Re: Multiple engines (was RE: Liquox)

At 12:01 AM 6/9/2004 +1200, you wrote:
>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