Ambitious Scratch-built Project: High-5

[DTH Rocket]

I'm a little concerned about this one. It's kind of ambitious for my experience level, hence I am seeking input. It's only my second high-power rocket, scratch built, and made without any special materials or precision tools, so I can only work with so much (and I'm only 16, so I can only work with so much cash, too). I named it High-5 because its a five-motor cluster that goes high.

The electronics for staging are MiniTimer from Apogee Components. I've never used it before, but I still have at least a year before I'm ready to fly this rocket anyway, and I'm sure that on its first flight I'll use a single I-motor, possibly a J-motor.

I posted a few build pictures awhile back, but now all that has been lost, so here is the rocket in its current state. The lower portion has been fiberglassed, but the payload section and the upper section has not been glassed yet. A lot of the internal structures are in place, but the payload bay is completely empty (not the staging electronics bay). I've never even used my mini RRC altimeter for deploying parachutes, so I barely have an idea on how to put this payload bay together. It's an odd shape, the part in between the 3-inch section and the 4-inch section. I'll post more about this rocket another day.

I am currently 16 years old, and am hoping to be Junior L1 certified sometime next summer on a simpler rocket.

 

[DTH Rocket]

Aw, I guess you can't upload RockSim design files anymore.

 

[BsSmith]

I am currently 16 years old, and am hoping to be Junior L1 certified sometime next summer on a simpler rocket.

Why not certify on this? How many people can say they certifies JR L1 on a 5 motor cluster?

 

[DTH Rocket]

Why not certify on this? How many people can say they certifies JR L1 on a 5 motor cluster?

I'm a chicken. Sorta. Something in me tells me to start small.

 

[MarkM]

Something in me tells me to start small.

No, you're smart...not chicken

I see a potential issue with mounting your RRC in the transition (or just below, too). Ebay vent holes placed in transitions and the tube sections just below it are subject to turbulent airflow around them. This results in potential "odd" ebay pressurization issues. A barometric altimeter, which the RRC is, could very likely be confused by this resulting in an anomolous recovery deployment at apogee -- anything from too early, too late, or maybe not at all.

By convention, barometric altimeters should have their pressurization holes about 3 BT diameters away from the nearest nosecone or transition. At this distance, the turbulent airflow around the transition/nosecone has disipated.

You have 2 choices, IMO.
1) Be sure your vent holes are placed ABOVE the transition, not on it or below it. By placing the vent holes above the transition, you're away from the turbulent airflow caused by the transition. Just be sure the holes are 3 BT diameters from the nosecone.
2) Use an accelerometer-based altimeter; they are not affected by turbulent airflow since they deploy apogee recovery by detecting a decrease in acceleration.

Interesting build....

 

[DTH Rocket]

I see a potential issue with mounting your RRC in the transition (or just below, too).

No, the barometric altimeter and vent holes will be just above the transition. The payload bay includes the transition and the segment just above it. I read about this problem in Modern High-Power Rocketry 2. Thanks for the input. It's more advice like that that I need. I don't have an expert on hand, so I'm left to go-it-alone usually, which isn't always safe.

 

[MarkM]

No, the barometric altimeter and vent holes will be just above the transition.

Then you should be fine. It wasn't clear in your original post. MHPR 2 is a great book. Lots of useful info.

EDIT....just noticed the new photo. You only have one vent hole? That's typically not a wise design. If there happens to be unusual airflow or the airflow is disturbed around that hole during flight you'll have a problem. You should use 3 or 4 smaller holes which have the total equivalent area of that one hole. I use 3 in all my rockets using altimeters.

 

[DTH Rocket]

You only have one vent hole? That's typically not a wise design. If there happens to be unusual airflow or the airflow is disturbed around that hole during flight you'll have a problem. You should use 3 or 4 smaller holes which have the total equivalent area of that one hole. I use 3 in all my rockets using altimeters.

I haven't put any vent holes in yet, but I just looked it up in MHPR2. One suggestion is to have a 1/4 inch hole for every 100 cubic inches of payload bay area. I calculated it out, and it turns out I have 500 cubic inches in my bay!

A=Bh = 7(pi*3^2)+6(pi*4^2) (I have two diameters in the payload bay) = 499.5 inches^3.

So by that formula I should have 5 1/4 inch vent holes. Does that sound about right?

 

[Indiana]

Aw, I guess you can't upload RockSim design files anymore.

You can if you zip them with winzip first.

 

[MarkM]

So by that formula I should have 5 1/4 inch vent holes. Does that sound about right?

No...way, way too big, especially for 5 of them. On 3" airframes I use 3 7/16" vent holes for an altimeter bay 7" long.

Take a look HERE for a great table and formulas. Five vent holes is total overkill. Use 3 or at most 4.

One of the main reasons your calcs are wrong is you used the diameter of the payload bay to calculate area. Volume of a cylinder is pi*radius^2. I'm a bit confused on why your altimeter bay is 13" long. You also need to confine your altimeter to a smaller volume. You should have bulkheads separating the altimeter from the parachute bays. Even at your long ebay your volume is ~ 130 cu in.

 

[DTH Rocket]

No...way, way too big, especially for 5 of them. On 3" airframes I use 3 7/16" vent holes.

Take a look HERE for a great table and formulas. Five vent holes is total overkill. Use 3 or at most 4.

Did you mean 3/16? Or 7/32? Because 7/16 is almost half an inch.

 

[MarkM]

Did you mean 3/16? Or 7/32? Because 7/16 is almost half an inch.

Sorry, my bad. Neither. It should be 7/64.

For your ebay as it is now, I'd use 3 holes which are 5/32" each. Drill them into the 4" section.
If you want to use 4 go with 9/64" holes.
Either way, be sure they are spaced equally around the tube.

 

[BAR0051]

That is very ambitious and I would never use such a complex rocket for a cert flight. I was more nervious about my first dual deplolyment flight than I was for my level 2 cert. I did a few dual deployment flights and was comfortable with it before I did my level 2 cert flight. I would not dream of using electronics and motor clusters for a cert flight unless I had done it several times before and was sure of sucess.

 

[Handeman]

I think there would be less confusion and better advice if you also included some pics with the rocket apart so we can see how the Alt-bay and payload sections are related.

There is some considerations with the staging electronics. Here I'm making an assumption. You are going to use staging electronics to light the outer motors and the rocket will lift off on only the central motor. I assume this is the way you will configure this rocket because that is usually the safest method.

Using that method, there are two things that MUST happen for a safe flight.

1. The central motor MUST be large enough to fly the rocket safely by itself. This is in case of electronic, battery, igniter, or any other type of failure that could occur to the air staging/ignition system after the central motor lifts the rocket off the pad.

2. The central motor MUST provide enough thrust to generate enough Gs to trip the G-switch on the staging electronics. Having the best electronics, igniters, etc, will do you no good if the staging electronics are never activated and start its timing because it never sensed launch. I watched two cluster rockets crash for this very reason. The first got lucky, the chute deployed and just as it was hitting the ground, the main chute snapped open, triggered the G-switch and the outer motors fired, sending the rocket back up in the air dragging the main chute. It ended up only braking a fin. The second was a large rocket that only got 1 - 2 hundred feet up without the outer motors and came crashing down. No chutes opened, but there were lots of pieces.

My advise would be look at the specs on the G-switch on your staging electronics. It should list a min. and max G range for the switch. Below the min. is where the switch will NEVER switch, above the max. is where it will ALWAYS switch, and in between is where it MIGHT switch. You want your central motor to provide enough Gs to be a safe margin above the max. value. If the switch is rated 3G to 6G and you want a 50% safety margin, your central motor should generate at least 50% more then the 6Gs, or at least 9Gs at launch.

Don't ever calculate the safe liftoff/fight of a cluster rocket on all the motors lighting.

 

[Handeman]

That is very ambitious and I would never use such a complex rocket for a cert flight. I was more nervious about my first dual deplolyment flight than I was for my level 2 cert. I did a few dual deployment flights and was comfortable with it before I did my level 2 cert flight. I would not dream of using electronics and motor clusters for a cert flight unless I had done it several times before and was sure of sucess.

This is where we differ. I see the electronics and clusters as a challenge and would personally welcome that for a cert flight. My cert was a mailing tube rocket with home made nosecone and chutes, with DD and a long burn I motor that got it to 4,200 feet. I wasn't "sure" it was going to work, but I was confident I had done everything to make it work. What didn't worry me at all was if I was going to get the cert or not. If I didn't, then I just repair/build a rocket and try again.

That is why I'm looking at an L2 cert flight on a L or large K, something that will bust Mach. I've never done that before and its all about he challenge for me. Everyone has to approach it in a way that makes the hobby fun and exciting for them. Just stay safe.

 

[MarkM]

That is very ambitious and I would never use such a complex rocket for a cert flight.

By his initial post, it seems as this is a rocket for AFTER he certifies.

In terms of clustering, don't forget that at Cert Level 1 you are limited to 640 Ns total impulse.

 

[BAR0051]

By his initial post, it seems as this is a rocket for AFTER he certifies.

In terms of clustering, don't forget that at Cert Level 1 you are limited to 640 Ns total impulse.

My bad, it was BsSmith that suggested using it to cert. Just my $.02 worth.

 

[MarkM]

My bad, it was BsSmith that suggested using it to cert. Just my $.02 worth.

For what's worth, I do agree with you in terms of keeping certs simple, especially the L1. Many don't feel that way, but to each his own.

 

[DTH Rocket]

Hey Handeman, you were right in your assumptions. The full potential of this rocket is a central K and 4 outboard G's or maybe even H's. The K motor in that case would definitely lift the rocket on its own, as I am anticipating the max. weight to be about 10 pounds fully loaded. On its first flight, however, I'm not going to fly it complex--just a central motor. Before I launch this rocket with the outboard motors I want to 1) test the staging electronics on a smaller multistage rocket, and 2) get my Junior L1 on a single I-motor in the rocket.

 

[Handeman]

Sounds like a plan. The central K will definately do the job. Sounds like a central I is about as small as you want to go.

Good Luck