This is a post to respond to another blog post, which advocates an idea that has almost a surprising degree of merit when held up in a fair economic comparison as a component of other architectures that people are already talking about.
One of the incredible mentions in the Elon Mars concept was a thousand spacecraft in orbit ready for the Mars launch…
(as an aside, the thumbnail above is terrible, because it actually comes from the profile picture of a commenter)
Okay, let’s rundown the basics of this idea. Here is my quick sketch.
This is mainly diagraming the orbits, and the most important things are far from being represented here. The real orbital action will happen at the tip top (the apogee) of the elliptical orbit.
One thing that might strike your eye is the relatively large total number of burns involved. This is quite counter to “Mars Direct” type approaches, which would reduce this particular diagram to potentially just 1. The reference-point for the proposal is primarily the Elon Musk proposal of the Interplanetary Transportation System (ITS), or whatever it will eventually be named. However, I want to note that the ITS would involve orbital refueling, so some of the simplicity may be deceptive.
There is also more than 1 type of vehicle involved. I would call these:
- Initial multi-stage launch and injection vehicle
- Some tiny rocket to accomplish circularization
- A reusable departure stage
- Some kind of tiny rocket or aerobraking shell for Mars arrival (unclear at this point)
Still, this is neglecting the interesting stuff at the high Earth orbit, which consists of an odd sort of flotilla of cargo in a staging lattice.
Timing — The Main Weakness
I don’t believe it was actually specified what altitude the parking orbit would be at, but the wording of “one to four days” was used, which already gives a pretty concrete range of altitudes. During this time, the reusable departure stage needs to swing by the Earth (firing its engines) to put the cargo on its Mars injection trajectory, but after that it would likely need to do another orbit around Earth in this elliptical orbit so that it could actually sync up and meet with the flotilla again.
This would be very difficult to achieve in one to two days because of a conflict with another objective — minimization of the propellant used for the circularization burn. Raising the apogee of an elliptical orbit is useful work, because it contributes to the momentum vector actually needed to get to Mars. The circularization burns, on the other hand, are completely wasted propellant. In order to maximize reuse, you want this as close to Earth as possible, but in order to minimize wasted propellant, you would want this as far away as possible.
Writing about this conflict seems to make one possible mitigating innovation relatively obvious. You could have 2 parking locations in orbits exactly 180 degrees from each other, both circular, and both at the same high altitude. This way, the reusable departure stage could depart from one with its payload, launch it, then meet up with the other parking flotilla to refuel and get its next payload.
This results in more duplication, but it would not negatively impact reusability, and on a sufficiently large scale there are essentially no drawbacks. Since you would be duplicating the propellant depot and the staging structures, there’s no obvious economic penalty because economies of scale are not relevant except for logistic and operational concerns.
Let’s Think About Schedules
In a certain way, this might fall short of Elon Musk’s ambitions when it comes to the manned shipments. I would argue that the reusable departure stage may still be useful as a first stage for a manned ship, but the stated goal of the ITS is to get people there in as little as 90 days (more realistically 120 or something along those lines). This actually changes the departure time that one might use, in addition to imposing a punishingly large Delta V requirement.
Assuming this goal of fast human transit times, the launch bottleneck would occur twice within a single cycle of launch windows. Low energy transfers would be favored for cargo, and a particular time window that balanced radiation tradeoffs would demand a flurry of manned missions be launched in rapid succession. These dual windows would actually increase the value of the departure stage’s reusability, and the cargo window would be quite generous indeed.
Economics of Boosters vs Stuff
Even at the moment, SpaceX is rumored to be aggressively pursuing 2nd stage reusability. Frankly, it’s not even clear to me what this would mean for geosynchronous orbits, which so many of their customers need. But assuming that some scheme for this is actually proven to work, this makes the architecture basically 100% reusable aside from the stuff actually attached to the payloads and used for Mars landing or capture (a reusable system around Low Mars Orbit would also be tremendously interesting).
The economic problem with space exploration is a combination of the high capital costs of the rockets, themselves, combined with a series of consequences that result in the payloads themselves becoming very expensive, keeping the industry locked in a death spiral of high-costs.
In order for any part of the architecture discussed here to make sense, we would need to be sending a large amount of mass of extremely low-value payloads. How else would it make so much sense to conserve rocket engine costs, if you just go on to blow the vast majority of your budget on the payloads. Nonetheless, I certainly see this making sense for a large Mars city. As the scale of your operations go up, the specialization involved in the components that need to be shipped goes down dramatically. Greenhouses would be an example of a component that is very inexpensive compared to robotics that we tend to send today. A manned presence would demand many such things, although other expensive stuff like space suits would still be a part of the mass sent.