On 3 May 2004 at 13:28, John Carmack wrote:
>
> >
> >How much of it is really that vanes are easier and how much is
> >differences in your own experiences/knowledge and time working out the
> >wrinkles? Or to put it another way - if you had started out with the vanes
> >and then switched to differential throttling would you have gone through
> >much of the same process? In reading your updates it seems like you have
> >worked out a lot of bugs that would affect both vehicles.
> >
> >The other more technical thing - I'm not recalling if you were still
> >using the solenoids and rapid on/off for throttling the differential
> >setup. The vanes are continuously variable (even if the controller
> >isn't), did you try a continuously variable valve setup on the
> >differential throttling vehicle?
>
> The last two vehicles have been continuously variable differential
> steering. The difficulty with that over solenoid operation is that
> solenoids give you instant response, where you can go from full on to full
> of in 10 msec at the actuator, while the ball valve driven big engines take
> 800 msec to go full open to full close. Even though you have the same
> amount of control authority with each engine supporting 1/4 of the vehicle,
> the responsiveness is much lower.
>
> The decision is a no-brainer if you have more complex engines, like our
> mixed monoprops or a biprop. For 90% peroxide it is less clear cut, but I
> still think there are compelling advantages.
>
> Our very first vehicle that just used four solenoid driven 90% motors might
> have been easier to put together and get working than the jet vane vehicle,
> but every vehicle after that would have been better off with vanes. The
> solenoid design can't scale up, so it would have been better to start with
> something that could.
>
> The jet vanes don't have any cross axis coupling. Our differentially
> throttled vehicles have four axis (pitch, roll, yaw, lift) coupled onto four
> engines. The computer deals with it ok, but having to balance and correct
> for it reduces the responsiveness of the system.
>
> The response to vane tilt is pretty much linear, unlike engine
> throttling. Again, the computer dealt with it ok, but it is nice to have it
> go away.
>
> The jet vanes provide instant torque on the vehicle, without needing to
> accelerate liquid or pressurize the engines.
>
> Making four or more engines run exactly (or very close to) the same is often
> a bit of a challenge. We have done it seven or eight times during our
> development, and we usually have a "problem engine" of some kind, and it
> sometimes changes after they have been run in. We know a lot more about how
> to do it reliably today, but it still isn't without difficulty.
>
> Vanes are much easier to manually check out -- tilt the vehicle and watch
> which way they turn. Building a jet vane system is rather like building an
> RC plane.
>
> Much less plumbing. Our test vehicle still has some hoses because of the
> dual tank design, but with a single tank, the engine will just be a straight
> shot through the cutoff and throttle valve, which will probably get us at
> least 10% better engine performance.
>
> Scaling is lots easier, you just build one bigger engine instead of four.
>
> There are some manufacturing and handling benefits to having multiple
> smaller engines, but the variability that we currently get overwhelms it for
> us.
>
> John Carmack
>
That's all very well. But I have to ask myself: Where are the existing
rockets that use jet vanes for steering? AFAIK, they all use engine
gimballing -- which suggests that vanes have some shortcoming.
(Yes, I know I'm in PCBH territory here. It's a region well known to
me. <g>)
Chris
--
PCBH -- politely corrected by Henry
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