What's the Risk.??

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Sluggo

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I was reading a thread here last night and they were discussing one of the Star Wars models. They were talking delay times. A member was advising to go with a 4 second delay rather than a 7 or 10 second delay. I need to be educated in this department. I've been thinking a longer delay would most always win over a short delay because of speed, zippering. So, what are the risks of longer delays.??For instance....

Say I put a model up and it has a 10 second delay with little wind. Say the rocket loses momentum and has made the turn towards earth. Does that risk a faulty parachute deployment.?? I've heard Tim at Apogee say chute deployment speed should be near 50mph. The graphs that I've seen show deployment between 50 and maybe 100mph(maybe 90mph.)

What is the risk of higher delay times is my question.

Also, I'm finishing a LOC Patriot. 2.6". LOC says the rocket is stable on E and F power motors. They also say to add weight if flying a higher powered motor. Apogee says add 4 or 5 ounces of weight in the nose. Can you tell me exactly where to put the weight.?? Can I tape it to the outside near the center hole at the back of the nose or do I have to drill a hole and get the weight up inside the nose, up inside the very tip.??

So 2 questions here. Where to put nose weight and the risks of longer delay times. Thanks. IMG_5933.JPG
 
Too long of a delay brings three potential problems, which vary in severity depending on the circumstances:
1) If the rocket is going too fast when the chute opens, it can zipper the airframe (careful design can minimize this risk)
2) If the rocket is going too fast when the chute opens, it can cause damage to the chute (well built chutes will usually not have a problem)
3) If the rocket follows a non-optimal trajectory, there is a risk of it coming down ballistic before the chute ejects.

You want the main separation event to happen as close to apogee as possible, while the rocket is traveling slowest. Within 2 seconds of apogee is a good rule of thumb. If you don't want the main parachute to open at that time (to minimize drift), there are multiple ways to delay it: dual deploy, JL Chute Release, cable cutter.

Nose weight is optimally placed as close to the tip of the nose as is practical, where it will have the maximum effect. With hollow nose cones, that usually involves either clay shoved through a small hole in the base, or some combination of epoxy and a metal weight poured in through a larger hole. With solid nose cones, weight can be attached to the shoulder as there might be no good alternative. But you'll need to use more weight to achieve the same shift in CG.
 
I was reading a thread here last night and they were discussing one of the Star Wars models. They were talking delay times. A member was advising to go with a 4 second delay rather than a 7 or 10 second delay. I need to be educated in this department. I've been thinking a longer delay would most always win over a short delay because of speed, zippering. So, what are the risks of longer delays.??For instance....

Say I put a model up and it has a 10 second delay with little wind. Say the rocket loses momentum and has made the turn towards earth. Does that risk a faulty parachute deployment.?? I've heard Tim at Apogee say chute deployment speed should be near 50mph. The graphs that I've seen show deployment between 50 and maybe 100mph(maybe 90mph.)

What is the risk of higher delay times is my question.

Also, I'm finishing a LOC Patriot. 2.6". LOC says the rocket is stable on E and F power motors. They also say to add weight if flying a higher powered motor. Apogee says add 4 or 5 ounces of weight in the nose. Can you tell me exactly where to put the weight.?? Can I tape it to the outside near the center hole at the back of the nose or do I have to drill a hole and get the weight up inside the nose, up inside the very tip.??

So 2 questions here. Where to put nose weight and the risks of longer delay times. Thanks. View attachment 444174

I would suggest modeling the rocket using Open Rocket and then run various simulations with different motors to determine the weight and motor delays.
 
For nose weight you can pour lead shot or BB's through the hole in the base of the cone and then follow with epoxy. Or you could attach quick links and nuts to the loop at the base of the cone. That's what I do. The quick links allow an adjustable weight system. More weight for bigger motors, less for smaller. Your ejection charge should fire when the rocket is at it's slowest speed. Thrust curve can help with picking the right delay.
 
Opening on impact is generally considered poor form.

Also a fire risk, as if it makes landfall prior to ejection you have delay burn (does not do much to propel but easily enough to light dry brush.)

With a long enough shock cord, especially with some elastic, early ejection has fewer risks than late.

Although not as much of a fire hazard, the second worst situation is also the most darkly comical: ballistic reentry with ejection 1-2 feet prior to impact, driving that nose even harder into the ground.
 
What about altitude and delay choices. Would it be true to assume that a 2500ft flight can have a long delay time.?? For instance.....

I'm going to fly the FlameThrower which is a 2 motor cluster. Apogee shows 1900ft. on 2 E30-7's. Seems to me a flight that high could handle a 10 second delay. Yes or no.?? Thanks.
 
“Handle” is probably not the best word.

The electronic deployment guys (generally non-black power motors, most high power and mid power, few if any low power) have an advantage. Their electronics “detect” apogee (usually......or at least HOPEFULLY) and deploy the chute or streamer (or nose cone and possible drogue if dual deploy) at the optimum moment, when the rocket has close to zero upward or downward velocity. Stress of the chute and recovery harness is mainly due to the kinetic energy imparted by the ejection charge to the nose cone (with any ballast weight IN THE CONE) after the shear pins separate.

Those of us mortals flying black powder (addedendum: or fixed delay Q-Jets) have to “guess” at the best delay for a certain rocket on a certain motor (sometimes intelligently with Rocketry software, sometimes based on instinct or experience, more often than not because the kit face card says, “Recommended motors are X, Y, and Z,” and sometimes the guys who write the face cards are out of their ever loving minds, but that’s another thread.) Problem is that, as you can imagine, guessing is by definition an uncertain art or science. The is compounded by the fact that black powder motors don’t come with an infinite number of delays. Example: there is a C6-3, a C6-5, and a C6-7. If your “Guess” is an even number, you have to choose your poison, a bit early or a bit late. Problem is also compounded by the fact that the motor delays printed on the motor package and the motor itself are themselves, SWAGs*. Fortunately being off a second usually isn’t a big deal, if it’s late 1 second at 32 feet per second squared, it’s going 32 feet per second (plus a horizontal velocity if it didn’t go up perfectly straight and vertical , which many rockets not flown by @JimJarvis50 do.) TWO seconds off and your looking at 64 feet per second. Actual numbers are going to be a bit less, depending on the drag your rocket has. And if you pick a 2 longer delay, and you just happen to get to the motor that JUUUUUSSSSTTT passed Estes Quality Control at one second long, and you are three seconds late, you are at up to 96 feet per second and have dropped 145 feet.**

And while the face card (usually cheerfully over-optimistically) SAYS you are going to hit an Apogee of say 400 feet, if you used expoxy instead of white glue, went a bit heavy on the paint or primer, didn’t get the fins on perfectly, or the guy that wrote up the face card was munching on funky brownies when he quoted the altitude that day, or more commonly it was a mildly windy day and your rocket weathercocked, so all that thrust was divided between vertical velocity and horizontal velocity, your rocket may be only 200 feet up.

No problem as ejection is only 145 feet late. But wait! There’s more! It still takes a finite time for that parachute to unfurl (especially if you it’s plastic, you packed it the night before, you didn’t use talcum powder, and your packing skills are less than optimal.). Add another second or more to your effective deployment, during which your rocket is still descending although with the nose cone out likely it is STARTING to decelerate a bit.

IMO it is better hedge on early rather than late. Both early and late have roughly the same risk of a zipper, broken shroud lines, or broken shock cord or the famous Estes Dent (google it), but late deployment adds risk of ballistic recovery, field fire, and if rocket weather cocks FAR more distance travelled and greater chance of rocket landing outside the field and possibly not recovered.

Here’s a recent example of a less than optimal choice of delay that had a rather entertaining but apparently not too objectively unfortunate outcome I am borrowing from @Patternflyer

https://www.rocketryforum.com/threads/ooops-saturn-v-on-wrong-motor.163598/

** https://keisan.casio.com/exec/system/1224835316






*SWAGs, Scientifically Wild Bottomed Guesses

Read Estes Motor Chart, line 1
• Delays have a tolerance of plus or minus 10% or 1 second, whichever is greater.
https://estesrockets.com/wp-content/uploads/Educator/Estes_Engine_Chart.pdf
 
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Thanks for taking the time to scare the living daylights out of me BaBar.!! Not really but you certainly stated some food for thought. I appreciate your reply.

To be honest with you any motor that I pair with a rocket is going to be a motor according to Apogee Components. I have tried twice to upload the trial period for RockSim which is a program my computer doesn't like. It could harm my device. Ya, right.

So am I on the right track if I trust Apogee numbers.?? I will only launch with calm winds and blue skies.
 
Thanks for taking the time to scare the living daylights out of me BaBar.!! Not really but you certainly stated some food for thought. I appreciate your reply.

To be honest with you any motor that I pair with a rocket is going to be a motor according to Apogee Components. I have tried twice to upload the trial period for RockSim which is a program my computer doesn't like. It could harm my device. Ya, right.

So am I on the right track if I trust Apogee numbers.?? I will only launch with calm winds and blue skies.
You are most kind.

I probably pick on the face card listings bit much, too many memories of the ExoSkell, Venus Probe, and more recently debate on Estes Saturn V. Assuming your build it stock, you don’t intentionally overbuild, you are probably fine with Apogee recommended motors. Hardest thing really depending on where you live is to have the patience to wait for winds at or under 5 mph.

If your system doesn’t like RockSim, other options are OpenRocket (lots of experts here, I ain’t one of them!) and ThrustCurve.org.

Wishing you Straight trails and Short Walks!
 
What about altitude and delay choices. Would it be true to assume that a 2500ft flight can have a long delay time.?? For instance.....

As with most things rocket, it depends.

The built in delay of a motor is agnostic of the ultimate altitude of the flight.

The delay depends on how long it will coast to apogee. That will be different for different motor/rocket combinations. One motor in the rocket could need a long delay for 2500', a different motor may need a middle or short delay

You can approximate this with a hand calculation, or use Rocsim or Openrocket (there's an installer for Openrocket on the forum at the top of the electronics/software section)
 
The deployment speed is almost the same for after the apogee and before it. Except for air resistance, the rocket is deaccelerating at 32 ft/sec per second upwards motion due to gravity, and then on the way down it accelerates down at 32 ft/sec per sec. So 2 seconds late is little better than 2 seconds early. The one real disadvantage of early is it won't make full altitude.

The number I've heard is not at 50 MPH, you want 50 MPH or less. For most rockets -- fiberglass, kevlar and a heavy duty 'chute can certainly survive more. Higher speeds increase chance of rocket damage or complete recovery failure (crash). If ejection occurs long before or after apogee, there is also a chance of it flying through the parachute.

On some longer rockets with a perfectly vertical flight you can get a tailstand where it starts coming back down backwards and then goes sideways and finds a stable sideways attitude, falling with high drag, but that can not be relied upon. However, Open Rocket can actually predict these. If you get a simulation without a parachute that lands a long way away even without wind, it's because it thinks it turns into a glider. It doesn't usually go straight down either -- in OR it can glide sideways over twice the apogee, while in real life it's probably more like half that, but it still can be more drift than a chute. I've even seen OP predict the rocket gliding through a loop!

A higher thrust, faster burn motor will need a longer delay because more of the flight is coasting, for the same altitude.

For your unconventional rockets that break most of Openrocket's assumptions, a guess is as good as you get.

For rocket shaped rockets, it's very close to money

I've yet to do anything I couldn't find a way to simulate it. Trust but verify. Asymmetrical cross sections could be an issue -- I simmed a symmetrical rocket as asym and it made no difference.
 
The one real disadvantage of early is it won't make full altitude.
Absolutely true. However, unless you are going for an altitude record or personal best with an altimeter on board, for your typical sport flyer, and given as you have said the harness forces are likely similar early vs late, I’d rather pop the chute early and have a shorter walk, than make altitude, arc over, and pop late, likely with some horizontal velocity (e.g., weathercocking) that is going to make a longer walk and possibly land not in the field.
 
The built in delay of a motor is agnostic of the ultimate altitude of the flight.
Perfect word choice, but made me think that, as the rockets leave the pad, they are a lot like soldiers in foxholes in battle.

There are no atheists!
 
BABAR .... I'm laughing but it really isn't funny. I've often thought of the atheists last thoughts. Those poor souls. You can't put someone down forever only hoping that he/she will love in your last moments. It won't happen. Back to rockets now.....
 
Bottom line: Apogee recommendations are a good starting point. Thrustcurve.org is extremely valuable for finding motors with sufficient thrust to get the rocket moving off the rod. Its delay estimates will only be as good as your Cd guesstimate, though. A sim program will give you more detailed results.

Ultimately, there's always a bit of guesswork involved in setting delays, but using the information you have available should keep you from making too severe an error.
 
I usually go for a lower delay in case of unexpected poor motor performance, severe weathercock lower altitude, clustering with a motor that doesn't ignite, and sometimes during test flights of unproven designs where drag is excessive or simulation is inaccurate.
 
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