Not that I claim to know anything, but: I'm wondering if John is walking (or flying) the inevitable path to gimballing? Starting with multiple engines seemed really marvellous (and it was) but he found that jet vanes were immensely more satisfactory in practice. The first successful mass produced liquid fuelled rocket used jet vanes, but they have pretty much vanished from the medium to large sector and been replaced either by vectoring using fluid injection or by gimballing. What are the advantages of jet vanes over gimballing?
Perhaps: - Less actuator force? - Quicker response? - Includes roll control? - Engine rigidly fixed to frame. - ???
All of those are indeed benefits of jet vanes.
The bottom line is that the half dozen times we fired the big vehicle with four engines, we had noteworthy problems every single time getting the engines to all warm up at the same time, perform identically, and have sufficient responsiveness and authority. In contrast, we have done over 30 jet vane flights with excellent results.
We have reason to believe that our current build method of heavier retaining plates and welding the top hot pack plate in under significant hydraulic pressure does help the startup procedures, so we would probably have less trouble with multiple engines than we did a couple months ago. It will be an interesting test to see how the new vehicle works out, because it is using one of the older engines from the big vehicle's differential incarnation.
We have considered making a cluster of four 12" engines with interconnect pipes between them underneath both the hot pack and cold pack to provide some level of balancing, but I would still expect some problems with that arrangement over a single 24" engine. We don't see any particular problem in fabricating a 24" engine, it will just cost about $30,000 and take a month and a half.
At this point, since it looks likely that Burt is going to win the X-Prize, we are going to stay focused on the 12" engines and waivered flights without a launch license until one of the commercial spaceports actually gets their environmental work done so we can bypass paying WSMR $1 mil+ for doing our flight tests there. If Burt crashes or otherwise fails in his attempt this month, we may consider racing at it, but there probably isn't enough time in any case.
John Carmack
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