Rod/Rail Length?

[spiff95]

I'm building my first scratch-built rocket. I'm just returning to the hobby after a many-year hiatus, and I've got a few (hundred) questions.

I'm curious- how long does a rod/rail need to be to stabilize the flight of a 4' tall rocket that weighs 3.5 lbs? I'm using a cluster of four engines with 29mm diameter. I can utilize E-G impulse engines (though I'm starting with E's for my first couple of launches).

 

[sodmeister]

You may get several opinions ,but the general consenses would be 6 feet long.There is also rail speed ,but without getting into that too much ,6-8 foot rails are what i use for light stuff to the bigger 7-8-9 pound rockets.

paul t

 

[powderburner]

Welcome back to rocketry, spiff95

There are several factors that help determine rod length, but the short version of the story is: your rocket needs to be going fast enough for the fins to be aerodynamically effective (and your rocket, therefore, self-stabilizing) by the time it releases from the end of the launch rod.

That magic "velocity of stable flight" is itself kinda murky. Some people will say 20 mph, or 30 mph, or some such answer, but this seems to be just another rule-of-thumb. I have never seen any analysis that roughs out why one speed is a minimum critical velocity for safe flight.

Aggravating factors: If it's a windy day, you need more launcher length. If your rocket has a long tail and the launch lugs/buttons stand way up the launcher (reducing the length of travel where the launcher contains and guides your rocket), you need more launcher length. If you have a (relatively) heavier rocket or a lower thrust-to-weight ratio (that takes longer to accelerate to flight speed), you need more launcher length. If your launch lugs/buttons are widely separated along the length of your rocket (leaving only the aft attachment connected to the launcher when the fwd launch lug leaves the rod), you need more launcher length. If the top of your launcher is at eyeball-level when the kids run up to the launch pad, you need more launcher length....but this is for ground safety, not rocket performance.

As in many matters, for a launcher, longer is better. Unfortunately, when it gets too long it can begin to get wobbly and bendy at the moment of launch, which kind of defeats the purpose of greater length. You may have to go to larger launch rod diameters for greater stiffness, or all the way up to a rail system.

Hey, once you buy one, you're set for the rest of your rocket hobby.

 

[Bazookadale]

If you plan to cluster 4 G motors I would advise a 1010 rail of at least 6" - longer is better. Even a heavy rod could be prone to whip.

 

[bobkrech]

I'm building my first scratch-built rocket. I'm just returning to the hobby after a many-year hiatus, and I've got a few (hundred) questions.

I'm curious- how long does a rod/rail need to be to stabilize the flight of a 4' tall rocket that weighs 3.5 lbs? I'm using a cluster of four engines with 29mm diameter. I can utilize E-G impulse engines (though I'm starting with E's for my first couple of launches).

You should run a sim to determine the launch rod velocity but a 6' rail should be adequate, however you will need to be Level 1 High Power certified to launch the rocket since the rocket weighs more than 1500 grams, and likely has more than 125 grams of propellant.

Bob

 

[scsager]

This seems like a pretty ambitious project for a first scratch build, but don’t let my opinion deter you.
If you don’t already have a simulator try “Open Rocket” - completely free, download at this link. Great for designing scratch builds and understanding how they might fly. :gavel:
As already stated your parameters take this outside the realm of “Model Rocketry” and make this a Level 1 High Power rocket. More info about the differences between Model and High Power rocketry can be found at the NAR website.

The cost for an 8 foot rail is only a few dollars more than a six footer, I would opt for the 8 footer if you have room to transport it in your vehicle. An example at this link.

Hope this helps :2:

 

[spiff95]

I used OpenRocket for the design, and it estimates the rocket will weigh just over a kilogram, including engines. I figure after it's built I will weigh it empty to make sure the weight is accurate. I estimated closer to 1300 grams. I see where the rub comes in is the weight of the propellant.:blush:

No matter, I was planning on joining NAR and pursuing a high power certification anyway. I have been looking at their website and Tripoli's in the last week or so, and I'm looking forward to learning something new.

BTW, thanks to one and all for the advice in your replies. It seems the way to go is a rail. Does anyone have a design for a scratch-built pad using a rail?

 

[sodmeister]

Scroll down a bit(in this topic forum) and you will see launch pad and how to make discussions topics.Also do a search for more info.

Paul T

 

[WillMarchant]

This does seem like an ambitious project if you're just getting started back in the hobby. My concern would be in clustering four 29mm composite motors reliably. Have you been having good luck getting a single motor to light exactly when you want it to? Not to be a wet blanket or anything just friendly concern...

 

[terryg]

You cannot get your level 1 on a clustered flight. If you want to use this design, consider a cluster design that has a central motor. This way you can fly a certification flight and work your way into clustering gradually.

 

[Micromeister]

My general rule of thumb when designing Clustered BP Upscalea, LMR (Large Model Rockets)or anything over 1 lb or a single D12, They fly from 1/4" x 72" Stainless Steel launch rods or Alum. Rails to ensure they reach high enough exit speed to keep them going in the straight and true direction intended

 

[spiff95]

This does seem like an ambitious project if you're just getting started back in the hobby. My concern would be in clustering four 29mm composite motors reliably. Have you been having good luck getting a single motor to light exactly when you want it to? Not to be a wet blanket or anything just friendly concern...

Good point. I guess my decision to cluster engines was to achieve enough thrust to attain a minimum altitude of 1000 ft. The sims I've run in OpenRocket show an estimated altitude of 950' using E9-8's. It's not a big deal if I don't cluster. I'll just use what I can legally use and accept the altitude it attains. Then, when I'm ready to do a qual flight, I can use an engine of proper impulse to qualify.

I haven't started construction yet, precisely because I have a lot of things to learn. I've already bookmarked a discussion thread on CG/CP, and am looking at several build threads. Once I'm set on the final design, maybe I'll post a build thread of my own.

 

[spiff95]

You cannot get your level 1 on a clustered flight. If you want to use this design, consider a cluster design that has a central motor. This way you can fly a certification flight and work your way into clustering gradually.

Also a good point! I'm probably going to abandon a cluster for this rocket and build a lighter, smaller rocket for my cluster experiments (that way I can use smaller, cheaper engines to learn with).

 

[bobkrech]

Good point. I guess my decision to cluster engines was to achieve enough thrust to attain a minimum altitude of 1000 ft. The sims I've run in OpenRocket show an estimated altitude of 950' using E9-8's. It's not a big deal if I don't cluster. I'll just use what I can legally use and accept the altitude it attains. Then, when I'm ready to do a qual flight, I can use an engine of proper impulse to qualify.

I haven't started construction yet, precisely because I have a lot of things to learn. I've already bookmarked a discussion thread on CG/CP, and am looking at several build threads. Once I'm set on the final design, maybe I'll post a build thread of my own.

The E-9 is just about the porest choice you could make for a 4 motor cluster on a rocket that might weight more than 1.5 kg. The E9 has a peak thrust of on 20 Newtons. Even if all 4 light, the peak thrust is only 8o Newtons. Your rocket probably weights 15 Newtons, so if all motors light the peak thrust to weight ratio is only 5.3:1, and the average lift-off thrust to weight ratio is half that or a mere 2.7:1. The peak acceleration is only 4.3 g, and if there is a breath of wind, the rocket will weathercock severely, and will not reach the expected apogee.

Here's the sim results using 4 E11-4s in a 3" minimum diameter rocket.

Total launch mass = 1500.0 Diameter = 75.0 millimeters, Initial Cd = 0.750
Stage 1 has 4 engines of type Estes E9-4 for an impulse of 111.47 Newton-Seconds
Total Impulse = 111.47 Newton-seconds Ejection 8 seconds after launch. Parachute diameter = 1000 millimeters.
Flight duration 35.7 seconds.
Max altitude = 165.99 meters (544.6 ft) at 7.3 seconds.
Peak speed = 45.28 meters per second (101.3 MPH).
Maximum acceleration = 4.32 g.
Turn over at 7.3 seconds
Chute deployment at 7.06 m/S, shock = 1.00 g.
Impact at 5.9 m/s.


With one motor not igniting, the altitude is halved, and the ejection is 2 seconds after apogee, however if the rocket weathercocks, it will crash before ejection. If 2 or more motors fail to ignite, the rocket crashes.

This rocket needs motors with a minimum average thrust rating of 30 Newtons to have a safe flight if one engine doesn't light.

Bob

 

[bguffer]

4xEstes E9 has over 125 grams of propellant, making that motor configuration a high power configuration. FAA waiver would be required to launch.

 

[cbrarick]

Ok, not wanting to put water on the fire, but check out Loc's Viper IV. https://shop.locprecision.com/product.sc?productId=101&categoryId=13
It's a four engine cluster rocket (hey, why reinvent the wheel) that flys great on four D12-7's 1000 feet really isn't hard on it, either. My club (CATO) drag races them all the time - some people also fly D12-5's. Never seen them on a 3 second delay, though. It would go off while the rocket is still coasting up.

JMHO

rick

 

[bobkrech]

Ok, not wanting to put water on the fire, but check out Loc's Viper IV. https://shop.locprecision.com/product.sc?productId=101&categoryId=13
It's a four engine cluster rocket (hey, why reinvent the wheel) that flys great on four D12-7's 1000 feet really isn't hard on it, either. My club (CATO) drag races them all the time - some people also fly D12-5's. Never seen them on a 3 second delay, though. It would go off while the rocket is still coasting up.

JMHO

rick

Rick

That's an excellent suggestion for spiff95.

The LOC Viper IV is a great 2.6" 4x24mm motor cluster rocket that weights 18 oz. (0.51 kg) and flys well on inexpensive BP motors. https://www.locprecision.com/uploads/IS-24_Viper_IV.pdf Some LOC sugggested motors are: (4) D12-5; (4) D21-7; (4) E9-6; (4) E30-10; (4) E15-7; or (4) F21-8. All except the (4) E9-6 are less than the 125 g. model rocket propellant limit, so no FAA waiver or high power certification is required, and @~$50 is very reasonably priced. It can also be flown with (2) E30-7 motors on smaller fields.

The LOC 4-29 SS is a scaled up 3.1" version of the Viper 4 with 4x29 mm motors in a 3.1" airframe weighing 29 oz. (0.82 kg) https://www.locprecision.com/uploads/IS-36_429ss.pdf This would be similar to the 4 x 29 mm clsuter rocket spiff95 was proposing with a significantly lower weight. Some LOC recommended motors are: *(2) E30-4, *(4) D12-5; *(4) E30-7; (2) F50-7; (4) F50-10; and (2) G80-10 [* requires 24 mm adapter] however (4) F50s exceed the 125 g model rocket propellant limit. It would also fly great on Roaderrunner and 1G and 2 G Pro29 motors as well. This is a very versitle rocket as it can be launched on either 2 or 4 engines as either a model rocket or on a big field as a L1 high power rocket, and @ ~$75 it's a steal.


Bob

 

[spiff95]

Well, it looks like I'll go with a single engine config on my scratch-built rocket and check out the LOC rockets. Thanks to one and all for the informative and thoughtful replies. I can see I've got a LOT to learn (especially concerning engines) and this seems like a good place to do it.

Thanks again...

 

[Peregrine Falcon]

Spiff, these guys have a LOT of good advice, I've spent hours just reading through different posts on this forum and learned a TON. I'm not that far away from where you are, just getting back into rocketry, scratch-building for the first time.

There's a link to this file in another thread on this forum but I'll re-post it here. This file was GOLD for me when I wanted to know exactly where my limits were as I wanted to stay under L1 certification limits for now.

 

[spiff95]

Spiff, these guys have a LOT of good advice, I've spent hours just reading through different posts on this forum and learned a TON. I'm not that far away from where you are, just getting back into rocketry, scratch-building for the first time.

There's a link to this file in another thread on this forum but I'll re-post it here. This file was GOLD for me when I wanted to know exactly where my limits were as I wanted to stay under L1 certification limits for now.

Thanks Peregrine, I appreciate the info!

 

[hardinlw]

Seeing Bob's comment about using 2 engines in a Viper4 triggered a thought. You absolutely have to plug the open motor mount tubes or the parachute won't eject. Also, with a cluster you have to fill the gaps between motor mount tubes so the ejection gasses that should pop the chute can't escape out the rear.

 

[cjl]

I haven't seen anyone mention it yet, but if you don't have any experience with clusters, I would HIGHLY recommend the Pro29 motors over the equivalent Aerotech ones. Aerotech makes nice motors, but the included igniter is a fairly standard igniter, requiring high current and not lighting instantly. CTI's Pro29 motors use a low current igniter that works much faster, so you have a much better chance of success if you use those (if you don't have much AP clustering experience).

 

[Peregrine Falcon]

I haven't seen anyone mention it yet, but if you don't have any experience with clusters, I would HIGHLY recommend the Pro29 motors over the equivalent Aerotech ones. Aerotech makes nice motors, but the included igniter is a fairly standard igniter, requiring high current and not lighting instantly. CTI's Pro29 motors use a low current igniter that works much faster, so you have a much better chance of success if you use those (if you don't have much AP clustering experience).

Thank you! I'm planning on attempting an AP cluster rocket next summer and was worried about such things. I was planning on using AT's 24mm hobyline cases for it but might consider going with Pro24's based on what you said above. I like the variety of motors I can get for the AT hobyline cases though. Would not using the stock igniters fix the issue you mentioned? What igniter would you recomend purchasing "aftermarket" for launching AP reolads?

I am going to get cluster experience with a 2x18mm black powder only rocket; build thread to start shortly after I finish off a few other loose ends related to nylon parachutes and my launching system.