I've suddenly taken a vicious interest in model rocketry.

[Starwhip]

Over the summer, I have a new goal: Build a custom model rocket.

I found three old (well, not literally old, but from a few years ago) C6-5 engines in my closet today. Did some research, and figured out a good general design. I'll keep it a secret for now.

Anyway, one thing I find annoying is that the EXACT model I have (Of rocket engine, that is) (A060705) has had a few rather bad reviews of either not firing the ejection charge, or not meeting thrust requirements.

But how much do you have to fail in supplying thrust when you have an engine TWR of 25? I think it should be fine.

Here's my logic:

Engine average thrust = 6N

1N = 0.225 Lbs

6N = 1.35 Lbs

Therefore, the absolute maximum rocket weight is around 0.614 Lbs (for a TWR of 2.2)

But with only 1.4 seconds of burn time that's not going anywhere.

So what should the TWR be? I can't find it anywhere online.

Other things:

I don't want it to slam into the ground like every single rocket kit I've ever built. I'll design a nice parachute that WON'T not-deploy and break the rocket. I'm thinking ribbon/tiny 'chute that pulls the whole thing out to the right configuration.

I want a good altitude as well. At LEAST 1,000 feet.

And if not altitude, having it just WORK would also be nice. As in it goes up, coasts, pops out the parachute and comes back down in one piece. Would be best.

So yeah, tips and such would be nice, but mainly I need to know the TWR of the rocket. Because 2.2 is not going to work.

[xenomorph555]

Iv'e also taken a large interest recently, although i'm building everything including the engines and fuel, so far it has gone very well.

As for your question on TWR, it may be okay since those engine can be pretty good, although I don't use lbs for measurement because i'm british so don't take my word on it.

As for parachutes I can not help you, finding a system that works has been the bane of my existence although as I said I create the engines (with charges), you should have it easier with the estes you have.

Good luck

EDIT: oh how big is the rocket?

Also you should install an altimeter, that way you can check how high you went.

[BagelRabbit]

Okay, I'm decent at model rockets (HPR L1 certified, and I've flown a GoPro and Jeb to over 1600 feet,) so:

When you're building and flying model rockets, you ideally want a 5:1 TWR. That is to say, the rocket should weigh less than half of your prediction. You're right, a 0.6 pound rocket on a C6-5 would hit the ground long before it deployed its ejection charge. When the charge goes off on the ground, it's kinda funny, but then again you want a successfully-returned rocket.

If you want the rocket to reach over 1,000 feet, though, you need to have a very lightweight rocket indeed. The aerodynamic, relatively sleek Estes Alpha III reaches about 1100 feet, and it weighs only 1.2 ounces wothout the motor.

On to recovery!

A "pilot" streamer or parachute will not help deployment, unless you have your main parachute wrapped inside of some sort of deployment bag. If you're just planning on placing some recovery wadding (remember the recovery wadding!!) in the tube, a regular, standard parachute will work.

TIPS FOR PARACHUTES!

Be sure to put in enough recovery wadding to protect the tube. For the size of rocket you are talking about, this runs from 3-5 sheets. Don't pack in the wadding too tightly.

After you're satisfied with the wadding, gently wrap the parachute. My preferred method is to fold the parachute into eighths (in half three times), then to lay the 'shroud lines' on top of the parachute, fold the top down, roll and then gently wrap excess shroud lines around the parachute. (Sorry if this isn't too easy to understand. It made more sense in my head.) Make sure that the parachute will slide out easily.

Put the nose cone on. If anything is too tight, don't force it. The most times I had to repack a parachute was five times, but a great deployment made the entire effort worth it.

Do you have any other questions? I would be more than happy to answer them.

[EDIT: About your "top-secret" rocket design:]

That seems nice 'n' stable, as long as you place the fins at the bottom and the nose cone at the top. I wouldn't recommend doing anything particularly special until you've gotten the hang of rocket building.

(aka: no fin area close to the nose to make the rocket unstable, which means ABSOLUTELY NO TRIPLANES!)

Edited June 6, 2014 by UpsilonAerospace

[Starwhip]

Like I said, the design is secret:

I have no idea what I'm doing yet, really, but I know enough about engineering and real-life aerodynamics to make a good shape. So... something like 15 inches long, with 3 or 4 fins: Curved, about 3 inches (root length), 5 inches long (downward from topmost root) plus 2.5 inches wide (outward), sanded into an aircraft wing-like profile for gyro-stabilization.

Totally secret, I know

Anyway, for the TWR all I really need is some kind of measurement. Any unit is fine. I'll convert it. It would be a smallish rocket, so I'd expect a TWR of around 10 or 15.

The thrust of the engine is 612.4 g, or as I said before, 6 Newtons.

Wow, super-ninja'd

[TechnicalK3rbal]

Well, if you're using an A-motor, you probably don't even need a parachute. I made one that uses C's, and it lands without a chute or a streamer.

[Starwhip]

Specs on the specific C6-5 I have:

Weight: 24.7 Grams

Burn time: 1.4 Seconds

Derived Specs:

1.4 Seconds for a 6 Newton thrust = 8.4 Newton-Seconds of Total Impulse

6 Newton Thrust = 1.35 Lbs or 612.4 Grams

Again, I don't know exactly what I'm doing, but testing will solve whatever issues pop up. But word of caution for myself: Mistakes cost money, and there is no "Revert to VAB" button in real life.

Get it right the first time.

Measure twice, cut once.

...

Unlike our robotics team, in which we consistently cut twice before measuring once!

I should fix that next year.

[BagelRabbit]

Well, if you're using an A-motor, you probably don't even need a parachute. I made one that uses C's, and it lands without a chute or a streamer.

Please read what Starwhip has said before posting, this comment has absolutely no relevance, as it is talking about the wrong motor and a (lack of) recovery system that Starwhip has said that he would not use.

Also, breaking the NAR safety code (rule 10) is punishable with a fine.

[Starwhip]

Technically, recovery systems don't need parachutes/streamers.

According to Wikipedia (The source of ALL KNOWLEDGE!!!)

Model rocket recovery methods

Model and high-power rockets are designed to be safely recovered and flown repeatedly. The most common recovery methods are parachute and streamer. The parachute is usually blown out by the engine's ejection charge, which pops off the nose cone. The parachute is attached to the nose cone, making it pull the parachute out and make a soft landing.

Featherweight recovery

The simplest approach, which is appropriate only for the tiniest of rockets, is to let the rocket flutter back to earth after ejecting the motor. This is slightly different from tumble recovery, which relies on some system to destabilize the rocket to prevent it from entering a ballistic trajectory on its way back to earth.

Tumble recovery

Another simple approach appropriate for small rockets  or rockets with a large cross-sectional area  is to have the rocket tumble back to earth. Any rocket that will enter a stable, ballistic trajectory as it falls is not safe to use with tumble recovery. To prevent this, some such rockets use the ejection charge to slide the engine to the rear of the rocket, moving the center of mass behind the center of pressure and thus making the rocket unstable.

Nose-blow recovery

Another very simple recovery technique, used in very early models in the 1950s and occasionally in modern examples, is nose-blow recovery. This is where the ejection charge of the motor ejects the nose cone of the rocket (usually attached by a shock cord made of rubber, Kevlar string or another type of cord) from the body tube, destroying the rocket's aerodynamic profile, causing highly increased drag, and reducing the rocket's airspeed to a safe rate for landing. Nose-blow recovery is generally only suitable for very light rockets.

Parachute/Streamer

A typical problem with parachute recovery.

The parachute/streamer approach is used most often in small model rockets, but can be used with larger rocket models given the size of the parachute greatly increases with the size of the rocket. It uses the ejective force of the motor to deploy, or push out, the parachute or streamer. The parachute is attached to the body either directly, by means of a ripcord, or indirectly, when it's attached to the nose cone, which attached to the body by a ripcord. Typically, a ball or mass of fireproof paper or material is inserted into the body before the parachute or streamer. This allows the ejection charge to propel the fire-proof material, parachute, and nose cone without damaging the recovery equipment. Air resistance slows the rocket's fall, ending in a smooth, controlled and gentle landing.

Glide recovery

In glide recovery, the ejection charge either deploys an airfoil (wing) or separates a glider from the motor. If properly trimmed, the rocket/glider will enter a spiral glide and return safely. In some cases, radio-controlled rocket gliders are flown back to the earth by a pilot in much the way as R/C model airplanes are flown.

Some rockets (typically long thin rockets) are the proper proportions to safely glide to Earth tail-first. These are termed 'backsliders'.

Helicopter recovery

The ejection charge, through one of several methods, deploys helicopter-style blades and the rocket autorotates back to earth. The helicopter recovery usually happens when the engine's recoil creates pressure, making the nose cone pop out. There are rubber bands connected to the nosecone and three or more blades. The rubber bands pull the blades out and they provide enough drag to soften the landing. In some rockets, the fins are used as the blades as well. In these, the ejection charge pushes a tube inside that has tabs sticking out of the rocket that hold the fins during launch. Then the tab releases the rubber band-pulled fins than pivot up into helicopter position.

[Camacha]

Also, breaking the NAR safety code (rule 10) is punishable with a fine.

That all rather depends on the country you are in.

[Starwhip]

More quick calculations:

Assuming a 5:1 TWR, Total Rocket Mass must be a maximum of 122.48 grams.

Engine weight: 24.7 grams.

Thus, the assembled rocket (without motor) must weigh under 97.78 grams.

EDIT:

Parachute design tips. What the heck do I make it out of?

I think that a parachute diameter of about 10 or 12 inches should bring it down safely. Maybe a little too safely. Meh. I'll do some testing (AKA dropping it out of windows).

Edited June 6, 2014 by Starwhip

[Redjoker]

I would think that a trash bag or some other kind of plastic sheeting would work fine and this looks like it should help you design your parachute http://www.apogeerockets.com/downloads/Newsletter149.pdf

In short, they recommend a diameter of 19 inches for a 100g rocket.

Edited June 6, 2014 by Redjoker

[BagelRabbit]

...EDIT:

Parachute design tips. What the heck do I make it out of?

I think that a parachute diameter of about 10 or 12 inches should bring it down safely. Maybe a little too safely. Meh. I'll do some testing (AKA dropping it out of windows).

10-12 inch parachute should be fine.

I would recommend that, more than any other part of the rocket (except for the body tube, maybe), you buy a genuine Estes parachute. They're nice because they're made of quality materials, and will withstand many launches (my most-flown parachute has more than 30 launches on it). You might make one out of plastic bags, but they generally don't work as well.

As for the NAR safety code, I have never seen a rocket that "returns safely and undamaged and can be flown again" (direct quote!) without a parachute on a C motor. There's a first time for everything, but I've seen at least five B- or C-motor powered rockets auger into the ground in a very unsafe way that is generally frowned upon. It's possible to safely return a rocket to the ground without a parachute, but I wouldn't do it my first time... or my second time... or my third time...

[TechnicalK3rbal]

Also, breaking the NAR safety code (rule 10) is punishable with a fine.

There is a paracute, however, it remains unused, and the rocket returns unharmed and can be used again.

[DerpenWolf]

I'm actually looking into making a two stage model rocket that uses a Arduino micro with an accelerometer for staging, I'm probably going to Need to check the local laws first though.

[TechnicalK3rbal]

I'm actually looking into making a two stage model rocket that uses a Arduino micro with an accelerometer for staging, I'm probably going to Need to check the local laws first though.

You actually don't need that for staging, you can use the ejection charge of the previous stage to ignite the next one.

[BagelRabbit]

You actually don't need that for staging, you can use the ejection charge of the previous stage to ignite the next one.

True, unless DerpenWolf staging composite model rocket motors (which I assume he's not.)

Here, have a good newsletter on how to stage model rockets!

http://www.apogeerockets.com/Tech/How_2-Stage_Rockets_Work

[Armchair Rocket Scientist]

All right. First of all, I strongly recommend downloading OpenRocket. It's an open source simulation program for model rockets, and it allows you to verify that your rocket will actually be stable, etc. Unstable rockets are VERY dangerous.

As for TWR, you generally need to be moving at least 13 m/s at the end of the launch rod/rail for the rocket to be stable. With a 'C' engine, you will most likely use a launch rod about 1 m long. This means that your rocket must accelerate at 85 m/s^2. Add 10 m/s for the force of gravity, plus a safety factor, and your rocket needs a minimum TWR at launch of 10. If your motor produced constant thrust, your rocket would have to weigh less than 61 grams. Fortunately, real rocket motors do not produce constant thrust. According to this: http://www.thrustcurve.org/motorsearch.jsp?id=21 the C6 has a "spike" of much higher thrust at the start of its burn, with a maximum thrust of 14.1 N. With a real thrust curve, simple math won't allow you to predict how heavy the motor can safely lift. However, my simulations show that the maximum safe liftoff mass for a rocket with a C6 motor is about 100 grams. With a low-drag rocket, an ejection delay of 5 seconds is appropriate for this mass. However, higher-drag designs may require you to use C6-3 motors instead. Again, get OpenRocket and simulate your design before you build anything.

[Starwhip]

I tried out OpenRocket, and it seems as if the rocket I had designed would work.

Here would be the specs:

Length: 24 cm

Max Diameter: 2.54 cm

Mass with motors: 58.1 g

Apogee: 425 m

Max Velocity: 122 m/s (Jeez, Mach 0.36)

And it's stable. So it would work.

I'll keep experimenting. I'll see how that one works, then maybe build a more powerful one for a higher altitude. Perhaps the expression MOAR BOOSTERZ works here too!

(As in a larger or more powerful engine.)

[Skyler4856]

This is a tad bit off topic, but I am designing a rocket sled.

[Armchair Rocket Scientist]

I tried out OpenRocket, and it seems as if the rocket I had designed would work.

Here would be the specs:

Length: 24 cm

Max Diameter: 2.54 cm

Mass with motors: 58.1 g

Apogee: 425 m

Max Velocity: 122 m/s (Jeez, Mach 0.36)

And it's stable. So it would work.

I'll keep experimenting. I'll see how that one works, then maybe build a more powerful one for a higher altitude. Perhaps the expression MOAR BOOSTERZ works here too!

(As in a larger or more powerful engine.)

Those stats sound about right: they're very similar to my Hatchet, which is the same diameter but a bit longer.

A word of caution: unless you have very good eyesight, you will not be able to see a rocket that size at that altitude. To avoid losing the rocket, make sure that you keep your descent relatively fast (minimizes wind drift and time spent at out-of-sight altitude). For a rocket that size, you can probably land safely with a streamer instead of a parachute. The streamer should also be either highly reflective or a bright color such as fluorescent orange, for maximum visibility both in the sky and on the ground. For the vast majority of rockets, the deployed parachute or streamer will have a much larger cross-sectional area than any other component.

In addition, for larger (and more expensive) rockets you can take other measures to making losing the rocket less likely. For example, personal keychain alarms are compact enough to fit in many low-power or mid-power rockets, but loud enough to be heard from over 100 feet away. While rockets often land farther away than that, if a rocket lands in tall grass being able to locate it by sound is very useful if you know the general area. Larger rockets may disperse colored powder or smoke for better visibility in the air, and be equipped with radio transmitters or even GPS modules.