[F-FT] More L1 questions

OK, building my 1/4 scale PML Patriot for level 1. Also building up the
Black Brant Vb in case I chicken out on the Patriot ().
In the instructions for the Patriot, it talks about drilling a bleed
hole for air in the payload section, and it infers that this is to keep
overpressurization from being a problem during flight (I realize that
you need a hole for altimiters, but I'm not discussing that here).
The first question that this raises in my mind is whether or not you
would also need a similar bleed hole in the main airframe, below the
payload section. It would seem that if the air pressure is going to
cause a problem with the nose cone separating from the payload section,
there would equally be a problem with the payload section separating
from the main tube.
The second question concerns the angle at which such a hole should be
drilled. Should it be straight (perpendicular to the tube), or should
it be angled? If angled, should it be angled pointing up or down?
My 'guess' would be that you would want to angle it down, such that the
upward flight does not cause airflow to move INTO the payload section or
body tube, but that's just a guess.
Any on-topic responses would be most appreciated.
David Erbas-White
Reply to
David Erbas-White
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I have never found it to be neccesary.
Now you may expect ~14 responses calling me an idiot.
Reply to
Tweak
Dave, If you are using motor ejection; drill a 3/32 hole in the main airframe and the payload section, both on the launch lug side, (cosmetics). The bleed holes will allow for air pressure equalization, between the inside of the structure with the ambient air pressure, preventing the nose cone and or the payload section from being ejected before the ejection charge fires. I originally certed with a Rocketman WA-HOO and experienced the nose cone separation on my first L-1 attempt. After that embarrassing experience, I used three screws to hold the nose cone in place, plus a bleed hole.
Drill the holes perpendicular, that's normal SOP.
Fred
Reply to
WallaceF
Your probably right as long as you don't go to high or not so high to fast. In my case I used a Rocketman Wa-Hoo powered by a US Rockets H-160 and, the rocket weighed less than 4 pounds, if I remember correctly, (I know it wasn't very heavy). The rocket went high and fast. Then on the other hand, the nose cone could of been spit because of the relative short recovery harness and the opening forces of the R-7 Rocketman chute. In any case I believe the bleed holes and screws to hold the nose cone on the payload section for a motor ejection flight, is good insurance. What can happen, it's only a rocket..(:-)
Fred
Fred
Reply to
WallaceF
Not from me...I think its more of a precautionary measure - on a rocket not flying all that high. On high-altitude level 3 or record attempt flight it might be a different matter. Whichever way, it would be unfortuate to have your flight ruined by the nosecone coming off during boost.
I would guess that the more joints you have in your rocket, the more spaces for air to leak out. A case of 'better safe than sorry' methinks...
As for the angle issue...I can't really see that being much of a problem, given that the hole will be small, and in theory at least the airflow will be flowing past its opening at 90 degrees.
Reply to
Niall Oswald
The vent hole is to allow the air pressure in the compartment to equalize, reducing the chances of a premature deployment due to air pressure differences.
If it's in a section of airframe that's someone held together (screws, plugged by a retained motor, etc) it's not necessary. It's only necessary at a separation point, to keep things from separating when you don't want them to.
Straight into the airframe.
-Kevin
Reply to
Kevin Trojanowski
I have had a few rockets drag separate at motor burnout, especially with quick burning fast motors. Never been able to attribute it to lack of bleed screws, though, as the nose cones (or payload sections) popped right at burnout). Maybe somebody out there with some spare CPU time could calculate the actual pressure increase on a typical flight, then we would have a better idea.
Reply to
Tweak
Perpendicular would be just fine.
Reply to
Dave Grayvis
Fred
I don't have my spreadsheets to hand (I can fish out the original equation if you like), but looking back at the code from my altimeter a flight to 3700m (12350ft) starting at 1000 mB will reduce the pressure to around 620 mB.
1000 mB is around 15 psi, so the pressure differential is about 5.7 psi. In a rocket of 4" diameter, that would mean around 70 pounds of force on a nosecone base (assuming a perfect gas seal all the way up).
The slope is linear enough to interpolate between this point and 0 altitude, so halve the altitude, halve the force etc.
Reply to
Niall Oswald
Only a couple things to add to the others:
1) In the main airframe you have a piston, shock cord, parachute, etc. which usually leaves very little air to pressurize so a bleed hole is not as necessary, although the same logic applies if you have a lot of free space in there.
2) Make your separation points plenty tight and don't get conservative with your black powder ejection charge. Doing so, you'll avoid most drag separation problems on these low altitude flights.
--Lance.
Reply to
LanceL
Quoting Phil Charlesworth on 8/10/2003:
From a pure physics point of view it is possible to calculate the pressure at any altitude relative to the launch site in terms of several other terms:
height h lapse rate L - the rate of fall or temperature with height (degrees/kilometre) temperature at launch site T (in Kelvin, not centigrade) gravity constant g (9.81 m/s/s) gas constant R (8.142 J/mole K) molecular weight of dry air M (28.9644 kg/mole)
Of these, only lapse rate and temperature at launch site are unknown. Both of these could be intellgently assumed. It should be possible to relate pressure to height without too much maths. The form of the equation should be
Pressure at altitude h = pressure at launch site x (1 - Lh/T)^k
where k = gM/RL (constant if we fix L)
I think I used a value of L of 8.5 degrees/km.
Reply to
Niall Oswald
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Reply to
Jerry Irvine
Can you find the H16o in the picture..(:-)
formatting link
Reply to
W. E. Fred Wallace
Because of your actions (or inactions) you do not have a motor now large enough to bend that trailer behind you.
Jerry
Reply to
Jerry Irvine
Guys, take it outside to the alley.
This is a flame-free thread.
David Erbas-White
Reply to
David Erbas-White
sorry.
Reply to
Jerry Irvine
I'm really not into anything much bigger than an O. The case is in the shed, compliments of Jim M..(:-)
Reply to
W. E. Fred Wallace
Sorry Dave, I got sucked in again..
Reply to
W. E. Fred Wallace
"Tweak" wrote in message > >
We had a club members 4" diameter hi thurst rocket do that twice.
we put vent holes in the main body the third time and it didn't separate that time.
Probably just voodoo, but you never know.
Reply to
AlMax
So what do we get in the real world, with gas leaking from various parts of the rocket? Also, the trip to ~12k is not instantaneous, so the leakage rate would need to be compared to the rate of pressure increase.
Anyone ever do any "leakdown" testing of rockets? I have only done it on cylinders, and I don't think my compression gauge reads that low.
Reply to
Tweak

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