We’ve been busy -- there’s a lot of engineering and design work going on with the Southern California Selene Group. We had a very productive all-hands team meeting on Saturday, March 1st, and we will be having another on Saturday, March 22; meanwhile, there have been lots of smaller meetings going on. Right now, most of our focus is on the electronics (in particular, implementing the landing radar), the propulsion system, and in considering other launch options. To that end, we welcome to our team Stan Kent, who is a propulsion expert. (You can read his bio on our team pages.) Phil Donatelli is also advising us in the propulsion arena. Our project manager, Ron Symmes, in addition to all the design work, has been working on getting us facilities, and in investigating alternative launch options.

Before I write anything else, we all want to express our sadness at the passing of Arthur C. Clarke. Our team leader, Harold Rosen, is the recipient of the 1990 Arthur C. Clarke Award, and a long-time friend. Stan Kent, who also knew Arthur well, is sending a condolence note on behalf of our team. In a preface to his 1992 book, “How the World Was One: Beyond the Global Village,” Arthur wrote: “Dedicated to the real fathers of the communication satellite, John Pierce [Telstar] and Harold Rosen [Syncom], by the Godfather.” Always a sense of humor! I think that I have read all his books, as I am sure many readers of this blog have done as well. Stan has suggested, and we all agree, that we put some kind of memorial words on our lander in honor of him. But perhaps the best honor we can give this pioneer and visionary is to have us – or any one of the teams competing – win the Google Lunar X PRIZE.

With that in mind, here is what we are now doing. I’d like to write here about a potential major design change to our propulsion system, and give you some of the background as to how it came about. Even if we decide against this new system, I think you will find it instructive to see what some of the trade-offs are, and how our team operates.  

First, some background. There are two basic types of liquid propulsion systems under consideration here: a monopropellant system (using hydrazine), and a bipropellant system (which uses an oxidizer, nitrogen tetroxide; and a fuel, monomethylhydrazine). A monopropellant system is simpler, and lower in dry mass. A bipropellant system gives a higher specific impulse  (For those readers who don’t know what specific impulse is, let’s just say that for a given momentum change (delta V) of the spacecraft, this delta V can be accomplished with a smaller mass of fuel if that fuel has a higher specific impulse – so the higher the specific impulse, the better!) There are also two different types of pressurization systems that can be used: a pressure-regulated system, and a blowdown system. Typically, a blowdown system is used with a monopropellant system, and a pressure-regulated system is used with a bipropellant system.

Our team members are well-versed in all these systems.

Our initial (preliminary) design – you can see this in our system and mission summary -- uses a hydrazine blowdown system, with a solid retro-rocket. Let’s start with the solid retro-rocket. Why was this chosen for our moon lander? Historically, looking back to the 1960s, a solid retro-rocket was used for the Surveyor moon lander. Syncom, as well as subsequent geostationary communication satellites, used a solid rocket apogee motor. Neither Surveyor nor Syncom used hydrazine, because the catalyst didn’t exist at that time – hydrogen peroxide was used for Syncom, and Surveyor used a bipropellant system. So, initially, in line with our “blend of the old and the new” design philosophy, we went with a solid retro-rocket.

Our team’s lead systems engineer, Dorian Challoner, wanted to cover the bases so he asked Robert Rosen to analyze the use of an all-liquid descent system. Dorian couldn’t ignore the fact that a liquid retro-rocket is more forgiving of descent timing errors and its use adds mission flexibility – with its use we may be able to pursue the 5000 meter roaming prize, as well as visit a historical landing site (within our reach is Luna 9, site of the first moon landing in February 1966). Meanwhile, Al Wittmann was dissatisfied from the get-go with our active interstage (an interstage having its own propulsion system). He didn’t like having fuel tanks and thrusters on the interstage because he thought we could incorporate those functions on the lander. But putting these functions on the lander, with the unspecified amount of fuel remaining (because it wouldn’t be jettisoned with the interstage) after the necessary orbital corrections, wouldn’t work well as long as we had a solid retro-rocket, because before firing it is best to have a known, fixed mass to decelerate (because this would establish its burnout altitude), making the vernier descent portion easier. (A caveat: many other team members have worked hard on, and weighed in with, their contributions -- I apologize that there is no room here to mention all aspects of who contributed what, when.) 

But going with a all-liquid descent system necessitates the use of a bipropellant system, because the specific impulse of a monopropellant system would be too low for the task. This (potential) design change has a ripple effect on the entire spacecraft design, as well as the timing of mid-course and other corrections. As one example of this, the interstage almost disappears: it is just a simple, cylindrical structure with a perigee motor at one end and the lander at the other end. It has but one active function: it uses small solid rockets to spin the spacecraft up to 50 rpm prior to the perigee burn. Another example is a simplification that Robert recently made. In our original system, we were going to spin up to 50 rpm before the perigee burn, and later to 100 rpm before the retro-rocket firing (in order to improve retro-rocket stability and the final, power-off stage of the touchdown). Robert pointed out that, if we go with this system, the much lower thrust of the touchdown and our new lander gear geometry give adequate stability at 50 rpm – hence, no second set of small solid spin-up rockets would be needed on the interstage.

On top of all this, we’ve also been looking at the tradeoffs between using a pressurize-regulated system versus a blowdown system. The latter, because it requires lots of storage space at low pressures, appears unfavorable at this point because it takes up too much volume (larger or more tanks). And there’s more. I can’t do justice to the complexity of the issues involved here, except to say that these arguments, design iterations, and analyses have been percolating in a stimulating intellectual ferment that is truly a delight to behold (genius at work!).  This coming Saturday, we will hash all this out -- with Stan and Phil playing a starring role -- and make our decision.  

Deborah Castleman

Associate Team Leader