Rocket coupling and staging

[Muchoman798]

Hello There!

I'm planning to build a small rocket based on the book "I still have all my fingers." It's a comprehensive guide to the basics of assembling a small rocket complete with an ingenious apogee detector, a parachute charge, complete reusability, and homemade fuel (the standard KNO3 and sugar). It also has an emphasis on safety; hence the title.

What I really want to figure out is creating a coupler after creating the initial rocket. My basic idea is to take the idea of The King of Random's parachute charge (here, timestamped and all) and modify it to blast the upper stage away from the lower stage on a short delay (my main parachute won't be deployed like his; his is unreliable). I do have two problems with this idea, however. I want to salvage the lower stage as well as the upper stage, so how can I strap on a parachute? I also am not sure what to make the coupler out of. I need something sturdy enough to go up during the flight, but weak enough to actually decouple.

The materials I will be working with can probably survive a fall of a few thousand feet one or two times, and I don't care about the motor casings being reusable.

Note: Everything I talk about here is "hypothetical" and I do "not" want to do it, as it might be dangerous and destructive. "No one" should attempt this based on anything written here. I speak as though I want to do this simply because it is easier to type that way

[K^2]

You can use helicopter recovery for your lower stage.

[B787_300]

Muchoman, So what works well for HPR is for there to be a timer in the upper stage of the rocket that fires and sends an electrical charge to an igniter (in the upper stage engine) and a black powder charge below the engine to separate the 1st and 2nd stage.

For a Low Power rocket (LPR), which is what you should start with, you can have the booster tumble back down to earth. BUT it has to fall slowly. There is no easy way to do what you want to do with out some sort of on board electronics.

If you want to discuss more, There is a Model Rocketry Resurrection thread that this should be in.

(Total Aside here... If you look up how to do this in the High Power Rocketry (HPR) Magazines (Which i recommend you doing) they will normally use the terms Booster and Sustainer. WHICH IS TOTALLY WRONG! By definition a sustainer is lit BEFORE the boosters/first stage drop off to Sustain the acceleration.)

[mardlamock]

Use nose blown recovery.

[Armchair Rocket Scientist]

Hello There!

I'm planning to build a small rocket based on the book "I still have all my fingers." It's a comprehensive guide to the basics of assembling a small rocket complete with an ingenious apogee detector, a parachute charge, complete reusability, and homemade fuel (the standard KNO3 and sugar). It also has an emphasis on safety; hence the title.

What I really want to figure out is creating a coupler after creating the initial rocket. My basic idea is to take the idea of The King of Random's parachute charge (here, timestamped and all) and modify it to blast the upper stage away from the lower stage on a short delay (my main parachute won't be deployed like his; his is unreliable). I do have two problems with this idea, however. I want to salvage the lower stage as well as the upper stage, so how can I strap on a parachute? I also am not sure what to make the coupler out of. I need something sturdy enough to go up during the flight, but weak enough to actually decouple.

The materials I will be working with can probably survive a fall of a few thousand feet one or two times, and I don't care about the motor casings being reusable.

Note: Everything I talk about here is "hypothetical" and I do "not" want to do it, as it might be dangerous and destructive. "No one" should attempt this based on anything written here. I speak as though I want to do this simply because it is easier to type that way

Okay, first of all look up the laws on doing this in your country, any subnational jurisdictions, and municipality; the US is pretty permissive with sugar rockets but you have a bunch of different regulations to think about, including explosives regulations, aviation regulations, and fire codes. Doing amateur rocketry "safely" includes not being thrown in prison.

Also, I don't have the book: what is your "ingenious apogee detector" and how does it work? It's possible that there are some better ideas.

Now, provided all this is legal in your jurisdiction:

If you are in the US, I would STRONGLY recommend buying some model rocket motors from Estes BEFORE you test-fly experimental motors. This will let you determine that you can successfully fly a rocket safely without the extra failure modes of your experimental motors. Second, do NOT try to fly a multi-stage rocket until you have succesfully flown a single-stage one, AND ground-tested your motors in the staged configuration to make sure your booster can actually ignite the sustainer.

A few other points:

My basic idea is to take the idea of The King of Random's parachute charge (here, timestamped and all) and modify it to blast the upper stage away from the lower stage on a short delay (my main parachute won't be deployed like his; his is unreliable). I do have two problems with this idea, however. I want to salvage the lower stage as well as the upper stage, so how can I strap on a parachute? I also am not sure what to make the coupler out of. I need something sturdy enough to go up during the flight, but weak enough to actually decouple.

There are two problems here. First, your black powder ejection charge on the booster stage will just blow your upper stage off without igniting it. Build your booster like the Estes booster engines: an endburning propellant grain without any end cap; at the end of the burn the remaining disc of propellant gets so thin it breaks, sending exhaust gases forward and up the nozzle of your second-stage engine. This will light it much more effectively than the brief explosion of a black powder charge. Also, for a rocket this small, a delay isn't very useful.

Second, the "delay grain" method is reliable enough that on rockets of the size you will be building nearly everyone uses it, and it works quite well.

Now, I'm assuming your airframes will be cardboard tubes; if they aren't, seriously rethink your materials selection.

If your rocket is not minimum diameter, i.e. there is an outer airframe tube, and an inner tube that holds the motor casing, separated by rings, then you will make a coupler by taking a short length of tube, cutting a small amount off lengthwise, putting it back together and gluing it into the top of your first stage. It should stick up into the back of the upper stage at least 1/2 the airframe diameter, and extend as far into the lower stage. Make sure the rear centering ring on the upper stage is far enough into the airframe that the coupler will fit. Alternately, if you're using standard commercial tube sizes, just buy a coupler from the manufacturer.

If your rocket is minimum diameter, i.e. the motor case fits right into the airframe, then have your second stage motor stick out the back of the airframe at least 1/2 a diameter, preferable 1 diameter. Make sure you leave room on the booster to insert it all the way.

In either case, the joint between stages will stand up to the first-stage motor's thrust, but because it's only held together by friction they can easily slide apart when the first stage motor burns out and the hot gases push the first stage backwards and the second stage forwards.

A couple other tips: for your first attempts, do not use more than 20 grams of sugar propellant per motor. And do NOT, even if it's legal in your jurisdiction, use more than 125 grams of propellant in a flying vehicle unless you really know what you're doing.

For lower stage recovery, the lower stage will be unstable without a sustainer on top, and will be fairly light. It will simply tumble back to the ground from a low altitude. The only likely source of damage is thermal damage from the upper stage igniting.

EDIT: I don't know if your book mentions this, but always test motors from behind a blast shield. If your propellant grain has large voids or cracks, it may burn much faster than you expected, overpressurizing the case and turning it into a pipe bomb. Assuming you're using a cardboard or plastic casing and under 20 g of propellant, most of your shield can be plywood. For parts you want to see through, use polycarbonate plastic, NOT glass or acrylic. Oh, and don't use a metal case; aluminum cases are for people who really know what they're doing, and steel cases are for people who REALLY know what they're doing, aka professionals. (here in the US, even the launches that let you fly 300 lb rockets 20 miles up in the air don't allow steel cases).

Edited July 19, 2015 by Armchair Rocket Scientist

[SAI Peregrinus]

The book "How to Design, Build, and Test Small Liquid-Fuel Rocket Engines" (Out of print, PDF here) has a good test stand design. Note the use of a mirror to see the test, so that you can be behind strong blast shielding.

[kzauner]

The book "How to Design, Build, and Test Small Liquid-Fuel Rocket Engines" has a good test stand design. Note the use of a mirror to see the test, so that you can be behind strong blast shielding.

Yep. And go with time, invest in the cheapest HD-Video-camera you can get your Hands on, which can record @120fps and providing optical zoom. And a tripod. Such a device can be used for other things too. And if in doubt, use an even bigger distance between you and the object tested, You don't need to see anything 'live', as you have it on tape, anyway (and: slow-motion imagery of test-anomalies is cool, anyway).

Why the cheapest cam: So you never, ever get the impulse to 'save the equipment'.

take care!