Why Rocket Engines Remain Hard

Jeff Thornburg helped build Raptor at SpaceX. Now he’s building something new: a spacecraft designed for rapid maneuverability across orbits powered by solar thermal propulsion.

In this episode of Valley of Depth, we get into the technical, strategic, and commercial implications of in-space mobility and why it matters now. Topics include:

• The hardest parts of building a rocket engine (and why most startups get it wrong)
• The origins of the Raptor engine and lessons from SpaceX
• Why Portal is betting on solar thermal propulsion
• How Supernova enables fast, responsive movement across LEO, MEO, and GEO
• The case for refueling in orbit and designing without dependencies
• Why defense needs faster spacecraft and what commercial use cases follow
• How Thornburg sees Starship changing the economics of space

and much more…

• Chapters •

00:00 – Intro

01:44 – What drew Jeff to creating engines?

05:13 – Why is building rocket engines still so difficult?

11:50 – Jeff's time at SpaceX

17:52 – Stratolaunch and Commonwealth Fusion and why Jeff moved on

22:50 – Origin of Portal

25:25 – Commercial use case

27:00 – 5 year vision for Portal's architecture

32:01 – Pricing

34:52 – Solar thermal propulsion

38:14 – What comes after Portal's Supernova?

41:15 – Customer traction and Supernova flight

45:44 – Competition in solar thermal propulsion

49:20 – Fundraising prior to the first launch

51:18 – Portal in the next 10 years

54:53 – State of Starship

• Show notes •

Jeff’s socials — https://x.com/JeffThornburg

Antares’ socials — https://x.com/PortalSpaceSys

Antares’ website — https://www.portalsystems.space/

Mo's socials — https://twitter.com/itsmoislam

Payload’s socials — https://twitter.com/payloadspace / https://www.linkedin.com/company/payloadspace

Ignition’s socials — https://twitter.com/ignitionnuclear /  

https://www.linkedin.com/company/ignition-nuclear/

Tectonic’s socials  — https://twitter.com/tectonicdefense / https://www.linkedin.com/company/tectonicdefense/

Valley of Depth archive — Listen: https://pod.payloadspace.com/

• About us •

Valley of Depth is a podcast about the technologies that matter — and the people building them. Brought to you by Arkaea Media, the team behind Payload (space), Ignition (nuclear energy), and Tectonic (defense tech), this show goes beyond headlines and hype. We talk to founders, investors, government officials, and military leaders shaping the future of national security and deep tech. From breakthrough science to strategic policy, we dive into the high-stakes decisions behind the world’s hardest technologies.

• Payload: www.payloadspace.com
• Ignition: www.ignition-news.com
• Tectonic: www.tectonicdefense.com