Small wind tunnel for a model rocket

[leright]

I need to test a model rocket which will have average airspeeds around 200 mph. I am hoping to construct a plexiglass box about 9 feet long, 3 feet wide and 3 feet tall. For these dimensions and airspeed requirements it looks like I'd need a 160000 cfm blower. Is this something I can feasibly obtain? This is supposed to demonstrate a model rocket roll control system, which will be my senior design project.

Maybe I should look into a smaller rocket, eh?

Sorry, I'm not an ME so this might be a dumb question.

 

[FredGarvin]

160,000 scfm is about 20 Lbm/sec. That's a lot of air. I would seriously look into scaling if at all possible. If you can get the scale of the rocket down and make the test section smaller, you can take advantage of the neck down to get your velocity up. Either that or find someone who already has a wind tunnel.

Just to give you an idea of scale here, are you familiar with the blower units that janitors use to dry out carpets? Those put out, at best, about 1500 scfm. If you go to www.mcmaster.com and enter 3225k32 in the Find Products box, you'll see what I am talking about.

 

[berkeman]

leright, this is mostly for vertical flight, right? I think I remember your thread on the control and feedback system. If the rocket is going to servo mostly vertically, can you just make the diameter of the tunnel only a few times the outer fin tip distances? Can you get it down to more like 1 foot in diameter? I don't know if that helps enough, though.

 

[russ_watters]

I recently did a study on an air handling unit in a skyscraper in Philly. The air handling unit was roughly the size of a small apartment (but taller) and had a capacity of 110,000 CFM and a 125 HP motor. The fan wheel itself was 6 feet in diameter.

Yah, I'd consider scaling it down too...

 

[leright]

leright, this is mostly for vertical flight, right? I think I remember your thread on the control and feedback system. If the rocket is going to servo mostly vertically, can you just make the diameter of the tunnel only a few times the outer fin tip distances? Can you get it down to more like 1 foot in diameter? I don't know if that helps enough, though.

unfortunately, to house the electronics and servo motors/controllers the rocket 'tube' will need to be at least 6 inches in diameter I believe, and the rear fins plus tube diameter will be roughly 18 inches in diameter. I could get away with using a tube with a 20 inch diameter, I think. I could actually get away with using a 40,000 cfm blower if I were to do that. Ideally though, I would be able to get the wind speed to the maximum speed of the rocket, which is roughly 500 mph.

 

[leright]

I recently did a study on an air handling unit in a skyscraper in Philly. The air handling unit was roughly the size of a small apartment (but taller) and had a capacity of 110,000 CFM and a 125 HP motor. The fan wheel itself was 6 feet in diameter.

Yah, I'd consider scaling it down too...

lol, thanks, I will consider that. sorry, I just have no intuition or feel for these types of things.

Maybe I will look into renting a university wind tunnel instead.

 

[leright]

160,000 scfm is about 20 Lbm/sec. That's a lot of air. I would seriously look into scaling if at all possible. If you can get the scale of the rocket down and make the test section smaller, you can take advantage of the neck down to get your velocity up. Either that or find someone who already has a wind tunnel.

Just to give you an idea of scale here, are you familiar with the blower units that janitors use to dry out carpets? Those put out, at best, about 1500 scfm. If you go to www.mcmaster.com and enter 3225k32 in the Find Products box, you'll see what I am talking about.

ha, yeah, I'm going to look into using someone else's wind tunnel.

Problem is, I will need to empirically determine the airframe dynamics of the control system (relationship between roll speed and control fin angles) and this will require many different test runs. I assume renting a wind tunnel is not a cheap thing, even at a university.

Thanks everyone for your guidance.

 

[FredGarvin]

You never know! Play the "poor college student" routine to the hilt. You may be surprised. When I did my senior project, there were a couple of companies that donated time and a few items to our cause. After hours time on a small tunnel doesn't seem like it would be too much of a donation.

 

[xez]

Well just a couple of ideas that might help

a) get a real wind tunnel,

b) if not a, gravity might be your friend -- terminal velocity
might be around your 200mph velocity regime, so if you
can find somewhere to drop the streamlined thing off
a cliff while it's running a data collection / controls
actuation test that you can gather data from, that'll get
you empirical data that's at least in the realm of being
accurate for your problem. Intact recovery shouldn't be
that infeasible depending on your test site and setup.

c) flight test the thing under its own power
if that's easy; if you're collecting data and have appropriate
exercises programmed into the actuators, I'd think you'd
be able to gather some useful data before the velocity
decreased too much. Start out with neutral controls and
get increasingly adventurous as altitude increases then
let it parachute (or whatever your design does) home
after the high altitude / high velocity period is over.

d) If you tethered it to a rotating arm or cable it seems
like it shouldn't be too hard to use a conventional motor
to spin it up to reasonably high airspeeds safely.
Rather than using a giant propeller with high speed
blades to push air at high speed down a wind tunnel,
it simplifies things a bit to just use the airspeed
at the tip of a rotating arm itself if you can stand the
centripetal accelleration and the radius of body turn
in the airstream is within the realm of servo control that
you wanted to test anyway. Clearly unrealistic for 500mph,
but clearly do-able for 100mph (45m/s), and somwhere
between the two is your limit.

e) you can use a nozzle (just like ... a rocket, hey!)
to convert high pressure/temperature air to low pressure
high velocity air... So it could conceivably be within the
realm of engineering feasibility to generate low pressure
200mph to 500mph air on the output of a
static combustion chamber with a nozzle, but, of course,
you'd need to have some reasonable balance of
temperature vs. speed vs. pressure (which would need to
remain above atmospheric, of course). Certainly
nothing to attempt for a huge diameter / length of test
chamber, but possibly something that would be very
do-able for a very restricted size.

f) Would it be that bad to just use computer models to
estimate the forces involved, then to use some kind of
self-calibrating servo algorithm using feedback sensors
etc. to implement something like a PID loop or whatever?
How far to turn the fin? Well start turning it a little,
if that's not enough, turn it some more... etc.

 

[xez]

g) drop it from a small weather balloon and use
a terminal velocity fall for some calibration tests.
That's something you could repeat several times a day
if you had a safe test area with little cross-wind and
launched/recovered the balloon and test payload once
every hour or so and limited the altitude to some modest
few thousand foot level.

 

[momentum_waves]

The Russian's used large compressed-air banks to test supersonic flows. Could this concept be of use at lower velocities?

 

[cliff]

Fred Garvin; Can I get that SCFM to LBs equation from you? I've been looking for that for sometime.

 

[xez]

The Russian's used large compressed-air banks to test supersonic flows. Could this concept be of use at lower velocities?

Actually, that's a great idea for certain flow regimes!

I suppose I didn't put it out as an idea since they'd
been tossing around 100,000.00 CFM numbers a few
posts back talking about the "pie in the sky" dimensions
for an "I wish I had" wind tunnel. You'd run out of
compressed air pretty fast at 100,000.00 CFM, but then
that kind of flow is impractical for any simply built
wind tunnel, you'd need a full size jet engine or something
to get those numbers.

But if the tunnel test could be scaled down, certainly,
one could get a nice clean cool stream of several
hundred MPH air out of a few SCUBA tanks exhausting
2200 to 3300 PSI air through some nozzles into a
chamber a very small number of cubic feet (or less)
of volume.

You'd have no real control or stability of flow velocity
in that the tanks pressures would drain pretty quickly,
but for testing automatic servo mechanisms, I'd guess
you could learn a lot in only 5 seconds of test time.

And if you staged 4-5 tanks and had a few people open
the valves in a staggered arrangement e.g. start to
open the next tank valve when the first tank is
down to half-pressure, you could prolong the test time
and get somewhat more stable air flow rates since as
one tank emptied and lowered pressure you'd be adding
the boost of another.

And, actually, having quickly varying air-flow rates would
be perfectly realistic of a rocket with high accelleration
anyway since it'd be going from something like 0 to
200MPH in just a few seconds, so constant air-flow rate
would NEVER occur in a real world environment for his
setup anyway.

You can get SCUBA tanks of compressed air up to
120 CU FT capacity (in STP CF equivalents) with a peak
pressure in the 3200PSI range, so figure at least 60CF
will be available at a "pretty high" pressure and it'd
diminish in pressure for a single HP120 tank from there
as it got more empty.

You can get much bigger industrial gas cylinders that
have several hundred CF capacity at ~ 2000PSI max.
pressure, and it's not uncommon to put a few of those
in parallel for higher capacity.

So it's an option if you can figure out a way to use
pretty cold fast air in the range of 50CF to 500CF total
ambient air pressure CF capacity delivered at 2000-500
PSI pressure decreasing fairly rapidly in time.

You might need to use DIN valves (for SCUBA tanks)
or some other kind of valve that let's the flow rate you
need escape; the normal tanks have relatively narrow
orifices so that a large fraction of the tank can't escape
in less than several seconds.

Of course you'd also have do be sure to do this kind of thing
SAFELY so that you don't have any hazards from freezing
air, accidental air pressure surges causing ruptures of any
restricted flow areas (make sure the exit flow tubes
can NOT get blocked so that no high pressure can
develop), and that normal high pressure tank/gas
handling precautions are observed.

 

[FredGarvin]

Fred Garvin; Can I get that SCFM to LBs equation from you? I've been looking for that for sometime.

It is simply \dot{m}=\rho*Q where

\dot{m} = mass flow rate
\rho = density at reference condition
Q = volumetric flow rate at reference condition

 

[FredGarvin]

In regards to the air farm set up...it really is an extremely large endeavor to provide ample air like that. It is a good way to do it, but the facilities required are tremendous mostly because to get a usable/beneficial amount of air, you need a very large storage system.

For example (granted, a large example) is the air farm built for the Pluto Project that LLNL did back in the '50s:

Also required for the construction was 25 miles (40 km) of oil well casing which was necessary to store the million pounds (450 t) of pressurized air used to simulate ramjet flight conditions for Pluto.

We air start some of our engines and I have played around a lot with scuba tanks, etc...You really don't get that much usable air out of a scuba tank. They deplete pretty quickly. Even the larger K bottles at 5000 psi were pretty small.

The idea of the rotating arm and using the rocket's own engines is a great idea. You would have to strap a camera to the arm and the runs would be short lived, but it would definitely save you on the wind tunnel construction. It's definitely something to consider.

 

[Cyrus]

I don't see how you are going to control anything that's tethered to a rotating arm. Although, it would probably be good for ripping the rocket appart. I guess the Gyros will just have to ignore the massive accelerations...

You can't 'control' anything with that setup. What you can do, is try to fly the rocket like a control line model. In other words, you can only control up/down motion along the horizontal axis. Because the rocket has 4 fins, you can first get 2 fins working, then rotate the rocket, and get the other 2 fins working. So you will only be able to work out 2 control loops. But if it does not work, you are going to disintegrate your rocket as it flys into the ground at 100mph because it did not get enough up command. Also, when the motor runs out, it has no wing. In other words, it will fall into the ground like a brick and break off your fins anyways.