Rideshare launches have made access to orbit affordable, but they come with a catch: every passenger is dropped into the orbit chosen for the whole mission. Orbital transfer vehicles, often called space tugs, close that gap. They ride to orbit on a rocket, then carry satellites to individual destinations before releasing them.
How an orbital transfer vehicle works
A space tug is a small spacecraft with its own propulsion, power and avionics that carries several customer satellites. After separation from the launcher, it performs manoeuvres to change altitude, inclination or orbital plane and deploys each satellite where it is needed. Some vehicles also host payloads that stay attached for in orbit demonstrations.
What problems they solve
- Custom orbits: satellites can reach altitudes or local times that differ from the rideshare drop off orbit.
- Constellation phasing: satellites can be spread out along an orbit faster than with their own small thrusters.
- Satellites without propulsion: small satellites that cannot manoeuvre themselves still reach a suitable orbit.
- Hosted payloads: sensors or technology demonstrators can fly without a dedicated satellite bus.
Examples in the market
Several companies offer or develop transfer vehicles. D-Orbit operates its ION Satellite Carrier on rideshare missions, and Impulse Space has developed the Mira vehicle and the larger Helios kick stage. Launch providers also offer kick stages, such as Rocket Lab's Photon based platforms, that perform a similar role for dedicated missions.
Limitations
Transfer vehicles add another integration step, another set of interfaces and another spacecraft that must perform correctly. Large plane changes remain expensive even for a tug, so the achievable range of orbits is limited by the propellant on board. Customers should also clarify how long the satellite will stay attached before deployment, since this affects power and thermal design.
A historical note
The idea is not new. In the Space Shuttle era, communications satellites were deployed from the cargo bay and used attached upper stages to climb to geostationary orbit, as shown on the cover image.
Related reading
- Rideshare or Dedicated Launch? How Small Satellites Get to Orbit
- In-Orbit Servicing: How Satellites Are Refuelled, Repaired and Extended
- Delta-v Explained: The Currency of Spaceflight
Sources
- NASA Small Spacecraft Systems Virtual Institute: State of the Art of Small Spacecraft Technology
- D-Orbit
- Impulse Space
Cover image: Deployment of the RCA Satcom K-2 communications satellite from Space Shuttle Atlantis during STS-61B, 1985. Credit: NASA (61b-38-036w), public domain.