SLOW boost cluster?

[DJ Delorie]

An idea I've been toying with... anyone try to design a rocket with the SLOWEST boost velocity? I'm thinking a cluster of A10s to get up to a minimum safe speed, then an E9 (or F10) to sustain the speed for the remainder of the boost. The rocket's mass would have to just balance the E9's thrust, and you'd need an overstable model since it would be flying on the low end of the safety margin. I figure it would be a nearly 2 lb model. Another option is the (obsolete?) D3, or some other low-thrust engine.

 

[adrian]

I haven't intentionally designed anything like that, but one or two of my heavier scale models ended up that way.

The normal rule of thumb is that a rocket should have a thrust/weight ratio of 5/1. The minimum you should even consider is 3/1. Which means, if you're using an E9 with a nominal average thrust of 9N, the rocket should weigh no more than about 0.3 kg, or about 10-11 oz. Adding a few A10's will give the rocket some extra power right when it needs it, as it takes off from the pad. Ignite the whole lot together, do not ignite the A10's first and air-start the E9.

My GSLV does something similar, but with a C6-3 rather than an E9. It's a bit too heavy for the C6-3 alone but flies nicely on a C6-3 and four A10-PT's, even if it won't exactly break any height or speed records!

One other thing. You're planning a rocket which will be overweight, underpowered and overstable. Given half the chance, it will weathercock like crazy. Only launch it in zero wind conditions.

 

[vjp]

What Adrian said sounds right on the mark. Especially about igniting the A10's and E9 together. In my Vostok, the A10's kick the model off the pad quickly, and though the center D12-3 lights simultaneously, you can see in the launch video (follow my URL) that the D12-3 doesn't come up to full thrust until about 20 feet up, by which time the A10's are just about done their job.

Another thing, short models have a greater tendancy towards wobbling off course under low thrust than longer models do. Keep it long (and don't oversize the fins) and this should keep the flight straighter, which is always desireable but especially when you have low thrust and a long burn.

Nice work on that GSLV there Adrian.

 

[lalligood]

Alternatively, have you considered a high drag design like one of Art Applewhite's saucers, Qubits, Cones, or Hourglass rockets? They are very light but stay low because of the large vertical surface area producing lots of drag...

HTH,

 

[wwattles]

Yeah, I just saw that Art has posted on his "free stuff" page a pattern for building a 3-stage saucer...

WW

 

[DJ Delorie]

Alternatively, have you considered a high drag design

Well, my solar sailers tend to be higher-than-usual drag, and are heavy for the engine size, resulting in "majestic" boosts. I was just pondering the ramifications of designing towards the slow majestic boosts, rather than the zip-it-vanished boost. At least, in standard 10:1 rocket shapes.

This all started back at the cub scout launch, where they all had Flis Overdrives on A10s, which basically teleported themselves to altitude. Not much to watch that way, I thought A3's would be more "fun". The Rickter Recker is more fun on E9's than D12's as you can track it easier. Etc.

 

[jflis]

I fly my Pheord X150 with one of the mini saucers from the Triple Threat as a 2-stage all the time. Great flights to about 80 feet

Here she is on the pad: https://fliskits.com/photo_album/whitakers_2004/images/P6171445.JPG

and in the air: https://fliskits.com/photo_album/whitakers_2004/images/P6171446.JPG

 

[Karl]

I dont mean to hijack the thread but Jim...... Is it ok to use a 1/2 A motor with a ejection in the Tripple Threat sacuers?
Karl

 

[MetMan]

Originally posted by adrian
My GSLV does something similar, but with a C6-3 rather than an E9.

Yikes, Adrian! I hope you keep a fire extinguisher close by! Looks like a lot of dry brush there...

 

[jflis]

Originally posted by Karl
I dont mean to hijack the thread but Jim...... Is it ok to use a 1/2 A motor with a ejection in the Tripple Threat sacuers?
Karl

Depends on the ejection and I would have to see it fly. Trick is, you would want the ejection charge to go off while the model was still very high so as to prevent the charge from starting a grass fire...

...now, back to our regularly scheduled thread

 

[DynaSoar]

Originally posted by DJ Delorie
An idea I've been toying with... anyone try to design a rocket with the SLOWEST boost velocity? I'm thinking a cluster of A10s to get up to a minimum safe speed, then an E9 (or F10) to sustain the speed for the remainder of the boost. The rocket's mass would have to just balance the E9's thrust, and you'd need an overstable model since it would be flying on the low end of the safety margin. I figure it would be a nearly 2 lb model. Another option is the (obsolete?) D3, or some other low-thrust engine.

You're poking the 5:1 ratio rule right in the snout. I like that. As long as you get it up to stablility as it comes off the rod, you really don't have to keep it accelerating much at all.

I think I'd try something like a extra long stretched Fat Boy or something else with a lot of length AND width, with a Apogee E6 sustainer (a 6 second burn E), with a pair of C6-0 outboard boosters to give it an initial kick.

 

[adrian]

Originally posted by MetMan
Yikes, Adrian! I hope you keep a fire extinguisher close by! Looks like a lot of dry brush there...

That was UKRA 2004, where considerably larger and more powerful rockets than mine were launched from the same field without setting light to it! But yes, I believe there was water available just in case.

Originally posted by jflis
Depends on the ejection and I would have to see it fly. Trick is, you would want the ejection charge to go off while the model was still very high so as to prevent the charge from starting a grass fire...

Judging by the official page, "very high" is not a suitable description for any Triple Threat flight. The A10-3T is one of the recommended motors. A 1/2A3-2T would deliver much lower thrust, but would keep pushing the saucer up for about 0.4 second compared to the A10-3T's 0.25 second. Do the Triple Threat saucers coast much after the burnout of an A10? If not, the 1/2A3-2T might actually be safer than the A10-3T, especially given the shorter delay charge. And, as it provides lower thrust for longer, it would give the model a longer, slower boost, thereby bringing us back on topic.

 

[jflis]

love how you came full circle on that one...

 

[Karl]

Originally posted by adrian
That was UKRA 2004, where considerably larger and more powerful rockets than mine were launched from the same field without setting light to it! But yes, I believe there was water available just in case.

https://www.macs.hw.ac.uk/~adrian/ukra2004/satmod/77turbo.jpg
Yep I do remember water being there, fortunately no-one had to use it
Karl

 

[lalligood]

Originally posted by adrian
Do the Triple Threat saucers coast much after the burnout of an A10? If not, the 1/2A3-2T might actually be safer than the A10-3T, especially given the shorter delay charge. And, as it provides lower thrust for longer, it would give the model a longer, slower boost, thereby bringing us back on topic.

High drag models (such as saucers, Qubits, cones, tetrahedrons, spools, etc.) have almost ZERO coast after burnout. It's not uncommon for large HPR high drag models to even experience apogee *before* motor burnout. Long burn low thrust motors will pretty much always provide greater altitude than short burn high thrust motors (assuming equal total thrust) for high drag designs.

HTH,

 

[jflis]

Ditto what lalligood said