[sugpro] Re: More on Arcas
- From: "Steve Peterson" <dmarc-noreply@xxxxxxxxxxxxx> ("steve_peterson")
- To: sugpro@xxxxxxxxxxxxx
- Date: Fri, 5 Jan 2024 06:47:42 -0800
On 1/2/24 9:22 AM, Richard Nakka wrote:
I am somewhat surprised at the low efficiency of the Arcas rocket,
having an efficiency of only 7.5 ft/N-s (note: I calculated 6.4),
considering it had a very respectable mass fraction of 56% and a long
(29 sec) burn time. Curious, why do you suppose the Arcas efficiency
is low?
I have some additional comments on Arcas efficiency, especially in
connection with Figure 4.
In the Arcas development report, there is a graph of velocity vs. time
that looks very similar to the curve of the pure end-burner in my Figure
4. The similarity between the two curves is not too surprising since
Arcas is a pure end-burner. Both curves feature a long ramp up, very
triangular shape. In Figure 4, I show that by changing the thrust
profile to have a higher initial thrust, the velocity profile changes to
one that produces greater altitude than is produced by a pure end-burner.
The Arcas developers tried to get around this low initial velocity by
using a piston launcher, and spent a lot of time, apparently, trying to
get the initial velocity to be higher by modifying the piston apparatus
(primarily to control dispersion, not to gain additional altitude), but
the initial velocity (especially when compared with the peak velocity)
was still very low. It may be that a higher initial thrust would have
produced a velocity curve that would have resulted in higher altitude
which would have improved the efficiency. It also might not have helped
all that much if it resulted in high velocity at low altitude, thereby
increasing the drag. Without a lot of simulation, it isn't easy to see
how the tradeoffs would have worked out.
Another thought that occurred to me is that mass fraction, which can be
a very useful measure, is only one measure (that of how efficiently
structural mass is being used) and won't describe all there is to know
about a rocket and how it might fly. One could, for instance, have a
high mass fraction with a lousy aerodynamic design that would result in
a poorly performing rocket. In the case of Arcas, they did change the
nosecone L/D ratio from 1.5 calibers to 4.5 calibers because the drag
with the shorter nosecone was too high to achieve the desired altitude.
It's not clear whether further refinement was considered, but it's
important to realize that the issue was to meet a certain altitude
requirement, not to achieve the highest possible altitude.
Finally, they used spin stabilization which is known to hurt altitude.
The other rockets in my comparison table were not spin-stabilized, so if
that did have an impact, it would hurt Arcas by comparison.
--Steve
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