Boosted darts

[Anonymous]

Another NCR member and I are getting ready to start on a boosted dart project. We are considering either a 75mm or 98mm diameter booster in combination with either a 29mm or 38mm dart. Does anyone have any printed or online resources regarding darts? Has anyone in the club ever lofted one?

I believe we have figured out the interstage coupler, which should be the tricky part. Darts are famous for folding during the boost, but I’m confident we have that issue solved.

There is a lot of math associated with the drag separation, etc. My hunch is that if we go w/ the 98mm booster, we’d be best off going with the 38mm dart. This is due to the fact that the booster must transfer the energy to the dart at burnout, and a 4″ diameter booster will probably be too much mass to drag separate cleanly and quickly for a 29mm dart.

Early work shows the dart will have to weigh perhaps 7-9 lbs., depending of course on the motor. That much mass also steers us towards a 38mm dart… which of course, costs dearly in altitude vs. 29mm.

Thoughts?

JW

[Ed Dawson]

Way cool, John! I want to see this one fly.

A boosted dart has been on my list for a while. The problem is that I have too many other projects that are ahead on the priority list.

Just curious about you comment about the mass of a large booster affecting the seperation. Wouldn’t it be more of a question of drag? Isn’t point of separation is when the force of drag on the booster is greater than the mass times acceleration of the dart?

I’m sure there is some really cool math here and you could figure it out with fancy computers and calculus – but wouldn’t it be more fun to just build and fly a few rockets. You know, like trial and error. Or, just like in college calc, multiple choice.

[Ed Dawson]

BTW, A boosted dart contest would be way cool, too.

[Warren B. MusselmanModerator]

Bill Heath and I are also in the design stage for a boosted dart. In our case we’re looking at a 38mm dart and a 3″ booster. In fact, we’re building a 100% scale Loki Dart. (As a matter of fact, the original flew with 9000 nSec of total impulse in a 1.8 second burn and regularly reached altitudes in excess of 230,000′) In that case the dart weighed approximately 15# (weight varied due to different instrumentation – the earliest flew with just a radar chaff dispenser)

I have a number of websites and resources you may be interested in John.

Warren

[Anonymous]

Just curious about you comment about the mass of a large booster affecting the seperation. Wouldn’t it be more of a question of drag? Isn’t point of separation is when the force of drag on the booster is greater than the mass times acceleration of the dart?

Drag is surely a big chunk of the equation, but you also need mass to play its part. The perfect scenario is a light booster with big draggy fins and a slender, heavy dart.

Just as a piece of paper that is crumpled up and tossed will not go far, the same sized piece of paper with a golf ball inside (for added mass) will go much farther. Same principle with the dart — you want the lighter, draggier booster to get pulled away from the more massive, yet more slippery dart.

This will be very cool… I HOPE 😮

This will fly this summer. My thought is that I will use my SSSS bird for the dart. I’ll have to add ~6# of lead, though. YIKES!

[Anonymous]

Bill Heath and I are also in the design stage for a boosted dart.

Warren, when will you guys be ready? I don’t think we will be in the air much b4 July or August. Elvis Jr. is building the booster, and he is in school for a while….

[Warren B. MusselmanModerator]

I don’t see our bird flying until ’08 as we still have some issues to deal with such as what will we fly it on. The loki dart also requires a special helical launcher that spins the bird up to 60rpm or so during the launch as the fins aren’t quite big enough for stability without spin stabilization. Also, I want to fly it on a 3″ L or M motor, but we have to figure out how to keep the altitude down.

Warren

By the way, I’m thinking of turning my dart out of stainless steel pipe. with a solid stainless nosecone.

[Ed Dawson]

John,

I understand the analogy of the optimum weight of the combined golf ball and crumpled paper. Even in this case the ball will go further if it sheds it’s paper shroud.

But it seems like your talking about “gravity separation” instead of “drag separation”. Just like dropping a hammer and a feather on the surface of the moon, wouldn’t drag be the deciding factor?

Could you RockSim it by putting a 1/4A in the dart and then runing the optimal mass function?

[Chris LaPanse]

Bill Heath and I are also in the design stage for a boosted dart. In our case we’re looking at a 38mm dart and a 3″ booster. In fact, we’re building a 100% scale Loki Dart. (As a matter of fact, the original flew with 9000 nSec of total impulse in a 1.8 second burn and regularly reached altitudes in excess of 230,000′) In that case the dart weighed approximately 15# (weight varied due to different instrumentation – the earliest flew with just a radar chaff dispenser)

I have a number of websites and resources you may be interested in John.

Warren

I believe it actually was a bit more impulse than that. Looking at the specs here, it made 9.0 kN of thrust for 1.9 seconds, which is 17100 Ns of impulse. So, it used almost a full N motor. Considering that its ceiling was 180 thousand feet and it reached around mach 5.1, that’s quite a bit more speed and altitude per Ns than any current hobby N record though, and if you make a decent replica, you should be able to push the BALLS waiver on a big M or small N.

Note that for a rocket to be classified as an “amateur rocket”, it must have a ballistic coefficient that is lower than 12 lbs/ square inch, limiting a 38mm dart to 21 lbs and a 29mm dart to 12.27 pounds. You can still do a lot within this limitation though.

[Warren B. MusselmanModerator]

I was actually thinking about machining the dart body out of thin-wall stainless and using a solid nose. I have enough blueprints to actually duplicate the coupling transition if I can get the machinist where I work to turn his hand to it. Between the airframe, transition piece and nose, the dart will weigh in about 7#. Optimal weight is around 18-20# from the stuff I’ve read about the Loki.

I want to build a very lightweight booster of 3″ carbon aerosleeve with maybe a 3oz sanding veil. No phenolic inside. I have a deployment mechanism that involves logically ANDing the apogee outputs of an altimeter with a timer output and a microswitch that closes when the dart separates. With a machined aluminum transition section at the front and CF plate fins with say 3 layers of tip-2-tip carbon I can probably keep the weight below 5# not including the motor casing. The most important thing in drag separation is that the booster not only have a greater frontal area than the dart, it must have much less relative mass in order for the air drag on the booster to reliably separate them.

Are you sure about those figures? I’m using Morrow & Pines “Small Sounding Rockets” book and they say a total of 9,000 nSec of impulse. What you say makes more sense to me though.

Warren

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