Safe descent velocity? (LPR)

[4regt4]

I know, I know. This is going to be one of those "hold your rocket up to the keyboard so I can see it" type of things. I usually fly in a smaller field, and occasionally I'd like to upsize the motors a bit. I can probably get away with this if I bring the rocket down a little quicker.

Doing sims on the various Estes kits I've built seems that the included chute sizes give descent rates around 15fps. Yes, I know that safe (non-destructive) rates depend greatly on rocket weight (i.e. heavy rocket that might land on a fin needs to go a bit slower) and how robust it is. Assuming 3-5 oz, basic 3 or 4 fins, papered balsa or plywood. I'm thinking of bumping the descent rate up to 18 or maybe 20fps. For a rocket that I'm probably going to launch tomorrow, that would (according to OpenRocket) entail going from an 18" chute to a 15". I'm talking about the basic cheap-o plastic Estes chutes. The rocket flew an actual 576' on a D12-5 with a reasonably comfortable landing distance from the pad using the 18" in low wind. But I'm considering a Qjet D22-7, which sims to 770 feet. It might be a nail biter on the 18" chute with 15 seconds more descent time from that altitude than with the D12. Switching to the 15" gives a similar descent time with the D22.

Tomorrow's weather forecast has minimal wind in the morning.....

Hans.

 

[Zeta]

Hans your reasoning all makes sense. I would have no worry flying that plan. In low power there isn't a lot of risk, other than a possible fin repair.

 

[MidOH]

I like my LPR's to come down as fast as the club will allow. Streamers on almost all of my Bt20 and bt50 rockets. My Der Red Max has only a 12" Apogee chute. My Highflyer, just a 15" Top Flight or maybe even 12".

Just cut a massive hole in your Estes chute. Like half of it gone. When it shreds, get a 12" Topflight or Apogee.

I'm fine with damaged air frames on any size rocket, easy fixes. I just want it back. That's my first priority.

 

[tjkopena]

I would not even think twice about a ~20fps ground hit velocity for a typical LPR (which implies it's not especially fragile). My personal target is usually keeping it around 22fps (15mph), trying to strike a balance between a manageable hit but also getting it back on a somewhat small field. This is for a corn farm, so mostly reasonably soft grass+dirt landings. In winter when the ground freezes I'd try to come down slower if I thought about it but I probably wouldn't.

 

[Bill S]

I have found that generally Estes oversizes their parachutes. I fly for example, an Hi-Flier XL and Vapor routinely on an 15" nylon chute with a 4" spillhole, with no problems. Last weekend, I flew that Vapor on an 12" with a 2" spillhole because the upper level winds were a little odd and wanted the extra insurance of getting the rocket back. This was on a large field with knee-high grass so it was going to be a softer landing because of that. At my normal field with short grass and hard ground, I probably wouldn't have risked a 12" parachute.

I aim for no more than 15fps for moderately hard ground with some grass on it. If I were flying on just hard ground I'd go lower.

 

[tjkopena]

Trying to put some rationale behind the numbers, I guess one way to think about it is: "From how high would I be comfortable just dropping the rocket?"

Using some online freefall calculators, from a standing start and assuming no air resistance, you get numbers like:
* 25fps = 9.7'
* 22fps = 7.5'
* 20fps = 6.2'
* 15fps = 3.5'
* 10fps = 1.5'

I'd feel pretty ok dropping almost all of my rockets ~8' onto grass/dirt. For me (6'1) that's basically just holding it up at arms length and dropping it. The small LPRs I'd probably even be ok risking that on asphalt or similar.

 

[lakeroadster]

I know, I know. This is going to be one of those "hold your rocket up to the keyboard so I can see it" type of things. I usually fly in a smaller field, and occasionally I'd like to upsize the motors a bit. I can probably get away with this if I bring the rocket down a little quicker.

Doing sims on the various Estes kits I've built seems that the included chute sizes give descent rates around 15fps. Yes, I know that safe (non-destructive) rates depend greatly on rocket weight (i.e. heavy rocket that might land on a fin needs to go a bit slower) and how robust it is. Assuming 3-5 oz, basic 3 or 4 fins, papered balsa or plywood. I'm thinking of bumping the descent rate up to 18 or maybe 20fps. For a rocket that I'm probably going to launch tomorrow, that would (according to OpenRocket) entail going from an 18" chute to a 15". I'm talking about the basic cheap-o plastic Estes chutes. The rocket flew an actual 576' on a D12-5 with a reasonably comfortable landing distance from the pad using the 18" in low wind. But I'm considering a Qjet D22-7, which sims to 770 feet. It might be a nail biter on the 18" chute with 15 seconds more descent time from that altitude than with the D12. Switching to the 15" gives a similar descent time with the D22.

Tomorrow's weather forecast has minimal wind in the morning.....

Hans.

What's the field like where you fly? Nice, thick, freshly cut bluegrass or, like we have here, mostly rocks (Rocky Mountains)?

This one question makes all the difference in the world in regard to ground hit speed.

I design based on 15 mph or less, and try to design the fins hell-for-stout. Homemade Basswood plywood fins, and sometimes papered Basswood.

 

[4regt4]

What's the field like where you fly? Nice, thick, freshly cut bluegrass or, like we have here, mostly rocks (Rocky Mountains)?

This one question makes all the difference in the world in regard to ground hit speed.

I design based on 15 mph or less, and try to design the fins hell-for-stout. Homemade Basswood plywood fins, and sometimes papered Basswood.

Field: I forgot to mention that. It's a multi-soccer field. Mowed grass planted on reasonably soft sandy topsoil. My daughter is in town, so plan on showing her what dad's been up to. The weather guessers are forecasting clear skies with 1-2mph wind (which often means 3 or 4mph). Given that I think I'm upsizing motors today, it looks like I'll downsize the chutes today.....

 

[lakeroadster]

Field: I forgot to mention that. It's a multi-soccer field. Mowed grass planted on reasonably soft sandy topsoil. My daughter is in town, so plan on showing her what dad's been up to. The weather guessers are forecasting clear skies with 1-2mph wind (which often means 3 or 4mph). Given that I think I'm upsizing motors today, it looks like I'll downsize the chutes today.....

You're living the dream! Have fun.

 

[Spitfire222]

I want to reiterate something that another poster mentioned, which is to consider upper level winds. Going an extra few hundred feet in altitude could subject you to different and/or stronger winds, which can drastically affect your drift, even with a smaller parachute. This happened to me one time when I launched an Estes rocket on a C, and it landed right next to the launch pad. The next flight on a D had it drift much further away, almost outside of the field boundaries. Just something else to consider, good luck!

 

[4regt4]

To put a period at the end of the sentence...

No damage from smaller chutes. I supersized all but one rocket (it was 13mm, was already using the largest) and they came down without damage. Sort of......

The star of the show was a fairly light rocket that I was trying with a D22 Qjet for the first time. Shortly after leaving the pad (like 100ft up), it veered considerably to the NE, I doubt it was weathercocking, as the wind was minimal and what little there was, was coming from the NW. A bystander said "that's headed for the river". Sure enough, it came down some 600 feet away into the water. We arrived at the river bank just in time to see it float downstream and over the rapids. Biggest loss was the altimeter it was carrying. Not sure what happened, as all the other rockets landed within 50 feet of the pad. I'm suspecting perhaps the 3/16" rod was whippy, or maybe the foot of the pad on one side settled into the soft grass.

Der Red Max survived the 12" chute just fine. Well, the chute had a small burn hole. I've never been able to get the wadding right on this rocket.

Hans,

 

[Bill S]

4regt4, I would report that as an engine malfuction, as many of the newer Q-Jets have been having problems with uneven nozzle erosion. Lost an Estes Interceptor to that, as well as some funky flights. Maybe report that to Aerotech as well, maybe they can reimburse you for it somehow.

http://www.motorcato.org/

 

[tsmith1315]

FWIW, the Tripoli Safety Code allows landing up to 35 ft/s.

 

[Tractionengines]

The question is: Since you lost the D22 to the river, do you gave the 2nd motor from the pack? If so, what is the date code on the motor?
That will help see if the nozzle was "suspect".

 

[4regt4]

The question is: Since you lost the D22 to the river, do you gave the 2nd motor from the pack? If so, what is the date code on the motor?
That will help see if the nozzle was "suspect".

Inspecting remaining motor. Says "Warning Flammable". That's good to know.

Date code: 020922-01

Assuming that is a straight forward date and not some "code", it looks like it is a very fresh motor.

4regt4, I would report that as an engine malfuction, as many of the newer Q-Jets have been having problems with uneven nozzle erosion. Lost an Estes Interceptor to that, as well as some funky flights. Maybe report that to Aerotech as well, maybe they can reimburse you for it somehow.

http://www.motorcato.org/

I hate to report a CATO without the "specimen" to inspect. Don't know for certain if it was defective or not. I'll check the CATO website to see if there are similar failures for this particular motor.

This had a plastic "wedgy" thingy to hold the igniter in instead of the red plastic tube on the other motors that I have. I had a terrible time getting it to stay in. Suppose it's possible that I damaged the nozzle pushing and shoving that damn thing in?

Hans.

 

[Tractionengines]

This had a plastic "wedgy" thingy to hold the igniter in instead of the red plastic tube on the other motors that I have. I had a terrible time getting it to stay in. Suppose it's possible that I damaged the nozzle pushing and shoving that damn thing in?

Since it had the new Q-Pic ( plastic "wedgy" thingy ) to hold the igniter, it is for sure a newer motor, and should not have the soft nozzles that some early motors had. How much "pushing and shoving" it takes to damage the nozzle, I don't know. I guess it could get chalked up to learning curve.

 

[4regt4]

Since it had the new Q-Pic ( plastic "wedgy" thingy ) to hold the igniter, it is for sure a newer motor, and should not have the soft nozzles that some early motors had. How much "pushing and shoving" it takes to damage the nozzle, I don't know. I guess it could get chalked up to learning curve.

The Qpic wouldn't stay in. I pushed it up to it's shoulder, then simply turning the motor upside down it would just drop out. I'm wondering if there are different nozzle diameters and it simply didn't fit in this particular motor. Might have to go old school with a touch of wadding poked in with a toothpick.

Hans.

Edit: What I had to do was twist the wire leads together right in the nozzle area to "thicken" them so the plastic wedge would stay put.

 

[BigMacDaddy]

I like this table from this newsletter: https://www.apogeerockets.com/education/downloads/Newsletter496.pdf



Fast decent should work...

EDIT: Guess I was behind in the thread since I was still answering the original question.

 

[lakeroadster]

I like this table from this newsletter: https://www.apogeerockets.com/education/downloads/Newsletter496.pdf

View attachment 517107

Fast decent should work...

EDIT: Guess I was behind in the thread since I was still answering the original question.

This is a pretty conservative analysis, based on the of decent rate criteria established in the article.

"For slow descent, we’ll assume that your rocket can survive a fall to the hard ground without breaking from a height of two feet.

We’ll define a fast descent as being able to survive a fall from three feet above the ground." "a “fast descent” .. about 4.5 m/s" = 10 mph)".

 

[BigMacDaddy]

This is a pretty conservative analysis, based on the of decent rate criteria established in the article.

"For slow descent, we’ll assume that your rocket can survive a fall to the hard ground without breaking from a height of two feet.

We’ll define a fast descent as being able to survive a fall from three feet above the ground." "a “fast descent” .. about 4.5 m/s" = 10 mph)".

View attachment 517157

That is good to know... I guess a similar table with more aggressive descent numbers might be useful...

 

[Ez2cDave]

I know, I know. This is going to be one of those "hold your rocket up to the keyboard so I can see it" type of things. I usually fly in a smaller field, and occasionally I'd like to upsize the motors a bit. I can probably get away with this if I bring the rocket down a little quicker.

Doing sims on the various Estes kits I've built seems that the included chute sizes give descent rates around 15fps. Yes, I know that safe (non-destructive) rates depend greatly on rocket weight (i.e. heavy rocket that might land on a fin needs to go a bit slower) and how robust it is. Assuming 3-5 oz, basic 3 or 4 fins, papered balsa or plywood. I'm thinking of bumping the descent rate up to 18 or maybe 20fps.

Hans.

Hans,

In addition to descent rate, the size of the flying field also has to be considered, as you noted.

Wind is the biggest enemy of slow descent rates . . . The higher the wind velocity, the farther the rocket drifts downrange before landing.

Using 15 ft/sec as an example :

Deployed at 1500ft, a rocket will take 100 seconds to reach the ground ( excluding thermals ).

With a 5 mph ( 7.33 ft/sec ) wind, the rocket will drift 733 ft downrange.
With a 10 mph ( 14.66 ft/sec ) wind, the rocket will drift 1466 ft downrange.

Remember that this excludes thermals which will decrease the descent rate and increase drift distance.

You might consider using a "Spill Hole" in your parachutes, an "X-Form" parachute, or Streamer Recovery. A "Jolly Logic" parachute release device is another option.

Dave F.

 

[4regt4]

This is a pretty conservative analysis, based on the of decent rate criteria established in the article.

"For slow descent, we’ll assume that your rocket can survive a fall to the hard ground without breaking from a height of two feet.

We’ll define a fast descent as being able to survive a fall from three feet above the ground." "a “fast descent” .. about 4.5 m/s" = 10 mph)".

I think a key phrase here is "fall to hard ground". 4.5m/s, ~10mph, ~15fps might be dicey for many rockets landing on pavement. Looks like from my smaller chute adventures yesterday, around 20fps is no big deal on well irrigated cut grass. (that estimate is based on OpenRocket sims, I didn't actually measure it) On my more robust models, I might try descending even a bit faster.

Hans.

 

[Dane Ronnow]

I think a key phrase here is "fall to hard ground".

Fruity Chutes' Descent Rate Calculator will give you fall rates. (Pick one of their chutes that is closest to what you've got.)
https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm

 

[Ronz Rocketz]

I have had several Qjets veer off like that but with motors before 8/21 date code. Sometimes the initial thrust is too much for the rocket. This can occur on less aerodynamic rockets like the Super Mars Snooper. I'm guessing yours was a simpler rocket design.

If the Qpic isn't working, then I use a piece of masking tape to hold the initiator.

If you're worried about the other D22, then use it with the Mega Vortico. Always entertaining and Cato proof.

 

[4regt4]

I have had several Qjets veer off like that but with motors before 8/21 date code. Sometimes the initial thrust is too much for the rocket. This can occur on less aerodynamic rockets like the Super Mars Snooper. I'm guessing yours was a simpler rocket design.

If the Qpic isn't working, then I use a piece of masking tape to hold the initiator.

If you're worried about the other D22, then use it with the Mega Vortico. Always entertaining and Cato proof.

Thanks Ron.

I'm actually more concerned about an old batch of Estes D12. Back then they were in a 3 pack. One CATO'd, still have the other 2. Now I know what to do with them. Mega Vortico ordered! BTW, Rocketarium sells on Amazon, so I can do mini sized orders and not have to pay freight.

Hans.