[AR] Re: Spartan: Hovering hybrid
- From: David Hein <davehein@xxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Fri, 17 Oct 2014 09:31:56 -0700
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> 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> wrote:
Did Masten ever get the four engine version to hover? Has anybody?
>
>
>
>On Fri, Oct 17, 2014 at 9:50 AM, <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>
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
HerbertSent from my iPhone
>>>
>>>On Oct 16, 2014, at 2:44 PM,
"Troy Prideaux" <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>
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
>>>>
>>>>.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>
>>
>>
>>
>>
>>
>>
>
Other related posts:
