Thursday, October 07, 2010

Thoughts on a SpaceX Lunar Architecture

It's no secret that the SpaceX Dragon capsule has a very impressive heat shield believed to be capable of direct lunar return. Official statements from SpaceX that they intend to add deployable landing gear and leverage the thrusters in order to land on land in the future prompts an obvious suggestion: if it can land on Earth, could it land on the Moon too?

The Dragon capsule has thus far been launched on the Falcon 9 booster, and although that booster is able to put 2473kg into lunar transfer orbit, after using the Draco thusters on the Dragon to enter low lunar orbit the total mass would be under 1876kg.. this seems a bit light for a crewed configuration, especially when you consider that only 1422kg of it could be returned to Earth. And that's just lunar orbit.

We need a bigger rocket, and the official SpaceX plan right now is called the Falcon 9 Heavy. One should not be confused by the name, the F9H is not "heavy lift" in the sense often used by space advocates and policy makers - who should more correctly be using the term super heavy lift. So how heavy is the F9H? Comparison is usually done in terms of lift to LEO, but for our purposes lift to LTO is more interesting at 10622kg. A slight improvement on the Delta IV Heavy at 9984kg.

Now we can imagine a Dragon-lander flying direct from lunar transfer orbit to the surface, it would have a mass of 3766kg when it landed which is quite respectable. For a cargo flight this is fine, delivering 2737kg of payload, but it's unlikely the vehicle would have enough fuel left to attempt an ascent.

Having determined that a single stage direct descent vehicle is unlikely, we're now forced to choose a mission mode. The size of the launch vehicle has already dictated that LEO should be bypassed, so our choice comes down to lunar-orbit rendezvous (the mode used by Apollo) or lunar-surface rendezvous, aka, refueling on the surface. So much has been said about LOR already, so let's run the numbers for LSR.

Having landed a crewed Dragon-lander on the surface, and assuming no fuel is left, we would require 6014kg of propellant to return to Earth. This is not too bad, at 3 fuel landings, but we can do better. If we can carry just 540kg of fuel in reserve we can eliminate the third fuel landing. Another alternative is to throw 338kg of payload out.

Of the 2737kg payload delivered, we have to determine how much is needed for the crew and their supplies, and how much can be fuel. The pressurized volume of the Dragon is 10m^3 requiring 11.839kg of air to fill. Without an airlock we may wish to cycle that a few times, so let's say 118kg total. Next we need a one week supply of oxygen candles at 25.83kg per person, and LiOH to scrub the CO2 at 52.71kg per person. Finally there's food and water at 45kg per person. For a total of 488kg for a crew of three. Too easy! This leaves 1708kg for spacesuits and equipment.

Once on the surface, the crew would vent the chamber, get out and refill the fuel tanks. Having gravity, transferring the propellant is well understood. Return to Earth would be direct, with no need to enter lunar orbit or perform a rendezvous. As no parts fall off the Dragon-lander on the way it could be fully reusable, providing a stepping stone to in-situ produced propellants.

I estimate a Falcon 9 Heavy / Dragon-lander cargo configuration would cost around $40k/kg to the lunar surface. With the two fuel emplacement flights, this makes crew transport something like $130M/seat for crews of 3, but you could conceivably get that down to $55M/seat if you were delivering 7 at a time - most likely to some kind of base as they would have reduced volume for equipment.

Most of my calculations were done with this rocket equation calculator and I used an inert mass fraction of 0.15 for the lander.

Saturday, October 02, 2010

Revolutionary Thinking in Nuclear Rockets

A few months ago I started talking to Jim Dewar about his latest book, which I reviewed in August. My suggestion to him was that he needs to write a better introduction that assumes the technical knowledge of rocketry but not the nuclear industry. He took on the task and recruited a number of people to serve as reviewers, myself included. So far, it hasn't been published anywhere, but he's given me a copy and invited me to publish it here.

To avoid confusion, I've put it on my website:

A Technical and Economic Introduction to Nuclear Rockets

It's long but divided into sections, and I think Dewar has done a great job, so check it out.

Jim tells me he would like to hear feedback.

Friday, October 01, 2010

Affordable Deep Space Exploration

For too long the aspirations of NASA have not matched the budget allocated. Building the International Space Station has been a decade long effort, now finally reaching completion, and until just recently the plan was to splash it into the ocean before moving on to "the next thing". For a while, that was a return of humans to the Moon, but the recognition that the necessary budget would not be forthcoming has pushed that goal so far into the future that it doesn't even make much sense to talk about it anymore.

Today, the focus is on making a new heavy lift vehicle, finishing a big heavy capsule to go on it, and considering the possible missions that could be done with that hardware should it ever be finished. At the same time, technology development and commercialization of ISS resupply promise to free up some existing budget dollars to pay for the lunar landers and prepare for the next next thing: Mars.

This has prompted many to ask: what if we didn't need heavy lift? What if NASA could do deep space exploration without it? I know what you're thinking, propellant depots, right? Not this time. I've talked about propellant depots, enough, let's talk about something completely different.

In this paper David L. Akin has made the case for ISS crew rotation, two lunar missions per year, and a "flexible path" mission every second year, using only storable propellant stages and the existing Delta IV Heavy. He's done the sums and says the whole thing could be done for less than the current NASA budget for human spaceflight.

Here's how it works. First off, the Delta IV Heavy (or some similar launcher should it become available) needs to be human rated. That's expected to take $2B and 5 years. A five ton crew module is developed - it's about 70% larger than the Soyuz - for $2.5B. With just these two components ISS crews can be rotated and this will likely happen anyway under the commercial crew development program. One thing to keep in mind, though, is that the heat shield has to be able to do direct reentry from the Moon, something only the SpaceX Dragon and the Orion is planning at this time.

Simultaneously, an Orbital Maneuvering System is developed. This is similar to the service module on most crew spacecraft, in that it uses storable propellants and is expended after use. A common configuration is used for multiple maneuvers: lunar orbit injection, lunar descent, lunar ascent and trans-Earth insertion. A modification of the Orbital Maneuvering System is required to do the final landing on the Moon. Landing legs need to be added obviously, deep throttling and possibly restarts will need to be supported. These changes are significant enough that it deserves a new name, so Akin calls it the Terminal Landing Stage.

And that's it. The paper explains a few of the more interesting details. For example, for non-ISS missions LEO is completely bypassed, with direct lunar injection of the stages. All the rendezvous and docking happens in low lunar orbit, and by careful management of the loading of propellants stages, can be fired sequentially to generate the necessary delta-v for each maneuver - there's no need to transfer propellant from stage to stage.

Peak funding comes at the end of the development of the program (where it belongs!) and is less than $3B/year. Yes, that's right, Akin says we could have ISS crew rotation, 2 lunar missions a year and a flexible path mission every alternate year for less than what was being spent on the Shuttle program during it's peak. That's the value of leveraging existing hardware.

Read my lips: no new launch vehicles.

* Thanks to Ralph Buttigieg for sending me this very interesting paper.