Pilot Chutes and Mortars |
Pilot Chutes and Mortars |
Dec 21 2007, 07:51 PM
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#1
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Junior Member Group: Members Posts: 91 Joined: 21-August 06 Member No.: 1063 |
The large Pioneer Multiprob deployed parachute so it could descend to surface of venus.
I am trying to understand the whole mortor and parachute system. For those of you parachute experts. When a mortor fires to pull out a pilot chute. What happens to the mortor after its fired? I am guessing the mortor has a rope tail that it pulls out of the chute. Then the rope has a pilot parachute attached to it. And then what..the mortor just files off leaving rope still attached to the pilot prachut or it takes the rope tail with it? I have tough time finding info about the details of a mortor pilot chute relationship. I am not even sure if the mortor really does have a rope tail connecting to pilot chute. Also what does the mortar look like. Is this same method used for the Hyguns and Galileo probes? Thanks for any help. |
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Dec 21 2007, 09:46 PM
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#2
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Founder Group: Chairman Posts: 14432 Joined: 8-February 04 Member No.: 1 |
A mortar isn't an object in itself that would get ejected - it's a technique. As I understand it (and I probably don't) Imagine a cylinder and a piston. The parachute is packed above the piston - with a retaining cover, and a charge packed below the piston. The charge is ignited and the piston is pushed upwards ejecting the chute at high speed out through the cover and into the airflow. It's similar to the way the nose-cone and chute get deployed on model rockets. I think.
Some use a spring instead of an explosive charge - but with spaceflight you really REALLY want the chute to deploy, quickly, so a small charge gets used. Doug |
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Dec 22 2007, 02:46 PM
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#3
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Member Group: Members Posts: 611 Joined: 23-February 07 From: Occasionally in Columbia, MD Member No.: 1764 |
... I am trying to understand the whole mortor and parachute system. .... Is this same method used for the Hyguns and Galileo probes? (restrains impulse to comment on spelling) The Huygens PDD (Parachute Deployment Device) was a mortar on the top platform of the probe. A propellant cartridge is ignited by 2 NSI pyros and builds up gas pressure behind an aluminium sabot held inside the PDD with 2 O-rings. The pilot chute itself is packed above the sabot with a closure cap. The pressure accelerates the sabot, chute (in a bag, with the riser and bridle) and cap. When they have moved 42mm or so, they push out the breakout patch in the aft cover (remember all this stuff has to be protected from the heating of entry) which is held in place by shear pins. So, sabot, chute and cover fly out. The sabot is unrestrained. The cover is attached to the parachute bag. As the chute bag flies off, the bridle, then riser and lines come out, then the canopy itself which begins to inflate as the bag comes off. The bag, cover and patch are all connected together and fly off. Pilot chute inflation is complete 1.4 seconds after PDD initiation. Insofar as the thing is a tube with stuff hurled out by a charge, it's a mortar, but really it is a more sophisticated system with lots of bits all of which get designed with care and whose trajectories have to be modeled etc. System was designed by Martin-Baker (UK) who make ejection seats etc (though the people involved with Huygens split off to form their own company - Vorticity) Hope this helps. Couldnt tell you anything about Galileo - this level of detail is rarely found in public documentation for US systems, unlike ESA/Huygens. You can find some more general background in 'Planetary Landers and Entry Probes'. Knacke's Parachute Recovery Systems Design Manual is excellent, if dated and expensive. Ralph |
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Dec 22 2007, 02:50 PM
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#4
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Member Group: Members Posts: 611 Joined: 23-February 07 From: Occasionally in Columbia, MD Member No.: 1764 |
... Imagine a cylinder and a piston. The parachute is packed above the piston - with a retaining cover, and a charge packed below the piston. ..... Some use a spring instead of an explosive charge - but with spaceflight you really REALLY want the chute to deploy, quickly, so a small charge gets used. Doug has it pretty much right. But the issue is less deploying the chute quickly as it is getting the canopy clear of the recirculating wake of the probe. So you need enough momentum to get the chute into the clear air (rule of thumb - though there are now sweet fluid-structure interaction models that can explore it explicitly - is you need to be 9 probe diameters or so behind) to inflate reliably. |
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Dec 22 2007, 03:13 PM
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#5
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Merciless Robot Group: Admin Posts: 8783 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
Huh. So you're saying that 9 vehicle diameters is a more-or-less constant for avoiding wake turbulence, at least to the degree needed for successful deployment? That seems almost too small, esp. at supersonic speeds as during Mars EDL. It also seems peculiar that such a relationship would exist between vehicle diameter & deployment distance (I would have expected that velocity & atmospheric density would be more significant influences). Is this 'constant' basically something derived from experience, or via laminar-flow modeling?
-------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Dec 22 2007, 03:48 PM
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#6
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Founder Group: Chairman Posts: 14432 Joined: 8-February 04 Member No.: 1 |
Not clear of any turbulence, just the bit of it that's recirculating and might trap the chute ( I think )
Doug |
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Dec 22 2007, 06:08 PM
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#7
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Member Group: Members Posts: 611 Joined: 23-February 07 From: Occasionally in Columbia, MD Member No.: 1764 |
Huh. So you're saying that 9 vehicle diameters is a more-or-less constant for avoiding wake turbulence, at least to the degree needed for successful deployment? That seems almost too small, esp. at supersonic speeds as during Mars EDL. It also seems peculiar that such a relationship would exist between vehicle diameter & deployment distance (I would have expected that velocity & atmospheric density would be more significant influences). Is this 'constant' basically something derived from experience, or via laminar-flow modeling? 'seems almost too small' - based on what, may I ask ? I believe this is an essentially empirical relationship - as are most parachute things originally (hence I said 'rule of thumb') - only now is the field moving into a substantially model-based approach. I may be also conflating different requirements - the pilot and main (deployed supersonically) were supposed to be 10 calibers behind, whereas the later stabilizer chute only needed to be 7 calibers (the issue there being not so much inflation as degradation of the steady-state drag performance by being in the wake). I believe in the Galileo program originally the trailing separation was not as large and they discovered problems during testing which pushed them into the 9-10 calibers line/riser length You're right in that in an ideal world you analyze everything with CFD (no reason to force it to be laminar), then build it, test in a wind tunnel, then test in flight. But before/instead of going to that, you use the rules of thumb that prior missions give you. Prior experience may be deceptive of course (e.g. the effective porosity of a canopy will depend strongly on Reynolds number, so that a porous fabric that works fine on Earth with nice stable characteristics acts essentially impermeably in the low density Mars environment..) |
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Dec 22 2007, 06:36 PM
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#8
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Merciless Robot Group: Admin Posts: 8783 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
'seems almost too small' - based on what, may I ask ? Actually, based on aircraft departure procedures; the spacing between launches is hundreds of times the cross-sectional area of the aircraft themselves, but of course the primary sources of turbulence are the engines, so the airflow is considerably more chaotic. However, I'm more inclined to accept your premise based on fighter aircraft landing behavior. F-4s deployed a drogue chute upon landing that was extended behind the aircraft far shorter than the horizontal dimension of the vehicle; the Shuttle does the same. So, again I ask, how did the 9X diameter touchstone arise? It does not seem to be intuitive. -------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Dec 22 2007, 11:51 PM
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#9
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Member Group: Members Posts: 611 Joined: 23-February 07 From: Occasionally in Columbia, MD Member No.: 1764 |
Actually, based on aircraft departure procedures; the spacing between launches is hundreds of times the cross-sectional area of the aircraft themselves, but of course the primary sources of turbulence are the engines, so the airflow is considerably more chaotic. Primary source of *noise* is the engines, but as far as I understand it, aircraft takeoff or landing spacing is driven by the wingtip vortices (I've even hear the term vortex spacing) which can remain coherent for quite some time (after all the weight of the aircraft is being deposited into downward momentum in the air every second and since the tip vortices are separated by the aircraft span, the dissipative shear in the vortex system is comparatively low) QUOTE However, I'm more inclined to accept your premise based on fighter aircraft landing behavior. F-4s deployed a drogue chute upon landing that was extended behind the aircraft far shorter than the horizontal dimension of the vehicle; the Shuttle does the same. So, again I ask, how did the 9X diameter touchstone arise? It does not seem to be intuitive. You can ask again, but the answer is still the same - it is empirical. That doesnt mean it has a robust theoretical background, nor even that it is right. But a pilot chute absolutely has to work, or else mission loss. Drogue chutes for braking are helpful (after all, shuttle coped for years without one until they decided to copy Buran) but perhaps less mission-critical. Note also the wake behind the tail of a jet or even the shuttle is less likely to have a nasty street of vortices or large recirculating region than is the very bluff shape of an entry probe (which is blunt for aerothermodynamic, rather than aerodynamic reasons) So, the justifiably conservative practitioners of the black art of parachute system design adopt that rule of thumb. Question it at your peril. |
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