of hiding its Superman cape under the hood.”— Hart Aviation founder, Y Combinator
Hart’s arc — 3D-printed model to United Airlines pre-order — is a compressed tutorial in how hard tech compounds: each physical proof point funds the next, and the airline that finds you in the spam folder becomes your biggest validator.