Another Falcon Heavy...

[KilroySmith]

So, it's been in the back (and sometimes the front) of my mind for awhile to build a clustered, staged Falcon Heavy. Just read wrad's writeup here for his Soyuz that has a very similar build approach to what I was considering - the core-to-booster hooks even look nearly identical to the ones I fabricated a while back.

At the basics, I'm planning on MPR - an E12 in the core, a pair of D12's for the strap-on boosters (so they separate while the core is still burning), and a D12 engine in the sustainer. This means I don't have to deal with getting three composite motor rockets lit simultaneously, then airstarting a fourth - I've got enough complications in this rocket. The boosters will be attached to the core with hooks that engage while the booster motors are burning, and then slide free under aero drag when the booster motors burn out.

I've acquired sufficient BT-70 tubing (55mm) for the core, sustainer, and boosters, and 3" (76mm) tubing for the payload bay. The boosters/core will be about 2 feet (60 cm) long, and the overall rocket about 3 feet (100 cm). The boosters and sustainer will deploy chutes using the motor ejection charge.

The core is interesting. I could try and do 50 cm gap staging from the core motor to the sustainer motor, but I'm dubious about the likelihood of success - I've read a number of success stories on staging over this big of a gap, but I'm still dubious. Alternatively, I could pull the Missile Works PET2+ out of my other project and use it to stage, whilst allowing the core motor to eject the chute (or letting the PET2 do both). Hmmm.

Rather than angle the boosters through the CG to deal with asymmetric thrust in the case an engine doesn't light, I'm considering using a hold-down to keep the rocket on the pad until all three engines light. After considering a number of over-complicated designs, I'm now thinking of a switch behind a blast deflector under each engine on the launch pad - once all three engines are producing thrust and there's continuity through all three switches, I'll fire one of my remaining, precious fast-acting Q2G2 igniters to blast the hold-down open and release the rocket. With a good battery, this should happen within 10's of milliseconds of the three motors igniting so it shouldn't significantly impact altitude or flight time.

Anyone with thoughts on a hold-down mechanism, please feel free to share. I only need to hold down the center core (the boosters can't leave without it), but I need to withstand three E12's worth of thrust (about 100N / 33Lb peak). I'm thinking of a device with two pivoted arms; each arm has a smooth protrusion that engages the rocket through a hole just above the rear motor mount. The two arms are perhaps 6" (15 cm) long and held together by a pivot at the opposite end from the protrusion. In the middle of, and between the arms is a cylinder with two pistons to push the arms apart. In operation, a very small amount of BP would be in the cylinder, and upon ignition would provide sufficient force to separate the arms, pulling the protrusion out of the rocket and sending it on it's way.

For the sustainer, I'm considering pop-out fins that deploy at separation. That should keep the looks closer to a Falcon Heavy. Saw this thread; maybe I'll build pop-out fins for all four stages so there'll be no visible fins at all on the launch pad! Anyone seen anything closer to the scale I'm building at here?

Anyway, that's kinda where I'm at. Let me know what you think...

 

[BDB]

I’m interested. Good luck!

 

[BABAR]

50 cm gap is easy, especially with big nozzle sustainers like D or A motors. Need to place the vents just below the sustainer nozzle to route the hot gas to the right place.

How about a piece of elastic string attached to rocket and pad just beneath each motor? When motor lights, burns the string. If doesn’t light, doesn’t burn the string and rocket stays tethered.

Make sure you have a well secured pad

 

[Charles_McG]

So the boosters can't leave without the core, but can the core leave without the boosters? (What is the different between drag and differential thrust?) Worst case is core leaves without one booster, and the straggler lights late.

I had a BT-60 size Titan IIIe suffer a CATO of a C11 in a side booster during boost and it flew remarkably straight on the remaining two motors.

 

[cerving]

...Worst case is core leaves without one booster, and the straggler lights late...

I can confirm that this is a very spectacular, albeit short, "flight". I had a 3 x F24 do exactly that, although the two outboards were airstarted right after launch instead of being clustered. One lit immediately, but the other took it sweet time coming up to pressure... and lit right before the remaining stack hit the ground after making a very nice 180 degree arc, land sharking about 200'. Fortunately, it was well away from the flight line.

 

[KilroySmith]

Babar -
Interesting idea. Three strings, each tied to it's own spot on the core tube. I'll have to think about that - I'm worried about the amount of "slop" in the system, possibly allowing the three body tubes to separate unexpectedly. I only have about 3/8" of "hook" holding the side tubes to the core.

Charles - the side body tubes are intended to have a one-way hook to the center core. As long as they're under thrust, they're hooked. So, the center core could leave without one or both side tubes, but neither side tube could leave without the center core. Thus, I only need to hold down the center.

cerving -
I was initially going to build this with composite motors, but wanted to avoid two things:
1. The uncertainties involved in igniting them (as your experience shows)
2. The staging events occurring out of eyesight.

 

[Charles_McG]

Charles - the side body tubes are intended to have a one-way hook to the center core. As long as they're under thrust, they're hooked. So, the center core could leave without one or both side tubes, but neither side tube could leave without the center core. Thus, I only need to hold down the center.

So one booster could have a delayed ignition where the stack leaves it behind, it tips over, and then lights.

I built my Titan IIIe so that the stack would lift the boosters if they didn't light. Hooked both directions, as it were. I used C11-0 boosters (with a little extra loose BP on top of the grain) to separate the boosters while the core (E9 - never got around to trying the F10) was still burning. Looks awesome. Hard on the boosters, since it's a high speed deployment. At least it tended to snap the shock cord or strip the shroud lines rather than zipper the boosters. And they're just BT60, once the nose cone was off, they didn't really -need- a chute to come down gently.

So when I did my 1/100 Space Shuttle I tried a different approach. Thrust hook at the bottom, and a hollow brass pipe at the top of the booster with piano wire springs on the boosters. I ran a length of fishing line with end stops through the core with the end stops inside the boosters, tightened to hold the boosters on against the springs. I have an altimeter fire an ematch to cut the tether, and the springs push the boosters off the posts so drag can sweep them away. A8-3s in the boosters for effect and -3s so they have a chance to slow down before chute deployment.

 

[Charles_McG]

Shuttle example test

 

[wrad]

Got to check in as I got a mention, In many ways your project is similar to Joe Barnards falcon heavy with staging etc and hold down. I like the idea of the switches to avoid launch if one fails to light just like Joes last flight. Would you just wire 3 momentary switches in series with a battery and igniter or do something more complex?

 

[BABAR]

The boosters will be attached to the core with hooks that engage while the booster motors are burning, and then slide free under aero drag when the booster motors burn out.

...

Let's introduce Murphy's law.

Core booster ignites, boosters don't. Sounds like with your attachment scheme the Core and Sustainer will immediately depart the premises, leaving the boosters behind. Is the Core Booster/Sustainer stack WITHOUT the boosters stable on your core motor?

Murphy 2

Core booster ignites immediately, one or both boosters ignite late. With your attachment scheme, will Core and Sustainer depart without the boosters, leaving the orphaned boosters lately lit and unguided?

Murphy 3
Booster or Boosters ignite, Core doesn't. Is the booster's or boosters' thrust sufficient to get rocket off the rail (the hope is the answer is NO, rocket slides up a foot or 2 on the rail and slides back down. Reason the hope is NO is because if you are using BP staging, the sustainer ain't gonna light. Also, if booster(s) get it off the rail, will it be stable? Hopefully not, because I would rather have a low altitude unstable rocket that everybody has eyes on than one getting up a hundred feet or so, possibly lost to site, then coming down pointy end first, since if sustainer doesn't light, the Core Booster/Sustainer stack [or sustainer alone, if core booster drag separates] is coming in ballistic .).

Also, if you use black powder staging Core to Sustainer, how do deploy your Core Booster chute and at what velocity is the booster going when it deploys?