In article ,
Bruce Hoult wrote:
In article 419af6e1$1@darkstar,
(Mark James Boyd) wrote:
Thrust is cheap. The amount of fuel used is to a first approximation
independent of the thrust of the engine (in fact to a certain point more
powerful engines result in less fuel used). But an engine that will
give you only 200 fpm of climb will take *forever* to get you to any
reasonable flying speed.
http://groups.google.com/groups?selm=an_595515430
I looked at your previous calculations, thanks for the reference to
the earlier post.
I wonder if you could redo the numbers for the limiting (minimum)
case for launching a human. Lets say a 50kg launch. Then also
do the numbers for a 150kg launch. I'd love to see what this looks like for
10kg and 20kg of thrust.
I also looked at some of the other numbers in the post,
and they seemed a little off.
20kg of drag until liftoff speed seems a bit of an overestimate.
The Sparrowhawk would seem to have at most 5kg of drag
while accelerating to best L/D, assuming no
wheel friction, and this drag should increase as airspeed increases.
If a turbine is used, perhaps this isn't so bad, if it helps
compensate for turbine inefficiencies at low airspeeds? I dunno.
Also, maybe this was to compensate for rough ground?
You also stated that "exact L/D doesn't matter much at all while in
powered mode." For the numbers you ran, this looks true (5000fpm
climb!). But at very low thrust and the minimal thrust case,
the best L/D and speed at that L/D would seem to be quite important.
Speeds much faster than this should require significantly more thrust.
Here were some of the calculations, which you did and I found enlightening:
thrust 50kg 100kg
Ground run 204m 76m
Dist at low level 945m 420m
climb angle 8deg 17deg
powered time 104s 45s
climb rate? 1000 fpm?
I'd be intereted to see what thrust is needed if the weight is reduced to
150kg, and the ground run is about 600m, using a Sparrowhawk polar.
Then I'd like to see how this changes if the ground run is allowed
to be 1200m. By ground run I'm assuming we mean accelerating
to something between Vs and Vy. I'd love to see what the climb angle
and climb rate then become.
I'm interested in the minimum case because this is a natural starting
point. I've done these calculations and it seemed that 15kg of thrust
gave a ground run less than 600m, and a climb rate of more than 200fpm
(might have been 500fpm, but I don't recall).
There are also rockets. Lighter, simpler, less fuel efficient, and
(probably) cheaper than jets.
It looks like $20 for 4lbs of thrust for 8 seconds. Each launch looks like
at least hundreds of dollars (worth of commercial rockets sold
by Public Missiles, Ltd and the like).
Perhaps these can be constructed as reusable and experimental,
for much less cost, but I'm just not familiar with this.
If you can give us some estimates on costs and thrust and burn time,
that would be great 
Perhaps the largest barrier to this
is unfamiliarity and not knowing how such a burn is controlled.
How does one perform an aborted takeoff?
You seem to be assuming solid rockets. That would be a *very* bad idea.
Liquid rockets are reusable and use cheap fuels. Have a look at the
videos etc on XCor's web site (http://www.xcor.com/). They've built
liquid/gas fuelled rocket engines with thrust levels ranging from 15 lb
to 1800 lb. One of their 400 lb thrust alcohol/oxygen engines would
launch a typical glider with performance similar to a winch launch using
about 45 lb of fuel.
Interesting stuff. Maybe for the moment we look at the thrust
calculations, and decide later what makes the thrust
I've done calculations on takeoff performance several times over the
years, and posted the results on this newsgroup.
http://groups.google.com/groups?selm...12003%40copper
.ipg.tsnz.net
http://groups.google.com/groups?selm=an_595515430
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Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------
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Mark J. Boyd