Recomended max descent rates for LPR models?

[Goat]

Hey all,

What is the recommended max descent rate for LPR models? The usual Estes style cardboard and balsa types is what I have in mind.

I've searched around...but no joy. I'm thinking around 4.5m/s (16kph or 10mph) but would appreciate any guidance.

Cheers,

 

[cjl]

Honestly? I have absolutely no idea. I bring HPR in at 20-25fps, but LPR, I tend to go with the "that seems about right" method for chute size. It hasn't failed me yet...

 

[gpoehlein]

I'm not really sure either, but I usually let the model being flown and the field conditions determine what recovery system I use.

First, your field - is it a hard surface like concrete or asphault. A little softer like mowed short grass, or very soft like tall grass? With the latter, you can get away with a smaller chute, streamer or even no chute at all (faster descent). With a hard surface, you'll want a big chute with a slow descent rate. Also, consider the wind. If there is a good breeze blowing, a faster descent rate is desireable or your rocket way drift away. Likewise thermal activity - I managed to thermal away a double egg lofter (with 2 eggs) on a D with a 36" chute Sunday - it was still going up at 15 minutes when we lost sight of it.

My rule of thumb - for an Alpha or similar sized rocket, I usually go with a streamer or even nose blow recovery - but our field is an unmowed pasture. I go with a 12 or 15" chute in something the size of a Big Bertha. If you build them well, these little guys can actually stand up to a bit of punishment.

 

[Goat]

Awesome.

Thanks for the tips. That's quite something about the double-egger getting lost on a thermal!

I'm tempted to slap some fins on a tube...by slap I mean I'd still use double glue joints...and do some drop tests to see when they start to break. I'm phobic about losing the rockets. There is not really an easily accessible launch area that is much bigger than a school field in my area...so it's risky to fly above 200m with any wind.

Hence...I want to bring them down as fast as possible. I also need to get better at judging wind velocity and appropriate launch angle to compensate...a bag of motors and some experience and I imagine I'll be boomeranging them right back to my launcher

I did lose the first Alpha-III on a small streamer though. Granted I should not have flown it on a C motor, but still...it wasn't going to come down much faster than that. The bigger rockets I have I'll be trying them out with a 28cm (with 7cm vent) which I'm hoping is not too quick on the descent.

Cheers,

 

[Goat]

Honestly? I have absolutely no idea. I bring HPR in at 20-25fps, ...

This is interesting. Converting that into metric...I know, I know, but I'm Canadian...that's about 6 - 7.5 m/s (22 - 27kph). Your HPRs are way more valuable then my LPRs so maybe I'm being too cautious...

Cheers,

 

[cjl]

They may be more valuable, but they're also stronger. I could stand on my L2 rocket without damage, so it can take quite a hit (and it comes in quite fast too - 28fps on its last flight). My LPR definitely come in slower than that. For small fields though, I would lean towards streamers whenever possible. I have successfully brought in models up through about big bertha sized on 5' by 2" mylar streamers (they're really visible when you do this too). Of course, that only works for a grass or dirt field - at the speed they drop at, you would probably break something if you hit concrete or asphalt.

My general thought for LPR goes like this though:
<2oz, and it usually gets a streamer or a small (12" or less) chute.
2-4oz and it gets a 12-16" chute
5oz and up get an 18-24" chute (24" can carry a lot of weight though, as far as LPR is concerned - I would save them for the really heavy ones of 8 or 10 ounces).

These categories can go up or down a size, depending on how delicate the rocket is, how high it is flying, and where I'm launching from. As I said though, it's hardly scientific. When I'm building, I just sort of go by feel for which size chute seems right for the model. For example, even though my Fliskits Borealis is pretty light, it has a 16" chute because it is a high drag model that won't be going very high, and it has a lot of small tubes and detailing that could easily get damaged. On that one, I'd rather risk a longer walk than damage to the model. On the other hand, I've seen Estes Eliminators flown on a small streamer despite their 4.6 oz weight because they are fairly strong, as LPR rockets go, but they do get quite high (1400 feet).

 

[Zeus-cat]

I found a formula that generates good results. Chute size in inches = 9 + .75 * Weight of rocket in ounces with expended motors. I have compared the results of this formula with other prediction methods I have found and it is very close.

I don't have time to do the metric conversion right now, but I can do it later.

 

[Adrian A]

With your small field, you might have fun making your rockets dual-deployment. You can make an 18mm or 24mm dual-deployment av-bay using a Parrot altimeter. The whole av-bay would weigh about 15-20 grams and be less than 3" long. Then you could fly much higher and still bring it down gently on your small field.

 

[gpoehlein]

I'm tempted to slap some fins on a tube...by slap I mean I'd still use double glue joints...and do some drop tests to see when they start to break. I'm phobic about losing the rockets. There is not really an easily accessible launch area that is much bigger than a school field in my area...so it's risky to fly above 200m with any wind.

Hence...I want to bring them down as fast as possible. I also need to get better at judging wind velocity and appropriate launch angle to compensate...a bag of motors and some experience and I imagine I'll be boomeranging them right back to my launcher

I did lose the first Alpha-III on a small streamer though. Granted I should not have flown it on a C motor, but still...it wasn't going to come down much faster than that. The bigger rockets I have I'll be trying them out with a 28cm (with 7cm vent) which I'm hoping is not too quick on the descent.

Cheers,

Well, I have a good solution to the "losing rockets" phobia: paper rockets. You print these out on your computer using 67# or 110# cardstock, roll the tubes and nose cones and build like normal. You don't have to paint them because they are pre-printed with whatever graphics the designer wishes to use, so not having to paint and prep more than outweighs the extra time required to make the tubes, rings, noses and fins.

And since they cost next to nothing, I'm not as concerned about losing one - if it sails away, print and build another. If it crashes and crumples a tube, print that part out, cut away the damaged section and rebuild. If it burns, CATOs or blows out a tube, see previous sentence!

These things are a lot of fun and quite adictive - I build mostly paper rockets for my contest flying - it's cheap and fun! Best place to get started is the Ultimate Paper Rocket Guide:

https://rocketry.wordpress.com/ultimate-paper-rocket-guide/

There are more free models there than you can build in a year or two, at least! Give 'em a try - I think you'll find they're a whole lot of fun!

 

[Micromeister]

Or switch most of your models to Streamer recovery. I've been flying a Big Bertha on a streamer for years to bring it down quicker. a 4" x 60" Crepe streamer has brought her back undamaged for a very long time now. I've used as wide a 10" x 100" in some models but you have to match the model to the streamer.. sometimes you can get em TOO wide or long and the models...yes 4 and 5 oz models thermaling away or out over the woods

Or how about the old Streamer Chute. adding a 6,8 or 10" chute to top end your streamer. Haven't done it in year but is used to be a semi common practice.

 

[Goat]

Thanks for the replies guys!

Some very cool and interesting ideas...that will keep me busy for a long time!

Cheers,

 

[bobkrech]

There's two limits for safe recovery:

1.) The maximum speed under recovery that your rocket could hit someone and not hurt them.

2.) The maximum speed under recovery that your rocket stays intact.

The first criteria is hard to determine, and can be avoided by following safe range procedures, but the second is easy. All you have to do is drop your rocket (without recovery gear deployed) from a given high and see if it breaks on the type of surface you fly from.

The acceleration due to gravity, g, is 32 feet per second per second. The time it takes for a rocket to gain a given velocity is simply t = v/g and the distance required for this to happen is d = 0.5*g*t^2. Combining the 2 equations tells what high you need to drop a rocket from (without a chute) to impact the ground at a desired speed, so d = 0.5*g*(v/g)^2 = 0.5/g)*v^2 = (v^2)/64 = (v/8)^2 or for a given d, v = 8 * sqrt(d)

The table below indicates the velocity in fps obtained when you drop an object from a given height in feet. If your rocket is not damaged after a drop from a given height, adjust the chute diameter to obtain that descent rate.

Height, ft.Speed, fps
1 8
2 11
3 14
4 16
5 18
6 20
7 21
8 23
9 24
10 25
11 27
12 28
13 29
14 30
15 31
16 32

Many low power rockets will not be damaged when landing on grass at speed as high as 20 fps. This velocity is obtained in a 6' drop.

Larger rockets have more momentum and energy so they usually require a slower impact velocity. Most should be undamaged from a 16 fps landing speed. This velocity is obtained in a 4 foot drop.

Bob

 

[powderburner]

With your small field, you might have fun making your rockets dual-deployment. You can make an 18mm or 24mm dual-deployment av-bay using a Parrot altimeter. The whole av-bay would weigh about 15-20 grams and be less than 3" long. Then you could fly much higher and still bring it down gently on your small field.

I agree this could be a great way to go, and would allow you to have a "high-power"-style rapid descent from apogee and still have a safe and gentle landing, all in a low-power model rocket. Sounds excellent, especially for those of us that live in windy parts of the country.

But-

Correct me if I'm wrong, but wouldn't this approach also require a second ejection charge (not built into a motor) and the use of loose gunpowder? It would seem that if I purchase commercially-made black powder to use for this purpose, I am buying a low explosive. And the igniters that go between the electronic controls and the black powder also currently require permits and licenses. This moves me into the wonderful world of LEUPs, local fire marshals, records paperwork, storage lockers, random inspections, and all the other craziness that turns low-power model rocketry (relatively "care free") into a whole different animal (a cosmic-sized hemorrhoid?).

OTOH, if one of our electronics vendors sold small packages (1 gram?) of black powder ejection charges to use with the electronic controls, and an igniter that was matched with the electronics, then it seems that it would all fall into the legal loophole of "model rocket" product, designed as an integrated system from a single vendor, that does not require all the LEUP nonsense. So I have to wonder, why haven't any of the electronics vendors taken this additional step? (It would make ownership and use of secondary ejection systems much simpler and easier, and sales would probably increase.)

The only existing complete system like this that I know of is from AeroTech, and nothing against them, but their system is kinda big and relatively heavy (oriented toward high-power rocketry) and kinda pricey, at least compared to low-power rocketry in general. Surely someone could come up with an equivalent-function-and-appropriately-priced product for low-power??? I'm not in the "timer electronics business" but it seems to me that the timer would be the "hard" part, and several vendors have that done already.

And all my rambling is off-thread, so if we're going to discuss this idea we should probably start a new one-

 

[Adrian A]

I agree this could be a great way to go, and would allow you to have a "high-power"-style rapid descent from apogee and still have a safe and gentle landing, all in a low-power model rocket. Sounds excellent, especially for those of us that live in windy parts of the country.

But-

Correct me if I'm wrong, but wouldn't this approach also require a second ejection charge (not built into a motor) and the use of loose gunpowder? It would seem that if I purchase commercially-made black powder to use for this purpose, I am buying a low explosive. And the igniters that go between the electronic controls and the black powder also currently require permits and licenses. This moves me into the wonderful world of LEUPs, local fire marshals, records paperwork, storage lockers, random inspections, and all the other craziness that turns low-power model rocketry (relatively "care free") into a whole different animal (a cosmic-sized hemorrhoid?).

OTOH, if one of our electronics vendors sold small packages (1 gram?) of black powder ejection charges to use with the electronic controls, and an igniter that was matched with the electronics, then it seems that it would all fall into the legal loophole of "model rocket" product, designed as an integrated system from a single vendor, that does not require all the LEUP nonsense. So I have to wonder, why haven't any of the electronics vendors taken this additional step? (It would make ownership and use of secondary ejection systems much simpler and easier, and sales would probably increase.)

The only existing complete system like this that I know of is from AeroTech, and nothing against them, but their system is kinda big and relatively heavy (oriented toward high-power rocketry) and kinda pricey, at least compared to low-power rocketry in general. Surely someone could come up with an equivalent-function-and-appropriately-priced product for low-power??? I'm not in the "timer electronics business" but it seems to me that the timer would be the "hard" part, and several vendors have that done already.

And all my rambling is off-thread, so if we're going to discuss this idea we should probably start a new one-

The way I do all my ejection charges is using Pyrodex (on the shelf at my local sporting goods store, no questions asked) potted into small cardboard tubes with epoxy and ignited with Estes or Quest ignitors. Both types of ignitors are easily available without a LEUP, and both work fine with a Parrot altimeter. The tubes I use are 1/4" launch lugs from Apogee, cut to 3/4" -1" long, (for a 38mm HPR rocket) or the little brown tubes into which copperhead ignitors are packaged for smaller rockets. I make a little puddle of epoxy on a piece of plastic, put the ignitor with the head in the middle of the tube, and set the end of the tube with the wires coming out into the epoxy so that the epoxy seals the edge all the way around. Then after that cures, I fill up the tube with pyrodex, tamp it down, and cap it off with another batch of 5 minute epoxy. I usually make a dozen or so charges in about 1/2 hour.

As to the legal standing of motor ignitors and Pyrodex used for ejection charges, it's an unclear area that has been the topic of numerous other threads. IANAL, but my own personal take on it is that it would be reasonable to interpret the method I described as legal, and as long as I don't use it to commit a crime, nobody is going to mind. I would guess that much less than half of rocketeers who use electronic ejection have LEUPs.

 

[powderburner]

OK Adrian,

The only stuff I found on the Parrot was for altimeter electronics

https://www.rocketryplanet.com/content/view/2147/28/

Can you post a link to a version that does staging?https://www.featherweightaltimeters.com/

(is this it?)
https://www.featherweightaltimeters.com/

(thanks ahead of time for your help)

 

[cjl]

IIRC, the standard Parrot has 3 programmable outputs. You could definitely stage with it if you wanted to.

 

[Adrian A]

OK Adrian,

The only stuff I found on the Parrot was for altimeter electronics

https://www.rocketryplanet.com/content/view/2147/28/

Can you post a link to a version that does staging?https://www.featherweightaltimeters.com/

(is this it?)
https://www.featherweightaltimeters.com/

(thanks ahead of time for your help)

Yes, that's the correct link.

 

[Goat]

...All you have to do is drop your rocket (without recovery gear deployed) from a given high and see if it breaks on the type of surface you fly from...

Thanks Bob...this was the direction I was heading in to try and get some empirical measure. I was simply going to take a spare tube and some spare fins, weight the whole structure up to a about 60g or so, and start some drop tests.

It's still a little subjective though because controlling actual impact angles etc. is going to be tough to do. I think though, that as long as I can get a decent sense of a reasonable drop speed I'll be fine with that...

...thinking aloud now...perhaps assuming that snapping a fin is the real problem I'm looking to avoid then maybe a simple test jig of a block of wood with a fin glued on it...the block has a hole drilled through it...and let the whole assembly drop/slide down the length of a rod (like a spare launch rod). That way I can determine the descent rate and impact angle...

Hmmm...

Then, of course...Adrian's staged chute deployment sounds like a lot of fun too.

Cheers,

 

[RocketsNorth]

There is a great book titled Handbook of Model Rocketry by G. Harry & Bill Stine. In the book they recommend 44 sq ins per ouch of total decent weight.
I know from limited experience that the standard 12" chute on most LPR's under 2 oz cause massive drift. I had one drift over a 1/4 mile before finally landing on a roof. The problem I've found with streamers is that if you lose sight of the rocket on asscent it's really difficult to reaquire it on decent. I've lost a few rockets from both problems.
I now either cut a hole in the center of the 12" chutes provided or cut them down to 9" chutes by trimmimg an inch off each side of the permeter.

 

[BsSmith]

This is interesting. Converting that into metric...I know, I know, but I'm Canadian...that's about 6 - 7.5 m/s (22 - 27kph). Your HPRs are way more valuable then my LPRs so maybe I'm being too cautious...

Cheers,

HPR rockets are also built extremely strong (hopefully) so they can take higher forces from landings, with a few paint scratches. :bang: They also go much higher, so a higher decent rate is wanted.

LPR rockets barely need any recovery method, using only a nose blow recovery on a nice grass field works great. Rockets with a high length to width ratio (like the Estes Mean Machine) need something to keep the tubes from krinkling.

The way I do all my ejection charges is using Pyrodex (on the shelf at my local sporting goods store, no questions asked) potted into small cardboard tubes with epoxy and ignited with Estes or Quest ignitors.

I pack Pyrodex into a latex glove finger (my mom works at a medical center so we have a ton) and ignite it with "LEUP free" Ematches that we bought from Performance Hobbies last year. I only have 8 of those left so maybe I should try the Quest igniters instead, they look pretty nice.

 

[kramer714]

Might be a different way to go, Ever look at different kinds of rockets? Spools and Saucers both can be launched with just aero-brake or tumble and land very close to where they took off even in wind. I scratch build them, but I have seen Art Applewhite designs fly nicely from a kit.

Just a thought

 

[spacecadet]

Snapping a fin isn't such a big deal as long as it comes off neatly at the root. You just glue it back on again. That's why I stopped tabbing fins- a break is easier to fix than a bend. I lose a fair few on the way down, but I've never lost one on the way up- they need to be strong enough for that.
Most of my streamers tend to be about the fin semi-span by three or four times the length of the rocket.
I say, is that a new rule of thumb?