Flying Falcon Heavy

[cscott]

I decided to kit bash three of the SpaceX flying Falcon 9 kits together into a flying Falcon Heavy.

I'm using clear ring fins for the side boosters.  Each of the three rockets should fly stably on its own (just in case of premature separation).  I got the ring fin idea from the Odd'l Corkscrew (http://www.apogeerockets.com/Rocket_Kits/Skill_Level_1_Kits/Corkscrew_Powered_Flics_Combo_Pack).

The center stage is a two stage rocket, with a small C first stage allowing the side boosters to do most of the work, and a D second stage allowing a decent length flight after the side boosters separate.

I'm filling in a photo album of construction pics here: https://plus.google.com/photos/109541946294746531763/albums/6062272024521824609.

[cscott]

Here are (mostly) all the parts for the rocket laid out.  Starts with 3 stock flying Falcon 9 kits from SpaceX.
Plus a C->D engine mount adapter kit and four sets of "glider pod hooks" to hold the boosters on.  Also, two nose cones from the Estes "NC-60A" four-pack.  Not pictured: 1" ID 1-3/16" OD phenolic tube, 3" ID clear polycarbonate tubing, and a set of ejection baffles from apogee.

[cscott]

The Estes nose cone wasn't really the right shape for the Falcon Heavy side boosters, so I added a layer of Instamorph moldable plastic around it and put it on the drill press+dremel tool hack-a-lathe.

[cscott]

I modified the engine mount in the center booster to make it into a two stage rocket.  The second stage shoots down a phenolic tube so it doesn't fry the lower half of the rocket to a crisp.  Some holes in the tube prevent the Krushnic effect.  The first stage motor mount is recovered with a mylar streamer.

Accessible set screws ensure the second stage motor doesn't eject itself rather than the parachute.

[cscott]

Clear ring fins for the side boosters.

[cscott]

Here's where I'm at now.  The left side booster and center booster are more or less complete.  Still working on the right side booster, then I need to mount the glider pod hooks to hold everything together.

A construction article in Apogee's newletter (http://www.apogeerockets.com/education/downloads/Newsletter260.pdf) has a much more robust hook system to hold on the boosters.  I'm a little worried the little glider pod hooks might jar loose at lift-off.

[Ronpur50]

Wow, this is pretty amazing!  Boosters that will separate!  I can wait to see this fly!

[NovaSilisko]

Beautiful... I really need to get back into model rocketry, but it's almost winter time.

[Geron]

When and where are you going to fly it? Can we come watch!?

[cscott]

Finished cutting the ring fin for the right hand booster, hopefully I've have that all epoxied together tonight.

I started looking at how my center of gravity is turning out.  The center booster is going to require a bit more weight in the nose.  I experimented a little with putting a pair of 808 keychain cameras in the tip of the nose to capture the downward view, including booster separation.  That would provide some neat video and also help my CG at the same time.  I ordered a pair of 10mm "slightly defective" glass right-angle prisms from ebay to make the layout work; when they arrive I'll see if I can make that all work.  Otherwise I'll just stuff some clay up there...

When and where are you going to fly it? Can we come watch!?

Well, I have to get it finished first!  But I live in the Boston area, so probably I'll fly it somewhere around there.  I don't have specific plans, yet -- I generally try to arrange launches around the schedules of the small children in my social circle, who enjoy watching things go up in the air as much as I do.

[mvpel]

If I get my SM3 airworthy in time we can fly them together - I work in Woburn.

[cscott]

Finished cutting the ring fin for the right hand booster, hopefully I've have that all epoxied together tonight.

A few pictures of the right ring fin.

I'm working on the booster attachment hooks; I'll post pictures of those soon.

[cscott]

Here's a picture of the "glider pod hooks" which I'm using to attach the boosters to the center core.  I added a split tube to the outside of the hook pair -- this ensures that the hook stays together until the rocket leaves the launch rail.  This design depends on the side boosters carrying the rocket for the initial part of the flight, and then the center thrust increasing to detach the boosters.  The plan is to put D motors in the boosters and a C (or B?) motor in the center for the "first stage" flight.  The second stage of the center core will be a D motor, which will ignite after the first stage burnout to carry the center core away from the side boosters.  That's the plan at least!

[cscott]

Here are some details of the hooks mounted on the body tubes, and of the whole thing put together.  I still need to epoxy on the fin cans and add the legs.

[cscott]

Legs!

[cscott]

The center stage needs some nose weight to gets its center of gravity far enough forward to be stable, what with the extra second-stage motor in the back and the heavy phenolic tube.

My plan is to mount an 808 keychain camera and a prism up at the top of the fairing to capture some nice footage of the flight (and booster separation!) and balance the model at the same time.

Here's the idea:

[ravedave]

How do you get all three main engines to fire at once? Almost need a hold down or something.

[clongton]

Beautiful... I really need to get back into model rocketry, but it's almost winter time.

Build season.

[kch]

How do you get all three main engines to fire at once? Almost need a hold down or something.

You'll want a hold-down of some kind on the core (so you don't lift off on just the boosters) or some kind of backup 'chute-deployment method ... "thwok!" is not a particularly pleasant sound (been there, heard that).

[cscott]

How do you get all three main engines to fire at once? Almost need a hold down or something.

I've got a three part plan (belt & suspenders & ... something else):

a) each part (side boosters and center) are stable flyers by themselves, and have a launch rail to use, so if they prematurely separate i'll hopefully get three simultaneous successful rocket flights, not a crazy conjoined pinwheel of disaster.

b) the launch lugs hold the boosters together until the flight leaves the rail.  This ought to give 36" for the thrusts to equalize, if things don't light off exactly right.

c) I bumped up to a 24V ignition power source -- an electric lawnmower battery.  In my tests this seems sufficient for a *very* prompt ignition.

The engine on the center core is a the weakest that will allow it to safely launch itself (as per part a) while the booster engines are Es.  This also helps ensure that the center core doesn't race ahead and detach from the boosters prematurely.  (If we let T=the power of the D engine which powers the stock configuration, the overall initial thrust is thus between 4T and 5T, for an assembly which weighs 3x as much.)

This is the theory at least!  There's lot of experience launching cluster model rockets which suggests that uneven ignition shouldn't be an issue, given sufficient igniter power.   There are other tricks which are sometimes played, such as angling the thrust of the outer cores so that their thrust vector points toward the combined rocket's CG, minimizing the torque effects of thrust imbalance.  I didn't do any of those things, because I didn't want to compromise goal (a) (the belt).

We'll see how it works when I launch it.  Lots of things that could go wrong!