On Fri, 2003-08-08 at 12:20, Ian Woollard wrote: > David Masten wrote: > > On Fri, 2003-08-08 at 09:53, Ian Woollard wrote: > > > > > So just what is the advantage of aerospikes? So far as I can tell, > > > extensible nozzles have better thermals, are simpler to design, > > > > > I'm not so sure about simpler to design. There appear to be some fairly > > simple and elegant ways to do aerospikes that are not inherently more > > difficult to design and build than a traditional multiple engine, > > multiple bell design. > Aerospikes in general (as opposed to a plug nozzle) seem relatively > complex to me, particularly from a cooling point of view (although > ERPS have sidestepped that problem by using HTP, for a biprop it's > more interesting).
I am aware of a few simple, elegant concepts. But, they are trade
secrets for now, and we cannot yet know whether they really are
workable. After, I build it I'll let you know. (It will be a bit.)
> > > and can give as good or better ISP.
> > >
> > But with more weight to carry. Nor am I so sure of the as good or better
> > ISP, maybe for specific points of a flight, but not for an entire
> > lifting trajectory.
> 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. 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. You might be able to play around with mixture, but then you're
really screwing around with 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, 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?
> > 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.
Dave
--
David Masten <[EMAIL PROTECTED]>
signature.asc
Description: This is a digitally signed message part
