"Uberfast" deployment velocities

[Tau Zero]

I ran into difficulties with my "Prometheus (http://forums.rocketshoppe.com/showthread.php?t=4681)" design having excessively high deployment velocities on "B" motors. The attached "Retrograde" design suffers similarly. :mad:

Do "all y'all" think these are just occupational hazards of this particular size (ST-10 with 18mm mount), or *what?*

In other words, if you have any suggestions for fine-tuning this design, I'm all ears.


Thanks,

[RandyT0001]

I added one ounce of mass located in the ST-730 body tube just aft of the bulkhead and it dropped the rocket's optimal delay to 3.9 sec and it's velocity at deployment to about 11 ft/sec. I also changed the rocket's finish from 'polished' to 'gloss' to reflect the typical 'high end' finish that about 90% of rocket builders can achieve at best. Actually most will build to about a matt finish (filled and spray can painted) for sport launches.

[Mark II]

Have you tried it on a B6-6?

The "optimal delay" in RockSim is a moving target. Don't take the figure that it gives you for Bx-4 motors at face value. To zero in the true optimal delay for a given motor, you need to sim the launch repeatedly with that motor while working through a whole range of delays, including delay times that aren't actually available. At some point the delay that RockSim recommends will reach a plateau, and simming the launch with ever longer delays won't cause it to budge from that number.

Mark K.

[CPMcGraw]

Jay,

First thing I notice is that the rocket is still travelling upward at deployment, so that lets you know the delay values are short, and as Mark said, the optimal value is a moving target. However, it does give you a target range to work toward.

Notice that the A8-3 and C6-5 values are close to perfect. They are still moving upward, but they're closer to the right Dv than the B4-4 and B6-4. What you want to do is shift the A8-3 and C6-5 flights until the model is heading downward before they deploy. Since you cannot adjust the delay times, you have to adjust the Dv.

How? Shifting the Dv is also a product of how much drag the model has. Increase the drag slightly, and the model will begin to arc over sooner. What you want to achieve is to have the A and C engines deploy just after apogee, and the two B engines just before.

Scale up the fins a couple of percent and try running the simulation again. See my attached image for the simulation runs I got. I tried to bump the scale up slowly until I came into range, and the resulting scale factor was 25%. YMMV...


A8-3......220'......Dv 3 FPS
B4-4......517'......Dv 14 FPS
B6-4......525'......Dv 19 FPS
C6-5......816'......Dv 10 FPS

[Tau Zero]

I added one ounce of mass located in the ST-730 body tube just aft of the bulkhead and it dropped the rocket's optimal delay to 3.9 sec and it's velocity at deployment to about 11 ft/sec.Notice that the A8-3 and C6-5 values are close to perfect. [SNIP] Increase the drag slightly, and the model will begin to arc over sooner. What you want to achieve is to have the A and C engines deploy just after apogee, and the two B engines just before.

Scale up the fins a couple of percent and try running the simulation again. YMMV...Randy and Craig (and Mark ;) ),

Thanks for your input! I basically ended up adding a bunch of nose weight by swapping out the SE-10 screw eye with two SE-14's (in appropriate places) and a couple of WL-7 washers. That leveled out the deployment times, but obviously cut back on the altitudes.

I'm thinking if I do the same thing to my Prometheus, I might have to eliminate my tag line about "High Performance." :o :(

Craig, I was using Carl's standard "Recruiter" fins, so I decided that (British accent) "my talents lie in other directions." (A Bit of Fry and Laurie)


Herewith, "Retrograde" version 2. ;)


Thanks so much, guys! I appreciate the help!

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[CPMcGraw]

I thought I recognized those fins from somewhere... :p

But you understand what I was working toward, right? Slowing the rocket down just a tad by way of drag instead of by mass. Adding mass can bite you -- it makes the model fall faster, even under the parachute, which means higher impact V with the ground. By using drag, the increase in mass is minimal, so the descent rate remains lower.

[Doug Sams]

But you understand what I was working toward, right? Slowing the rocket down just a tad by way of drag instead of by mass. Adding mass can bite you -- it makes the model fall faster, even under the parachute, which means higher impact V with the ground. By using drag, the increase in mass is minimal, so the descent rate remains lower.I prefer adding drag instead of weight, too. An ounce of noseweight nearly doubles the weight. According to the Rocksim file, it was only 33 grams to start with.

BTW, not only does adding mass increase descent weight, it also increases separation forces imparted to the recovery harness. So, while that 30mph Dv is hard on the recovery system, so too is the extra ounce of mass at the screweye.

Doug

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[Tau Zero]

Slowing the rocket down just a tad by way of drag instead of by mass. Adding mass can bite you -- it makes the model fall faster, even under the parachute, which means higher impact V with the ground. By using drag, the increase in mass is minimal, so the descent rate remains lower.BTW, not only does adding mass increase descent weight, it also increases separation forces imparted to the recovery harness. So, while that 30mph Dv is hard on the recovery system, so too is the extra ounce of mass at the screweye.Craig and Doug,

Thanks for your input! After sleeping on it, I think Craig's idea of increasing the fin size by 25% is probably the best way to go with this design. As for the Prometheus... The 175% SLS version works just fine, but the 100% version still has relatively high deployment speeds.

Maybe the solution is a Really Big Streamer? (http://www.oldrocketforum.com/showpost.php?p=87828&postcount=11) (See third picture.) But that means finding one big enough to slow this model down... (thinking really hard)

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[STRMan]

I think the solution would be twin very small chutes with very long elastic shock cords. Pack one chute a lot tighter than the other, so they don't deploy at the exact same time. I always like the look of a rocket coming down on two chutes anyway.

[JRThro]

Yeah, definitely use bigger fins rather than adding big hunks of weight just to reduce the deployment velocity.

It's a nice looking design either way, Jay.

[Tau Zero]

Yeah, definitely use bigger fins rather than adding big hunks of weight just to reduce the deployment velocity.

It's a nice looking design either way, Jay.John and all,

Thanks so much for your comments! I took all of them under consideration, and decided to enlarge the fins by 137%, but then "thin them back down" to the original 3/32" thickness.

I also substituted many of the original parts with replacements from the "updated" RockSim files, e.g. using fiber instead of cardboard for the centering rings. :cool:

I suppose the only suggestion I would make is to use a nylon chute for the B6-4 motor :eek: , or just use a B4-4 instead.

Any additional comments before I release this into the wild? I mean, the BARCLONE Stable? ;) :D


After this, we tackle streamers for the 1.00X Prometheus. (sly look)


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[stantonjtroy]

Maybe everyone else is seeing something I'm not but based on the sim data shown, you don't need to change the rocket at all. 18-25 fps decent rate under chute is normal so having these speeds at deployment is ok. 29fps is a little fast ( I wouldn't call it uberfast). Follow Mark's advice; Change up to a B6-6. That should put you dead on the money. Remember the "up" or "down" arrow shown for deployment in Rocsim is realative. On any balistic curve (parabolic) there is only one infintesimaly small data point where vertical velocity is truely zero. A fraction of a second prior or after and your rocket is, in the strictest sence, still moving up or down and Rocsim will show this. The velocities you're getting aren't too bad at all (except the 29fps). For a good visual repersentation, have rocsim plot the data (set it to use Apogee and Deployment as reference lines). The closer those lines are, the softer the deployment. I think you'll find them quite close. My two cents.

[Tau Zero]

Troy,

Thanks for your comments! I just double-checked Retrograde Version 3 on a B6-6, and it shows deployment velocity at 41.63 f.p.s. :eek:

This version doesn't have all the excess weight of Version 2, so there's the current variables that we have to factor in. :D ;)


Thanks again,

[CPMcGraw]

...18-25 fps decent rate under chute is normal so having these speeds at deployment is ok. 29fps is a little fast...

At least for me, 18-25 FPS is on the high side of a safe Dv, given that most of our LPR stuff uses traditional plastic chutes with carpet thread shrouds. I don't like hearing my parachutes "explode open" with an audible pop when they're still 800-1000 feet up - this is a clear indicator the Dv is too high. What I and others here are saying is, why not design these rockets so that the Dv is as LOW as we can get them, instead of just "trying to keep it under the limit"? The lower you can get the Dv, the better the chance of a safe recovery. It puts a lower stress on the recovery system, and anytime you can do that, it's better.

That's what "rocket science" is about - making the rocket do what we want it to... :)

[stantonjtroy]

At least for me, 18-25 FPS is on the high side of a safe Dv, given that most of our LPR stuff uses traditional plastic chutes with carpet thread shrouds. I don't like hearing my parachutes "explode open" with an audible pop when they're still 800-1000 feet up - this is a clear indicator the Dv is too high. What I and others here are saying is, why not design these rockets so that the Dv is as LOW as we can get them, instead of just "trying to keep it under the limit"? The lower you can get the Dv, the better the chance of a safe recovery. It puts a lower stress on the recovery system, and anytime you can do that, it's better.

That's what "rocket science" is about - making the rocket do what we want it to... :)


I agree that the lowest Dv is ideal but keep in mind, at 1G (32fpsps) 18-25 fps velocity is less than 1 second pre or post apogee ejection. Even if the rocket deploys right at apogee, it will be .5-1 second into the down side by the time the chute unforls and fills. This puts the velocity at around 13-21fps. Not so bad. I'm mot sugesting that these highter velocities are aideal target; Rather, if you have a design that sims to it, a change in ejection delay time might be better than a physical change to fin area or mass. Especially if the info is sim only. Though sims are fairly accurate, the descrepencies could more than cover a 6-9 fps difference in Dv experienced in actual hardware flight. In the end I agree, Slower is better.

[Ramjet]

I would increase drag for sure. However, have you considered a forward mounted tab or fin vs making the fin larger? Or perhaps just some 3D detailing on the body surface up near Cg.
Enlarging the fin may make the rocket overstable, and thus very prone to weathercocking.
Just my 2 cents.

Ramjet