Super noob: How long should my rocket be?

imminenthippo · May 9, 2015

I'm building an egg lofter for an altitude competition, this is my first time. I'm trying out OpenRocket to model the rocket but it's been bothering me whether or not it matters how long my rocket is. What exactly does the length of the rocket affect? Whenever I shorten the rocket, OpenRocket just keeps telling me the apogee will be higher and higher. How short is too short? How long would be good?

Also, i'm not sure how large the inner diameter of my body tube should be. I want to easily fit an 18" parachute, but at the smallest possible diameter so the rocket is skinny as possible. My current diameter is 2.7cm, which I just picked arbritrarily. We're only allowed a B6-4 motor, which has an 18mm diameter

Last question how do i delete a thread?

samb · May 9, 2015

https://exploration.grc.nasa.gov/education/rocket/rktstab.html

Welcome to the forum ! The length of the rocket has the biggest affect on the balance point (center of gravity, Cg) of the airframe. The Cg must be ahead (towards the nose) of the center of aerodynamic pressure (Cp) for a rocket to be stable. The Cp is the point where the aerodynamic forces like lift and drag balance out. OpenRocket calculates both values; the Cg will only be accurate if you add the total flight-ready weight including egg, recovery device, and motor. The Cp is determined by the size and shape of the body and the size, shape, and location of the fins.

Based on your design parameters you will need to build light; a B6 is not a big motor to push an egg. I think a 24mm diameter body is needed to easily fit an 18 inch parachute. How are you planning to hold the egg ? Pictures of any components you already have would be great along with your .ork file. Good Luck.

Some of mine:

left to right: Quest Courier, Estes Eggscaliber, two versions of the Gassaway 2-minute Egg

Zeus-cat · May 9, 2015

Are you using a tower to launch the egglofter rocket? I just competed in my first egg lofting competition and we used towers. I did OK on flight #1, but broke my egg on #2 because the parachute failed.

I used a tapered design for my rocket. 18mm at the base so I could fit a C motor, but at least 30mm at the top so the rocket could have a plastic Easter egg on top. The egg went inside the Easter egg and was wrapped in bubble wrap. The egg holding part of your rocket will likely determine the diameter of your rocket.

imminenthippo · May 9, 2015

View attachment eggrocket3.ork this is my ork file right now, I'm not 100% on the measurements yet but this is the general idea. The fins, i don't really know if they're any good or not, how would you consider shaping them? I'm planning on using balsa wood for some parts possibly the transitions and/or payload. What do you think about that material for those parts? Or should i just go cardboard everything?

JStarStar · May 10, 2015

The tapered conical design Zeus-Cat has in the other thread might be the way for you to go.

18 inch chutes can be fit in an 18mm tube but it is a tight tight fit. As SamB says 24mm is probably a better pick if you don't go for a conical cardstock body.

Another possiblity for you is something like the Custom Rockets Elite.

This will give you about the best altitude performance possilble out of a B6-4.

The problem here is that the package comes down awfully fast on a 12-inch chute (about the limit for an 18mm tube). If you pack the egg well in the egg capsule you may survive, but maybe not.

One way to sidestep that problem is to build the rocket with separate recovery systems -- a 6-8" streamer for the rocket body and a 12-inch chute for the egg capsule.

The flameproof streamer subs for some extent for wadding; I put just a cotton-ball size pinch of wadding in above the motor, then pack in the streamer, then the chute wrapped in tight at the top right at the base of the egg capsule.

Using only a little pinch of wadding, when the ejection charge fires the streamer may get barbecued a bit but that usually doesn't cause major problems with recovery. A rocket that size can usually recover just fine with nose-blow recovery.

When the 12" chute has to provide air braking for the egg capsule plus the rocket body with the heavy burnt-out motor casing in the base, that's a lot of weight. The whole thing tends to drop like an anchor.

When you cut the rocket body loose and give it its own streamer, all the 12" chute has to do is bring the egg capsule down. That's much less of a challenge.

The problem here of course is you now need separate recovery crews for the rocket body and the egg capsule. But any egg payload on a B6-4 is probably not going out of sight anyway; you should be able to keep an eye on both pieces and they probably won't be drifting for miles.

scsager · May 10, 2015

imminenthippo said:
View attachment 262796 this is my ork file right now, I'm not 100% on the measurements yet but this is the general idea. The fins, i don't really know if they're any good or not, how would you consider shaping them? I'm planning on using balsa wood for some parts possibly the transitions and/or payload. What do you think about that material for those parts? Or should i just go cardboard everything?

Your design in OR looks nearly identical to the Estes "Scrambler" egg lofter, and at 119 grams - it's too heavy if you are serious about an egg lofting altitude competition using a single B6-4 motor.

Better / Alternate plans like this >>> https://www.nar.org/wp-content/uploads/2014/05/TwoMinuteEgg-B-C-EL-Gassaway.pdf

Are found on this page >>> https://www.nar.org/contest-flying/competition-guide/altitude-events/eggloft/

lcorinth · May 10, 2015

samb said:
https://exploration.grc.nasa.gov/education/rocket/rktstab.html

Welcome to the forum ! The length of the rocket has the biggest affect on the balance point (center of gravity, Cg) of the airframe. The Cg must be ahead (towards the nose) of the center of aerodynamic pressure (Cp) for a rocket to be stable. The Cp is the point where the aerodynamic forces like lift and drag balance out. OpenRocket calculates both values; the Cg will only be accurate if you add the total flight-ready weight including egg, recovery device, and motor. The Cp is determined by the size and shape of the body and the size, shape, and location of the fins.

Based on your design parameters you will need to build light; a B6 is not a big motor to push an egg. I think a 24mm diameter body is needed to easily fit an 18 inch parachute. How are you planning to hold the egg ? Pictures of any components you already have would be great along with your .ork file. Good Luck.

Some of mine:

View attachment 262788

left to right: Quest Courier, Estes Eggscaliber, two versions of the Gassaway 2-minute Egg

That looks like my parents' dining room table and chair.

... Brother??

JStarStar · May 10, 2015

I'm sure this has probably come up as an issue at some point in eggloft duration: what are the timing standards when the rocket separates into two pieces as I suggested in the post about the CRC Elite above? Is your time determined when the first piece hits the ground, or when the egg capsule touches down?

I ought to know this because I've flown some eggloft competitions but I've never used a 'separating recovery' type design so I guess I just never happened to think about it.

Zeus-cat · May 10, 2015

That’s a good question JStarStar; he may not be able to bring it down in two pieces. I think he needs to go back to the contest organizers and start asking some questions.

- can the rocket come down in two pieces?
- are they using towers to launch the rockets? If the answer is yes, then you don’t need a launch lug. This greatly simplifies the design of an egglofter.
- what are the rules for the competition?

samb · May 10, 2015

lcorinth said:
...

... Brother??

Brother rocketeer ! :grin:

samb · May 10, 2015

Yeah your design will result in rocket that will struggle of the pad and certainly not get much altitude on a B motor. Building an egg capsule from scratch using balsa and cardboard is an ambitious project for anyone IMO. I would start with a hollow plastic easter egg a little bigger than your egg to allow for packing material. The full tapered two minute egg design is a good one or just a straight 24mm cardboard body tube. 18mm motor mount with two cardboard centering rings. Search for "Stine clipped delta" for a good all-purpose fin pattern.

imminenthippo · May 10, 2015

scsager said:
Your design in OR looks nearly identical to the Estes "Scrambler" egg lofter, and at 119 grams - it's too heavy if you are serious about an egg lofting altitude competition using a single B6-4 motor.

Better / Alternate plans like this >>> https://www.nar.org/wp-content/uploads/2014/05/TwoMinuteEgg-B-C-EL-Gassaway.pdf

Are found on this page >>> https://www.nar.org/contest-flying/competition-guide/altitude-events/eggloft/

Thanks, I actually have no experience so I'm gonna need a lot of help. These look pretty good.

Micromeister · May 10, 2015

imminenthippo said:
Thanks, I actually have no experience so I'm gonna need a lot of help. These look pretty good.

B motor Egglofting is Iffy at best. Your Egg must be not less then 57g and not more the 63g with a diameter of not more the 45mm.

The model can be staged but who'd want to with B impluse? To have any chance of deployment the best motor selection would be B6-2 not 4. The model can be as short as you like a good 18mm Body lenght without the egg capsule would be about 10-12inches. Remember you want to build as LIGHT weight as possilbe for B-eggloft altitude.
One more very important thing to keep in mind, Our recovery chute will need to be made with over the canopy should lines and should NOT be 1/4mil mylar material. .010" (1/4mil) Drycleaner plastic bag is a good alternate used with either 13lb Kevlar or heavy carpet thread shroudlines taped over the top with silver trim monokote tape strips.

18mm body tubes will EASILY handle up to 24" diameter 1/4mil dry cleaner bag chutes with the extra bulk of over-the-Canopy shrouds in a 12" long tube.
I strongly suggest using one of Pratt Hobbies vacuum formed styrene Egg capsules over the Old Estes or Apogee capsules as Pratt hobbies cones are about 1/3 the mass.
Most competitors fly Eggloft from either Metal head zero volumn Pistons or Floating head pistons to Eek out every Ns impulse out of the motor.

Can't stress enough YOU MUST BUILD super light for B-Eggloft events.
Most Important is to be EGGstremely careful loading and packing our egg and chute...or the Yoke will be on you!

imminenthippo · May 10, 2015

So I should use a B6-2? That's fine, any B6-# is allowed. I like the two-minute egg model i got from scsager https://www.nar.org/wp-content/uploads/2014/05/TwoMinuteEgg-B-C-EL-Gassaway.pdf

I wonder if thick cardstock is enough, would this thin sheet of wood veneer hold up as a body tube https://i.imgur.com/g8pzvTY.jpg? I feel like my cardstock is too flimsy, lol. I can't buy any professionally made egg capsules, the whole thing is basically scratch built for this competition except the motor and mount.

Micromeister · May 10, 2015

imminenthippo said:
So I should use a B6-2? That's fine, any B6-# is allowed. I like the two-minute egg model i got from scsager https://www.nar.org/wp-content/uploads/2014/05/TwoMinuteEgg-B-C-EL-Gassaway.pdf

I wonder if thick cardstock is enough, would this thin sheet of wood veneer hold up as a body tube https://i.imgur.com/g8pzvTY.jpg? I feel like my cardstock is too flimsy, lol. I can't buy any professionally made egg capsules, the whole thing is basically scratch built for this competition except the motor and mount.

Cardstock body are MORE then enough for a competition egg-lofter! No they will not least forever but they are more then sufficent for 3-6flights. The B-Liner in the pervious post had 3 flights on it before I took that picture. There is nothing except the rolled cardstock tapered body between the 13mm motor mount and egg-capsule shoulder. They work PERFECTLY Fine. Remember Build as LIGHT as possible.

samb · May 10, 2015

Micromeister said:
...

18mm body tubes will EASILY handle up to 24" diameter 1/4mil dry cleaner bag chutes with the extra bulk of over-the-Canopy shrouds in a 12" long tube.
...

I've seen you and others make this claim and I certainly have no reason to doubt you BUT that skill in chute folding is something I have yet to acquire . A 12 inch chute in a 3/4 inch tube is enough of a challenge.

Imminenthippo, please forget the balsa veneer idea. My two minute egg models are made with photo quality printer paper stiffened with thin CA (super glue).

.

samb · May 10, 2015

NAR and international competitors use thin plastic egg capsules (like hollow plastic Easter eggs) that conform very closely to the size of the payload, leaving some room at the top and bottom for a little foam cushion. Fabricating this part from scratch using cardboard and balsa or plastic presents quite a challenge, especially for first time builders/flyers. The resultant part may increase the weight beyond the safe limits of a B motor to lift. Perhaps you can ask the contest organizers for a ruling on this point.

Micromeister · May 10, 2015

samb said:
I've seen you and others make this claim and I certainly have no reason to doubt you BUT that skill in chute folding is something I have yet to acquire . A 12 inch chute in a 3/4 inch tube is enough of a challenge.
.

LOL! Yes I know what your saying

but to be honest when flying serious competition I and many other NOT BTC's fly 30" and 36" 1/4mil mylar chutes in 18mm (BT-20) size PD models. 13mm BT-5 get 18" chutes as the norm 22" & 24" on max rounds, in fact before the motors went Oop several of the guys at NARHAMS were flying 10.5mm (T4) .448" Diameter models with 18" and 22" 1/4mil mylar chutes.

It is truely a skill that takes a little time to learn and practice but believe me any Egglofter MUST have at very least a 20" chute to ensure a soft touch down

OBTW: don't try using the usual Estes Plastic chutes with tape discs or worse holes punched in the canopy with tied on shrouds, all they'll do is become and instant Streamer or 6 string streamers.

imminenthippo · May 10, 2015

alright. I'll use photo paper.

imminenthippo · May 10, 2015

samb said:
I've seen you and others make this claim and I certainly have no reason to doubt you BUT that skill in chute folding is something I have yet to acquire . A 12 inch chute in a 3/4 inch tube is enough of a challenge.

Imminenthippo, please forget the balsa veneer idea. My two minute egg models are made with photo quality printer paper stiffened with thin CA (super glue).

.

alright. I'll use photo paper.

PhlAsh · May 11, 2015

Micromeister said:
Most Important is to be EGGstremely careful loading and packing our egg and chute...or the Yoke will be on you!

And be sure to post a photo albumen when you're done!

JStarStar · May 11, 2015

Zeus-cat said:
That’s a good question JStarStar; he may not be able to bring it down in two pieces. I think he needs to go back to the contest organizers and start asking some questions.

- can the rocket come down in two pieces?
- are they using towers to launch the rockets? If the answer is yes, then you don’t need a launch lug. This greatly simplifies the design of an egglofter.
- what are the rules for the competition?

https://nar.org/pdf/pinkbook.pdf

Under Rule 15.2 of the 2014 Pink Book, Timing Data-Separation: Unless specifically allowed by the rules of that event, no timed entry may separate into two or more unattached parts, or eject its motor.

So that takes care of the separation idea for NAR events, but if you're just egglofting for fun, you can do whatever you want.

Micromeister · May 11, 2015

JStarStar said:
https://nar.org/pdf/pinkbook.pdf

Under Rule 15.2 of the 2014 Pink Book, Timing Data-Separation: Unless specifically allowed by the rules of that event, no timed entry may separate into two or more unattached parts, or eject its motor.

Well since the OP stated in the very first post that he is entering a Altitude competition. Rule 15.2 does not apply. Since staged models are allowed I'd bet that seperating the payload and launch vehicle body might also be allowed. While I've never flown any Egg-loft event that way is not to say it can't be done.
For B Egg-Loft it might even be a good idea using a small streamer on the body and as large a chute as possible on the Egg-capsule.

bobkrech · May 11, 2015

imminenthippo said:
So I should use a B6-2? That's fine, any B6-# is allowed. I like the two-minute egg model i got from scsager https://www.nar.org/wp-content/uploads/2014/05/TwoMinuteEgg-B-C-EL-Gassaway.pdf

I wonder if thick cardstock is enough, would this thin sheet of wood veneer hold up as a body tube https://i.imgur.com/g8pzvTY.jpg? I feel like my cardstock is too flimsy, lol. I can't buy any professionally made egg capsules, the whole thing is basically scratch built for this competition except the motor and mount.

Actually it's overkill. I made one with two #20 weight Xerox paper sleeves. I flew C egg loft duration and used a C11. Used a 1 meter square chute with fishing line for shroud lines. Planned for and hit 120 seconds, winning by a large margin. No tower or piston either, just a 1/4" rod. First and only time I flew competition.

Bob

Super noob: How long should my rocket be?

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