What are Orbital Transfer Vehicles (Space Tugs)?
Matija Milenovic, Co-Founder & CEO of porkchop
In the last years, there has been a massive resurgence in interest in Orbital Transfer Vehicles (OTVs), which are also sometimes also referred to as Space Tugs. I’m speaking with an increasingly large number of self-proclaimed “non-technical” people who know a ton about OTVs, which is fantastic! I want to spread the word further, because this is a technology you don’t want to miss out on.
Given the rapid growth of the space industry in the past decade, OTVs have unsurprisingly gotten a lot of people excited. In this post, I want to explain why this is the case. It’s important to understand what OTVs are, how they generate income, and what the future of them looks like.
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.
The benefit of OTVs is that they allow satellites to outsource their orbital manoeuvres to another vehicle, thus reducing the cost & complexity of their own satellite.
The idea of an OTV has been around for a long time, even before the first satellite was ever launched into orbit. Over the years, the use cases of OTVs have changed over time — from in-orbit servicing, to transferring humans from one space station to another. The reason that there’s been a resurgence in interest is due to the idea of last-mile satellite deployment, particularly for huge batches of satellites which form constellations. I’ll elaborate on this further down below.
Given that one of the main jobs of an OTV is to move between various orbits, we often classify OTVs based on which propulsion system they use to perform such manoeuvres. There are two main categories:
- Chemical Propulsion OTVs use a technology similar to the engines you’d see on a rocket taking off from Earth. Usually this involves burning pressurised fuel, resulting in quite high thrust. The key downside is that chemical propulsion is inherently inefficient, meaning to achieve the same movement, you need to carry more fuel. More fuel means more mass that you need to carry into orbit, and at ~$10,000 per kg, this is a heavy price to pay. This can be a deal-breaker for price-sensitive customers.
- 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.
(To learn more about propulsion efficiency, see Specific Impulse and Delta-V)
As you can see, both chemical and electric propulsion have their pros and cons. Which one you should use primarily depends on how much manoeuvring the customer requires, how quickly they need to deploy their payloads, and how big their budget is.
The second important distinction that needs to be made with OTVs is whether it is expendable or reusable:
- Expendable OTVs are single-use. They deploy their payloads at whichever orbits the customers require, and then they de-orbit themselves. Sometimes they stay in orbit for a few extra months to host a payload, but this is a secondary use-case. Given that building, assembling, testing and launching an OTV isn’t cheap, expendable OTVs are quite wasteful.
- 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.
Finally, I want to note that the propulsion system an OTV uses does not necessarily dictate whether it can be reusable or not i.e. there can exist reusable electric OTVs, reusable chemical OTVs, expendable electric OTVs and expendable chemical OTVs. In fact, there are companies developing each of these types.
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.
Satellite constellations are collections of many satellites (often 100’s or 1000’s) which orbit in specific orbits so as to provide imagery or internet connectivity all over the globe.
This is nothing fancy or new. GPS is a constellation of 24 satellites which provide us with accurate navigation. SpaceX Starlink is a constellation of over 3000 (and growing) satellites which provide satellite internet globally. In fact, yesterday Apple’s newly-announced iPhone 14 lineup uses satellites to get users access to emergency services, even if they don’t have standard cell coverage!
(On a side note, I’m beyond excited for phones which can directly communicate with satellites. We’re living in the future!)
As you can see, we’re becoming increasingly reliant on constellations, and there’s a growing number of companies looking to deploy them for a huge variety of use cases. There’s a great database which tracks many of these constellations.
The issue is that we’re trying to deploy these constellations much the same way we’ve been doing since the 1960’s. Either these constellation satellites need to individually manoeuvre to their final orbit, requiring costly, complex and risky on-board propulsion, or they need to book an entire rocket launch to get a few satellites into the right orbit.
This is where OTVs will save the day. Imagine if satellite constellation operators didn’t need to worry about reaching their orbit, but could hail a ride there instead? On top of that, what if it was actually cheaper and faster for these satellites to be deployed compared to existing methods. Imagine a world where your phone not only uses satellites for emergency services, but which offered you 5G connectivity even if you’re in the middle of the ocean.
The value of OTVs lies in their ability to make access to space easier for companies wishing to deploy constellations. Also, since constellations aren’t deployed at once, but rather over 3–5 years, it means that OTV companies can have somewhat more predictable cash-flow over this period, along with recurring revenue when the time comes for parts of the constellation to be replenished.
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.
My reasoning stems from the fundamental value proposition that an OTV offers — transporting satellites from one orbit to another. It’s the same as any transportation vehicle (airplanes, cars, ships, etc.). When we look around, we can see that there simply aren’t any expendable AND profitable transportation vehicles. The same holds true for expendable space transportation. SpaceX transformed the industry by making their boosters reusable. We want to take this a leap further and reuse vehicles which only reside in orbit.
As for propulsion, I think that there will always be room for both chemical and electric propulsion OTVs. Both have characteristics which the other doesn’t, and based on our discussions with constellation companies, needs vary from company to company when it comes to speed v.s. price, and I cannot see this changing in the coming 3–5 years. Small, seed-funded companies will continue going for the lower-cost option, whereas growth-stage companies with a lot of funding care more about increasing market share.
The crux is that having reusable OTVs means that one could in theory have both chemical and electric OTVs on orbit, and only use those which customers require.
Reusable OTVs are by no means the no-brainer. The up-front R&D valley of death produced by the need to successfully and repeatably perform rendezvous and docking is a serious, but solvable, challenge.
Finally, it’s worth noting that the achilles’ heel of the OTV wave is how satellite constellations progress with regards to their funding and market adoption. If demand for satellite constellations ever drops off, so too will demand for OTVs. Again, this is why I believe that doubling down on reusability is the best way forward. Reusability (i.e. the ability to rendezvous and dock) also allows OTVs to also become in-orbit servicing vehicles, and open up a whole new market and set of customers to such companies.
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.
This is by no means an exhaustive course on all things OTV, but I encourage you to use the attached links as a starting point for further reading, if you’re interested.
And of course, if you’d like to learn more about what we’re doing here at porkchop, never hesitate to send us a mail. We typically respond in under 12 hours: email@example.com.