Something that’s been bugging me for some time is the confusion surrounding the term “space tug”. The term’s been used to describe at least two very different ideas for many years now. At NGEC-2, I tried to inject a little clarity into my working group’s discussions by drawing the distinction between “tugs” and what I called “ferries”, and I was wondering if others thought it was a useful distinction (and if anyone had a less snicker-drawing nickname then “ferries”–you would think the conference took place somewhere near San Francisco or something from all the chuckles that term drew…)
Under my proposed classification scheme, a “space tug” would be a spacecraft of some sort that primarily is used for maneuvering target spacecraft/objects in the near vicinity of a space station or another spacecraft. For instance, the CSI and CSI/SSL systems proposed for COTS 1 and COTS 1.5 would both fall under this category (and Orbital Express would also likely fit under this category). A “space ferry” on the other hand is a spacecraft that hauls other spacecraft, cargo, or people from one orbit to another in a reusable fashion. For instance, CSI’s or Space Adventures’ respective “Soyuz-Around-the-Moon” concepts would somewhat be examples of a one-use ferry.
Basically tug == prox ops, ferry == large orbit transfers.
Both are very important capabilities, but while they have some overlap in requirements, many of their requirements lead to very divergent capabilities.
Tugs for instance are explicitly designed for proximity operations in mind. A good tug system implementation would likely have one or more robotic arms for better handling, grappling with, and berthing target spacecraft. A tug likely doesn’t have a huge amount of propellant on board. Enough to move things around between various low earth orbits, and to maneuver around the station, but total delta-V capability is probably in the low-hundreds of m/s range. Tugs want to be very robust. The very low delta-V requirements actually make a tug very mass insensitive. So long as most of the things you’re moving around are an order of magnitude or more bigger than you, even doubling the mass of your tug has only a minor effect on the total propellant used for tug operations.
Ferries on the other hand are high-performance spacecraft. The delta-Vs necessary for a useful space ferry are on the order of 4-8km/s (though those last 4km/s are probably going to be “dead heading” ie. flying the ferry back to LEO with no payload attached). In the case of a chemically fueled ferry, this means it looks very similar to an upper stage–mostly take, one or two big engines, and some hardware on both ends. An inflatable aerobrake might not be a bad idea depending on how much it weighs. It might not really need much in the way of prox ops capabilities, just navigation and rendezvous capabilities. Ferries are typically going to be much bigger than their cargoes, while tugs will typically be much smaller.
Both ideas also provide different benefits.
The key benefit of tugs is that they enable launch vehicles and their cargoes to be much simpler. Instead of having to come up with a “last mile” solution for every new passenger or cargo spacecraft, you can have a standardized tug interface, and have the tug do all the hard work. That means that it becomes easier for launch providers to get involved in station resupply, because they’re now just taking a standardized container, launching it to a specific orbit, and holding attitude until the tug can swing by and pick things up. Right now, most crew or cargo deliveries to the station require a system that uses a complicated service module and prox-ops hardware to actually get to the station, which results in fairly poor launched mass to delivered mass ratios. What tugs allow you to do in the cargo case is to drastically reduce the amount of wasted mass required to deliver a given mass of cargo to a station. Instead of having your cargo vehicle be a fully capable spacecraft, all it is now is a pressure shell, with some tug interface attachment (probably something brutally simple involving a couple of “hand holds”), and a passive CBM adapter on the other end. If you’re launching to a station that’s in a resonant orbit that provides frequent “first or second orbit rendezvous” opportunities, you might even be able to dispense with the need for power, communications, or even much in the way of thermal management. In other words, the cargo container starts becoming a lot more like your dumb intermodal container that you see on earth (just much lighter…). Tugs can also serve an emergency role for spacecraft that do have their own prox-ops capabilities, by serving as a backup in case something breaks (or in case multiple docking attempts need to be made and the visiting vehicle runs out of maneuvering propellant). Tugs are also a critical enabler for propellant depots. For propellant deliveries, the propellant can go through relatively narrow tubes (compared to what a human could fit through for instance), which means that a tug could allow for a very simple and lightweight standardized propellant transfer interface to be developed that could just be welded into the delivery tank. This interface could be 100% passive–just some mechanical attachment points, and the quick disconnect ports for fluid and if necessary power. A tug with robotic arms could then take all of the complexity onto itself for the fluid coupling. Much better than trying to make an automated docking and fluid coupling system that has to fly on each and every propellant delivery.
In a nutshell, tugs allow you to take all of the most complicated parts of getting people, propellants, and provisions to a station, and offloads it to either the launch vehicle, or to a reusable vehicle that always stays in orbit, doesn’t have to reenter, etc. Why lug all of that hardware with you each and every time if you can leave it at the destination. Why require each and every company that wants to launch stuff to a station to then also have to come up with their own prox-ops solution? Solve the problem once, and then you don’t have to keep solving it again. If your delivered payloads start outgrowing your tug, the right option might be to build more of them and operate them in a group, instead of designing a newer, bigger model. I think tugboats do just that for very large ships here on earth–instead of building a super jumbo tug, they’ll often just use two or three smaller ones.
Ferries provide very different benefits. First off, and most importantly in my opinion is the fact that ferries (when combined with propellant refueling capabilities) allow you to launch a given exo-LEO vehicle on a much smaller, higher-flight rate vehicle. Dave Salt has on many occasions mentioned that an RLV with an 8000-9000lb payload capability could pretty much service the entire GEO satellite market. Most of the mass required in LEO (I know, many GTO launchers don’t even stop in LEO, but it’s still a useful point) to put a satellite into GEO is not the satellite, or its “beginning of life” propellants–it’s the upper stage, its propellants, and the circularization propellants on the satellite. By having a ferry that operates between LEO and GEO, that has refueling capabilities in LEO, you can launch the largest commercial and government exo-LEO missions without requiring anything bigger than a bottom-of-the-line EELV. In fact, you can even launch manned lunar missions using launchers no bigger than an Atlas V 401 or a Falcon IX (a “Phase One” Atlas V might be a little nicer, but not because of the extra payload to LEO, but because the ICES stages envisioned are scalable and potentially much bigger than a stock Centaur stage, and would thus make a great starting point for a passenger transport ferry). For geostationary satellites, ferries can provide an extra service. Because the ferry can deliver things all the way to GEO, the satellite they’re carrying could possibly forgo its “main propulsion system” and circularization propellant tanks in exchange for more station keeping tanks, more transponders, more solar panels or what have you. Or, you could leave the main propulsion system on, but have the capability to retire the satellite to a different, lower-value GEO slot, where it could spend its last few years before moving itself to a final disposal orbit. For instance, by the time a satellite is nearing 15 years on orbit, it may be a bit obsolete for first-world markets, but maybe it would still be useful for a different GEO slot servicing locations in the third world, or sparsely populated areas in the Pacific for instance (much like how passenger jets in the US are often “retired” only to be refurbished a bit and sold to third world countries at a much lower price). Either of these can help you get more revenue out of a given satellite launch. There are probably plenty of other benefits of ferries that I’m not thinking of right now, but those are just some thoughts.
Ferries can be based around either chemical or solar electric propulsion systems. Some cargoes don’t mind a slow spiral out through the van Allen belts, and thus can be shipped by the more mass efficient (and hopefully therefore more cost efficient) solar-electric “slow boat”. Other cargoes (people, cryogens, and possibly GEO satellites) can be shipped via a much faster chemical ferry. Sure, it’s less mass efficient, so you’re going to be paying for launching a lot more material, but the hardware is relatively cheaper, it can make more flights before being retired, and most importantly, you’re not cooking your payload for several weeks in the van Allen belts. For GEO satellites right now, most of their radiation exposure (for their entire 15 year operation timeframe) happens in just one or two passes through the van Allen belts, so minimizing the time spent there might give chemical ferries a leg up (contra conventional wisdom).
Anyhow, what do you guys think? Does drawing this distinction make sense? And does anyone have a term better than “ferry” for a reusable transfer vehicle? Every time I’ve tried to bring up the idea of a “space ferry” there at the conference, the term would draw smirks or chuckles, or comments along the lines of “I guess NASA Ames is close to San Francisco after all”…