What are Orbital Transfer Vehicles (Space Tugs)?

1. What are OTVs?

An OTV is a space vehicle capable of moving, and subsequently deploying, payloads from one orbit to another. The analogy of a tug boat tugging a ship is also useful when thinking of OTVs, hence the synonym Space Tug.

NASA’s old proposal for a Space Tug. Source.
  • Electric Propulsion OTVs also fire fuel out the back, but they do so after being accelerated using magnets, electricity or both. Electric propulsion systems are typically 10–100x more efficient than chemical, meaning OTVs using electric propulsion need to carry far less fuel for the same manoevure. The downside of electric propulsion is that it produces significantly less thrust, and therefore takes longer to execute a maneovure. This can be a deal-breaker for time-sensitive customers.
Left: Chemical Propulsion system firing. Right: Electric Propulsion system firing. Sources: [CP][EP]
  • Reusable OTVs are OTVs which can be re-used in orbit many times. They collect payloads sent up into orbit on a rocket by docking with them, transporting them to the desired orbits, and returning to dock with a new payload, and so on. Reusable OTVs are able to spread their costs across several missions, meaning they have the potential to be significantly cheaper per mission, but they have inherent up-front R&D costs compared to expendable OTVs, namely in rendezvous and docking technology. The below diagram explains how reusable OTVs work on a high level.
Reusable OTV Concept of Operations. Source — porkchop M Reusable OTV.

2. How do OTVs make money?

As I mentioned above, the reason that there’s been a resurgence in OTVs is due to the idea of last-mile satellite deployment, particularly for huge batches of satellites which form constellations.

SpaceX Starlink Constellation. Source.
iPhone 14 connects directly to a satellite constellation to provide users with emergency services, no matter where they are. Source.

3. What’s next for OTVs?

In my honest (but biased) opinion, the future of OTVs is reusable. As I’ve shared in other blog posts, we’re developing porkchop M, which is a reusable electric OTV, but I don’t want to use this blog post for promoting it.

When SpaceX introduced the Falcon 9 and Heavy vehicles, there was a sudden drop in $/kg to orbit around 2017. Source.

Conclusion

I hope this post is able to get people up to speed with OTVs, why they’re quickly gaining significant popularity, and why they will inevitably go reusable over time.

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porkchop

porkchop

porkchop is a Stockholm-based startup with the goal of establishing an interplanetary economy.