Camera and Propulsion System Design at the Southern California Selene Group
The Southern California Selene Group is continuing with its engineering and design work. Since my last blog report, we’ve had two very productive all-hands team meetings on Saturday, March 22 and Saturday, April 19, and we have another scheduled for May 10. (By the way, I’ve just put up a new photo album on our team page, showing pictures from our last three team meetings.) In between, as always, there are lots of smaller meetings going on – for example, at least once a week, Harold meets with Ron, Dorian, Robert, Al, Brian, Dan and others to go over key design issues.
Also, I’d like to mention that Harold’s brother Ben Rosen (former venture capitalist) has a blog, where he wrote a humorous and clever post about Harold’s pursuit of the Google Lunar X PRIZE. You can read the blog at: http://benrosen.com. The title of the blog post is “Spin Me to the Moon” dated 25 March 2008.
We are happy to announce the addition of three new team members: Phil Donatelli (he was just advising us before), Daniel Geng and Patty Pun. Their impressive bios can be read on our team web pages. We also have a new honorary member, six year old Lucas.
OUR HONORARY MEMBER. Lucas came to us by way of his mother Courtney, who contacted the X PRIZE Foundation, writing that her six year old son had a design for a moon rover, and asked if someone there could forward her letter to our team. Will Pomerantz did just that. To make a long story short, after much back-and-forth correspondence, we were finally able to meet with Lucas and his mother -- both delightful people. Lucas brought along the paper that had his rover design. Once he started talking, you could see that this was no ordinary child. He explained to Harold how the solar panels worked, and how the batteries would provide power during the new moon phase. Curious (and amazed!) at his abilities, we asked him lots of questions. It was amazing how much he knew, and seemed to understand. The one that blew us away was when we asked him what caused the seasons (it’s a sad fact that many adults don’t know the answer to this question). He thought for a moment, tilted his head, and said “well, it is due to the angle of the earth…” (!!)
Lucas and his mother Courtney, along with his younger sister Cate (at three, she is too young to join our team:-) stopped by towards the end of our most recent team meeting on April 19. Many of the team members were therefore able to meet Lucas, and interact with him. Al Wittmann discussed with Lucas some of the design decisions we had made at the meeting. Lucas followed what Al was saying closely, and asked relevant questions (often he was gently and unobtrusively guided by his very smart mother). Then, after Lucas mentioned the “Saturn VEE” (of course! why would he know about Roman numerals?), Rex Ridenoure showed him some pictures of rockets, as well as the Saturn V, that he had on his computer. We all enjoyed welcoming our honorary team member.
Now, back to design issues. Major design progress has occurred in two key areas: the camera system, and the propulsion system.
CAMERA SYSTEM AND ELECTRONICS. Harold has been working with Brian Bliss and our new team members Dan Geng and Patty Pun to finalize the electronics and design of our camera system. To meet all the Mooncast requirements, we wanted to create a camera system from commercially available components. We need this camera system to be high up in order to get good viewing (looking downward) of the top of the spacecraft, as well as (looking outward) of the distant lunar surface. So we decided from the start to locate the camera system at the top of the mast – the highest point of our lander. The design challenge was to make this fit within our stringent mass and volume constraints. We have also designed the appropriate mechanisms to provide the pan, tilt and focus adjustments needed to meet the Mooncast requirements. Our detector will be a single focal plane array whose associated electronics provides the versatility for the required self-portrait detailed images, near real time video, and high definition video – meaning, these various specified modes are provided electronically.
We like this simple and elegant design.
PROPULSION SYSTEM. Since my last blog post when I discussed propulsion design issues, we’ve gone through several design iterations regarding our propulsion system. (In fact, before reading further, you might want to re-read that blog post to review the issues involved.) We have now converged on a configuration that appears optimum: it’s a blowdown bipropellant system, with a hybrid descent system consisting of a solid retro-rocket significantly augmented by liquid propulsion thrusters.
Of course, the selection of this configuration is contingent on the timely availability (we don’t yet have complete data from the manufacturers) of the requisite thrusters and the particular solid rocket we have in mind. Also, this configuration has a slightly higher center of mass, and we haven’t yet been able to confirm its compatibility with this particular still-unpublished Falcon 1e launch vehicle requirement. With these caveats in mind, let me explain how we got to this choice.
SpaceX recently issued its new Falcon Lunar Capability Guide, and this proved to be good news for us, since the Falcon 1e is now shown to provide a higher mass capability into low Earth orbit. This affects the design decision of bipropellant vs. monopropellant. We had wanted to go with a bipropellant before because of the potential performance improvements provided by its higher specific impulse, but the original lower mass capability advertised for the Falcon 1e made this problematic. This is because the dry mass of the bipropellant system is much higher than that of a monopropellant system and, under the original constraints, that factor outweighed its specific impulse advantage. With the new numbers for the Falcon 1e, the balance has shifted. Our spacecraft design now readily accommodates the use of a bipropellant system.
The blowdown system has long been desired by us for its relative simplicity, but it necessarily occupies more volume. We looked carefully at this, but couldn’t fit it into our volume constraints if we used an all-liquid blowdown descent system. So we decided to go back and look at re-incorporating the solid retro-rocket. We found that we could do this by using a hybrid descent rocket system – meaning, a system where the solid rocket provides roughly two-thirds of the delta V required for the descent, while the other one-third is provided by the liquid propellant thrusters. This combination results in significantly lower “gravity loss” during the descent phase than was possible with the all-liquid descent system. (“Gravity loss” is the additional delta V required for descent, as compared to an ideal system that has infinite thrust.)
So, with less liquid propellant needed for the descent portion, the blowdown liquid tank volume can be smaller, making for an easier fit. This is great – we can now use the blowdown system.
While we still won’t have a known, fixed mass to decelerate before the solid rocket burns, the fact that after it burns, roughly a third of the delta V required for the descent is provided for by the liquid thrusters means that it is still much more “forgiving” of descent timing errors than our original system had been.
As Harold puts it, this overall combination (a blowdown bipropellant system, with a hybrid descent) provides the best performance, characterized by the “highest landed mass (excluding the propulsion system).”
OTHER MATTERS. Dorian has been in Europe for most of the month on business-related travel. In addition to all his other team work, Ron Symmes has been working various facilities issues, and since I last wrote, he had arranged for him and Harold to visit three such sites. Ron’s getting it all in place so that we will be ready to start building our spacecraft as soon as we have our funding in place. Well, in truth we will be able to start some construction even before that! We are getting lots of offers of free help. The network of helpful friends and colleagues that Harold and Ron (it seems that Ron knows everyone) have amassed over the years is truly gratifying.
TEAM SUMMIT. I will be representing our team at the Team Summit to be held in Strasbourg on May 20 and 21.
Associate Team Leader