The 4-engine configuration's ability to have an off-center CG could help with the propellant distribution problem.
On Fri, 17 Oct 2014 22:02:28 -0500 Ben Brockert <wikkit@xxxxxxxxx> wrote:
Pixel, Texel, and Morpheus all hovered with a gimbaled engine just a few inches from the CG. It worked. The bigger issue for squat vehicles is with propellant distribution rather than control authority. On Friday, October 17, 2014, David Hein <davehein@xxxxxxxxxx> wrote:Multiple engines make sense if the lander has a short and wide profile. The center of mass is very close to the engine, and a single gimballed engine would not have much of a moment arm to work with. Gimballed mounts work better when the rocket is long and narrow, and the center of mass is located at a larger distance from the engine. I would think that a 5-engine configuration would work better, where the center engine provides most of the thrust, and the 4 outer engines are smaller and mostly used for attitude control. With the 5-engine approach, variances in the thrust of the 4 outer engines would have a smaller effect on stability than with the 4-engine approach. Using a large central engine would require maintaining the center of mass over the main engine. On the other hand, the 4-engine configuration would allow for the center of mass to be off-center, and move as the burn proceeds, so it does have that advantage over using 5 engines. On Friday, October 17, 2014 9:17 AM, Jonathan Goff <jongoff@xxxxxxxxx <javascript:_e(%7B%7D,'cvml','jongoff@xxxxxxxxx');>> wrote: Korey, The four engine design was abandoned because the engines were crap and highly inconsistent from engine to engine. I can say that because they were my design and I was soooo glad when we finally killed those and went to the new engine design. We tried to move too fast from single engine sort of working on the test stand in a fixed configuration to four flight engines. We were also outsourcing our GN&C at the time, and there were issues with how that was handled. By the time I left in 2010, I think we had the GN&C and engines to a point where we could've done a 4 engine vehicle had we wanted to... we were just still at a point where a single engine vehicle was a lot easier to work with. Multi-engine liquid VTOLs probably only start making sense when: 1- Your engines have enough development on them that they are reliable and consistent from vehicle to vehicle 2- You are getting near a "step function" in the engine size versus development cost curve, where doing multiple copies of a smaller engine would be a lot cheaper than developing a big new engine. 3- You're getting the rest of your vehicle to flight reliability levels where engine-out actually makes a difference reliability-wise. Masten will get there if it keeps plugging along. And it could go there today, it just doesn't make sense for them at the moment. It probably makes sense at XS-1 scale though. Anyhow, all that is conjecture since I left there over four years ago. If they let Dave Masten respond, I'd love to see how his thinking compares to mine. ~Jon On Fri, Oct 17, 2014 at 8:07 AM, Korey Kline <k2@xxxxxxxxxxx <javascript:_e(%7B%7D,'cvml','k2@xxxxxxxxxxx');>> wrote: Did Masten ever get the four engine version to hover? Has anybody? On Fri, Oct 17, 2014 at 9:50 AM, <joesmith@xxxxxxxxx <javascript:_e(%7B%7D,'cvml','joesmith@xxxxxxxxx');>> wrote: Even Armadillo was able to hover. Masten as well. On Fri, 17 Oct 2014 00:38:03 -0700, George Herbert < george.herbert@xxxxxxxxx <javascript:_e(%7B%7D,'cvml','george.herbert@xxxxxxxxx');>> wrote: For some reason I jump straight to a single centerline hybrid with cold nitrous gas attitude control, but each to their own... George William Herbert Sent from my iPhone On Oct 16, 2014, at 2:44 PM, "Troy Prideaux" <GEORDI@xxxxxxxxxxxxxxxx <javascript:_e(%7B%7D,'cvml','GEORDI@xxxxxxxxxxxxxxxx');>> wrote: Jon, You’re thinking in liquid mode ie. LOX. You need to think monoprop oxidizers eg. Nitrous or Peroxide. If you look at Nitrous:PE for example, the performance for O:F ratios between 5:1 to 10:1 is pretty much the same. The internal bond energy of the oxidizer provides a significant portion of the propulsion energy. Saying that, I generally agree that typical hybrids aren’t suited for this application although if your expertise is in hybrids and you’re really comfortable with them… Troy ...Because once you've done all the work to figure out how to do fire and throttle valves, building another set with slightly different characteristics is just too hard. Much easier to deal with a chamber that changes geometry throughout the burn, and where you have no real control over mixing efficiency. I mean, cool if they can pull it off, but I've got to scratch my head on why they'd do it that way. There *are* places where hybrids might make sense, but I have a hard time believing a hovering vehicle is one of them. Unless bobbing around like crazy in a semi-controlled fashion is good enough. Sorry if that came off overly negative, I just wonder about people's design choices sometimes. ~Jon On Thu, Oct 16, 2014 at 8:12 AM, Mark C Spiegl <mark.spiegl@xxxxxxxxx <javascript:_e(%7B%7D,'cvml','mark.spiegl@xxxxxxxxx');>> wrote: I have no connection with these guys. They're building an LLC type vehicle using hybrid rocket motors instead of biprop. http://www.spartanproject.eu --MCS .
