> Russell McMahon wrote:
> >
> > In due course maybe.
> > But first he intends to take out all the manned "time to altitude"
records
> > in the next few years - but not with the craft you see there. That is
just a
> > concept investigator. They have 2 unmanned versions flying so far.
> > "From the people who brought you Doom" :-) (seriously!)

Roman said -

> As much respect as I have for the Carmack brothers
> and their amazing programming abilities I doubt John's
> (and staffs) mechanical engineering skills looking
> at some of their "flying machine" prototypes.
>
> Why put the payload on top and the four rockets on
> the base at floor level?? Surely hang the payload
> or pilot at the bottom and the rocket structure at
> the top, the rockets are quite far out from the axis
> so it will fix so many of their stability and landing
> ground effect feedback problems. I was designing flying
> machines from about 8 years old and never would have
> made a silly mistake like that. Always HANG your
> CG weight below your lift engines if at all possible
> guys.

I have very great respect for Roman's electronics abilities but in the case
of rocketry I have to (gently) suggest that John's understanding of the
subject is somewhat greater.

While intuitively it is obvious to many people that having your motors high
and your c of g low makes for stability, it happens that intuition is wrong.
This subject has been debated in many forums on many many many occasions and
it takes the non-believers quite a long time to come to believe that it is
the case. We could have just such a discussion here if people wanted but it
would be easier to trust me on this one :-).
For good descriptions search for "pendulum fallacy", which is the name given
to the incorrect intuitive belief. Robert Goddard's first rocket was built
with the motor at the top for this reason. He rapidly realised that his
original assumptions were eroneous and all his later rockets were of what is
now thought of as "conventional" layout.

But I know people wont believe me.

Re rocket motor count. There is indeed one large central motor which provide
most of the thrust and there are 4 smaller outboard motors which provide
orientation and directional control. This is the smallest number required
for sensible control - you could get away with 3 at 60 degrees provided you
were prepared to make some life threatening compromises in your available
control capabilities and strategies. See beloe re ducting.

John, as you might surmise if you think through what it takes to make the 3d
real time virtual worlds involved in his video games' virtual worlds , is
well versed in the mathematical requirements of controlling a real 3d body
in this world.  I have followed his descriptions for quite some while now
and don't consider there is an excessive amount of "what say we try this"
going on. In a real world with real rocket motors that you develop yourself
from scratch it is exteremly normal for theory and reality to not always
meet. A look at video footage of NASA's early days is most amusing and
spectacular. Large rockets looping the loop, flying horizontally, collapsing
in laaaarge fireballs and more.

Enough I think - you get the general idea.

> And why so many engines??

As above

> Why not a central lift
> engine, in the middle

provided

> with the payload divided in two
> and hanging lower and to the front and rear, giving
> neutrally stable lift with minimum attitude
> compensation needed??

Sadly this doesn't work - pendulum fallacy

> some ducting system for attitude adjustment etc.

They decided that 4 independent small rocket engines are easier than a
single ducted engine. It's an engineering decision. Any rocket I have ever
met with vernier guidance has used multiple thrusters.

> >From their web site I sensed a whole "we have no clue
> but we'll try this and see what happens" attitude.

They have more clues in this particular amateur area than almost anyone
else.
Not as many, yet, as the professionals.

> Their whole flying platform concept seems to be based
> on a totally ridiculous mechanical system trying to
> rely on very fast software to compensate for all
> its flaws and hopefully make it fly. :o)

The only way to slow the system working would be to increase the moment of
inertia which could be achieved by changing the shape. This impacts other
factors considerably. Their control system is now adequately fast to
maintain very creditable stable flight. (see latest video). Not quite as
good as the stability of a Sedgway scooter but that has the great advantage
of having the ground as a reference point.

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