David Masten wrote:
On Fri, 2003-08-08 at 12:20, Ian Woollard wrote:
If you have the 7th edition of Sutton, Figure 3-10 shows what effect
different expansion ratios have- the very nearly optimum ranges are
pretty wide even for a single area ratio, plugging in one or two
extension pieces should be very close to optimum over a very wide
range.
I'll have to go home to check Sutton.
Not sure how one would fix the chamber pressure and throttle back.
You can't. Throttling back lowers the chamber pressure, and exit
pressure. If you throttle back enough you don't want any extension.
I'd
have to think through the equations, but I'm pretty sure that if
pressure and expansion ratio are held constant, thrust will also be
constant.
I've been agonizing over these details for a week or so- it actually
goes up as you gain altitude (basically because the air pressure on top
of the bell and on top of the combustion chamber goes down, and the
inside surface of the bell is supersonic, so doesn't change pressure,
provided the flow doesn't seperate). But if ambient goes down and the
throttling is kept constant adding an extension raises thrust and ISP.
Like I said, I'm not sure. I'm still not sure. It seems to me that there
is a fair difference at lower chamber pressures. If you're talking
Shuttle type chamber pressure then I'll agree.
Yes, 100 bar or so perhaps, it still works at low chamber pressures but
you probably need lots of expansion ratios and big bells. But then you
need a wide aerospike too- ultimately you're limited by the pressure on
the base in both cases.
Yes, just a coaxial conical section that slides axially. The force on
the extension piece is low, and no seal is needed, and if it jams, you
just lose performance, but the vehicle should be fine.
How do you keep it in place, and how do you move it at the appropriate
time?
Off hand, run it on rails, use an electric motor to move it at a
predetermined atmospheric pressure or time. The force on the extension
is a only a fraction of the total thrust; like 10%.
The attraction of aeropikes is that you can design the nozzle such that
you only provide a nozzle for a fraction of the expansion and depend on
fluid dynamics to translate the remaining expansion to efficient thrust.
This can (theoretically) mean a much better thrust to weight ratio.
That's unclear to me, extension pieces needn't be heavyweight, and
should be uncooled (in most cases aerospikes have to have cooling) and
bell nozzles have very good thrust to weight ratios.
Perhaps this will be more clear (if your MUA does ASCII art correctly):
Aerospike nozzle: Bell nozzle of similar exit area:
__________________ /\
\ / / \
\ / / \
-------------- / \
/ \
/ \
/ \
/ \
/ \
_______________________________________________________ line showing
where exit area is for both nozzles.
Note the difference in amount of material. I believe a better figure is
available in Sutton, if not, Huzel & Huang has one.
It's not totally clear to me, you may well be right, but there's
several confounding factors. The metal is not the same thickness. Also
the thermals for a biprop aerospike can be pretty bad- the lower part
of the Bell is much, much cooler and lower pressure, so the area
exposed to the really hot gas is far less and thinner- the aerospike
will probably need regenerative cooling over the whole surface whereas
the regen can stop some way down in the Bell.
Dave
--
-Ian
"If you want to build a ship, don't drum up the workers to gather wood,
divide the work, and give orders. Instead, teach them to yearn for the
vast and endless sea." -Antoine de Saint-Exupery
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