ANNOUNCING NARAM-46

Calvinball.

Jerry Irvine wrote in news:01rocket- snipped-for-privacy@news.verizon.net:

What if they were very big mm's?

len.

How much did the original Grrr weigh? Wasn't it in excess of 12 pounds.

Remind me again what motors you used to try to fly Grrr. What is their MRLOW? What is their 5:1 thrust to weight ratio?

IIRC you used a cluster of 2 H motors. For comparison, an I154 (425 ns) at

5:1 can lift not quite 4 pounds. An I284 would lift just over 11 pounds. I don't thnk you had enough thrust to get the thing moving even if it had been a 4" diameter cylinder.

Grrr was more limited by its weight than its drag. Given enough power to overcome its weight, it would have been VERY drag limited.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

For drag limited rockets you want 3:1 or less. I have discussed here for a decade now a range of thrust to weight of 3:1 to 5:1 and misled posters have been repeating it as 5:1 minimum.

5:1 is FAST.

Jerry

Jerry Irvine wrote in news: snipped-for-privacy@news.verizon.net:

5:1 assumes a 3' launch guide. Longer launch guides make lower liftoff thrust to weight ratios acceptable.

In another post, Bob sez Grrr wasn't drag limited. I'm inclined to agree, even if it had gotten successfully airborne, its flight speed would not be such that drag would be the primary factor in its ballistics.

OK, I have to go lie down now, after agreeing with both Irvine and Kaplow in a single post.

len.

Calvinball.

Well, I would have to agree. A 2.25 lb 18" Saucer on a J330 is very fast, very loud and oh-so cool :)

I think I need to call Ken.

Patrick - Bob may not like it...but I will... ;)

Besides, by all accounts, the Grrr needs to get closer to horizontal and climb by making aerodynamic lift (i.e., more of a "rocket powered airplane" than a "vertical ballistic rocket"), at which point thrust/weight doesn't optimize the same way - one might want just enough thrust to keep the airspeed up, for as long as possible...

-dave w

You are fixated on me again George. Go take your meds. You need them badly.

Jerry

You need drugs. Or a friendship with George Gassaway :)

Jerry

Let's not call it that, but draggy rockets do tend to benefit more from low thrust motors than low drag rockets.

Gravity and inertia are still in effect. All rockets coast like a normal rocket and do not STOP in their tracks at motor burn out. A higher power, higher thrust rocket will still burn out at a higher velocity and coast a longer distance.

Optimally, you want a sustainer thrust that is twice the weight, regardless of drag. So, a Snitch should be fine on a D3 sustainer, if you can keep the flight pointed straight up.

That is only true if you fly most of you rockets using high thrust short burn motors. It is not true if most of your rocketry is with near optimal thrust, e.g. E R/C R/G on an E6.

No, they will coast exactly as dictated by the ballistic coefficient and the burnout velocity.

What needs to be understood is Optimal Thrust and how it applies to saucers. It is known as the Goddard Problem, and the solution can be quite interesting for real high performance rockets, but it is reasonably simple for most sport rockets. The problem is, with fixed total impulse, or fixed fuel mass with nearly constant Isp, what thrust applied over time will maximize apogee altitude? In our case the solution is an impulse thrust that accelerates the rocket to the optimal sustainer speed, followed by a sustainer thrust that keeps us at the optimal sustainer speed, followed by a zero thrust coast to apogee. our optimal sustainer speed is nearly constant and the optimal sustainer thrust is about twice weight regardless of the CdA, or rather for all values of CdA.

This makes determination of optimal thrust almost trivial. Optimal sustainer thrust is twice weight. Optimal velocity can them be found by solving, weight = 0.5*rho*CdA*V*V for V. The optimal impulse that must be expended to reach the optimal velocity is simply m*V.

If your total impulse limit is low and drag is low, V will never be reached, and there is no optimal sustainer arc in the solution. You want the highest thrust shortest burn time motor that you can get. One might even call these rockets low powered or under powered.

If your CdA is very large, V will be small, the optimal impulse will be small, and the sustainer arc longer. When both V and mass m are small, the required optimal impulse can be quite small, perhaps as little as is built into some long burn motors. Consequently, saucer type rockets perform best on long burn motors with thrust about twice weight and a small initial thrust spike. But...

More typical sport rockets are slender low drag, moderately heavy with ample total impulse. They have both a low thrust optimal sustainer arc and a substantial initial impulse requirement. It is not possible to design solid rocket motors to perform this way and even if you did (see the Astrobee D) it would only be optimal for rockets with a particular mass and CdA. Optimal thrust in most cases is best achieved with stages or clusters, and a low thrust sustainer. Even then we are limited to available motors and often have to resort to things like delayed staging.

Many of our competition events limit us to a single motor, and all we can do is pick one over the other. Experienced competitors know that low thrust motors are good. However, as they build lower drag models, with higher V, and their optimal motor has a shorter burn time. Long burn "contest" motors offer a more dramatic boost to novice competitors who build higher drag rockets.

Now flying with near optimal thrust on a typical slender rocket is good. It gets off the pad fast and establishes its momentum in the vertical direction. Disturbances will not have a great effect on the flight path direction, even with a long burn sustainer, although some roll is desirable.

A saucer however, has very little momentum, less stability, and is more easily disturbed so that the sustainer thrust can more easily change the flight path direction. While, a low thrust saucer may thrust through apogee, it does not meant that the thrust is too low, only that the burn time is too long for a slow moving saucer.

And please understand that it makes little difference if the rocket is a saucer, an R/C B/G (which includes R/G), or an altitude model, you need to consider the mass, CdA, and the implications for optimal thrust and motor selection.

Alan

Alan

I don't recall the Grrr airfoil being optimized for lift. Or drag or just about anything else. It was a slab of house foam insulation.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

All that $#!+ we had to listen to last year about bad motors, old motors and the whole works and he did not even airfoil the wings? C'mon Bob, is this true?

Patrick

Well, perhaps, but I did fly a 6lb liftoff wt glider on an old I154 back at LDRS XV. This was one of the old 70g slug motors instead of the newer 62.5ers. It took off quite nicely, although youd still want a WL motor in a big glider for the effects.

Interesting youd bring that up. I flew two gliders on I284s, both were 11 or

12lbs liftoff IIRC. Youd be surprised how fast they move, they sure dont stagger off the pad!! Neither did a J415 in 12lb glider either (fluttered but held), nor did a J350 in two 14lb gliders (that motor seriously kicks a gliders *ss off the pad, too fast actually), so there is some leeway in motor choosing.

Best bet is to ignore the average impulse, and concentrate on the first half of the burn, what THAT averages.

Idve loved to seen this, was it 4 flights? Slap an I284 in it if its 12lbs, and it will get up ok if the wings arent 2" thick.

AstronMike

incorrect. it weighed 14 (?)pounds on lift off the first flight. it boost PERFECTLY stable to 150-175 feet. the only reason it crashed was that I was waiting for it to climb near 300ft like the now clearly inaccurate sim said it would get and it "surprised" me when it stopped dead at 150-175 ft. by the time I realized it was stopping this soon it was too late and I entered the flat spin where the aerodynamic quirk of the large wing to "relatively speaking" small stab (it was actually much bigger than needed for stable flight)

had I know it was going to stop at 150ft I would have nosed over at 100ft or so and got my cert right their on the very first flight.

that was an H55 single use with H128 reload on that flight.

that night the nsoe section was damaged to the point of why bother so I lopped it off. (it was just a peice of foam) but I needed an excuse to lighten it up. I also dumped the landing gear (no where to land on that field anyway) and chopped off the stab and added new stabs to the tips. this would prevent me from ever getting into a shadow stall again accidentally (since I know about it now I would actively avoid that type of flight profile but why not fix it while I am rebuilding)

overall flight damage was almsot nill.

As for Motor Lift off weight and Thrust to weight rations. they are completely irrelevant. a thrust to weight of 1.1 to 1 is fine. this is a radio controlled rocket glider. those figures do not apply the same to this kind of model and you know that. I have active guidance. its not ballistic.

by the way my initial thrust to weight for the first 2 seconds I believe was

4.5 to 1 for the next 3 seconds it is over 2:1 more than sufficient for me. in fact much greater than I normally fly but needed for such a short (5sec) burn.

so bite me. if I made a 4" airframe at 14 pounds on that motor combo with active guidance it would go well over 1000 feet.

On the second flight again it boosted PERFECT and stable. control was positive (it rolled a little and I corrected) I figured I would get

175-200ft (which I am guessing I got) at which point I actively pushed the nose to horizontal and slight below. after picking up some speed I pulled up and it responded and also tipped a little left which I corrected and it responded. when I got to a speed I liked I pulled up some more but that was all their was. it came in at a very shallow (20-30 degrees) dive and landed (I refuse to call it a crash as it landed "safetly" and in control. but not enough to certify (and I agree with that call)

I had goofed. in my paranoia (brought on by the likes of you on rmr mostly) I was fearful of changing the CG since it wa a know safe boost CG. I gambled I would have enough elevon authority to compensate for it. I gambled wrong. I later found out from george and others something I was not aware of. elevons like I use are less effective toward the root on a delta style wing so I needed to make the elevons not only larger in area but larger toward the root than at the tips.

for the third flight nothing else needed changing. I adjust the CG where it needed to be and had upgraded the elevons.

Third flight it jammed on the rail. (this was H180 I think with 38mm long burn Aerotech H I want to say H65 ?)

I was thankful it jammed since the H65 did not light. it would have lobbed up 30-50ft on the H180 and then flopped.

4th try. last try. (I had scrounged the last possible usable motor :-) it cleared the rail and I immediate knew I was screwed for I heard the H128 (this time) burn out and I did not hear the continuing burn of the H65. as predicted it hit 40-50ft and just stopped and hung for a second before tailsliding 15 feet and then dropping the nose HARD into the ground.

that damned H65 did not light again. and this time I had installed TWO wuickburst igniters BOTH of which fired !!!!! to no avail (thanks george ranchor :-) hehehe)

The Design was fine. The MODEL was fine. The MOTOR selection was FINE (not ideal but MORE than acceptable and usable considering the circumstances (no way to modify for hybrids as it was built when I found out about the I90)

oddly enough as hard as it hit the damage is surprisingly little. a little carbon or glass and I could have the nsoe fixed in 10 minutes. it is no load bearing just a peice of foam. oddly enough the tails would trake longer to make (the carry rig was falling apart so I tore off the tails stabs to fit it into the carrier for the trip home)

if I had motors I could have this bird flight ready in a single day if I wanted to.

It was just plauged with little glitches mostly outside my control. How many of you had made a 12pound glider that is 11ft long with a 6+ft wingspan fly on a pair of H's

I successfully did it TWICE. I just had technical issues with the glide transition which I worked out successfully before motor problems plauged me.

It was VERY clear that it was a DRAG limited model. at that mass (all flights after the 1st were 12.5 pounds or lighter) all sims said I would get

300ft or more.

to lay on the fact that it was a drag issue even MORE I had to increase the CD to 25 (no decimal their) in rocksim to get a proper altitude sim. clearly a CD of 25 is simply impossible. I figure I am just doing something rocksim is incapable of simming. but now that I have FLOWN it a few times doing mental sims for motor selection and design selection is much easier and more reliable now. I have a basis on which to work from. anything I get from rocksim I will use as a relative percentage.

so if my new model sims to say 400ft I know that is 30% more than the old models sim so it will probably net about 225-240 ft in real life.

if I can reduce the mass to 10pounds and use an I90 I sim out at 400ft with a H128 airstart I get over 500ft which means I should be able to get

400-450ft out of it (or 300ft without the airstart)

I plan to build with the airstart in mind but first flight on just the I90. for the airstart if I want more "airtime" I plan to do what george did. have a rail connected switch that will only fire the H128 if it clears the rail. so if the I90 fails to light teh H128 will not even try. and if the H fails all together so what I already know it will boost fine on the I90 alone (since at that time I will have already done an I90 only flight)

Live learn. I did the first model under limited condition. but at the time I had both the money and the time. not something I have in unison very often so I gave it a shot and was very pleased with the results. I learned a lot and had a lot of fun. and got some truly cool pictures and videos from it and a lot of good laughs.

If this bothers you then tough. you can go screw off in your own corner and keep flying your happy meals. next year I will bring Grr back and people will go what happy meals ? hey did you se the Grrr.

Stitch will pilot again but this time Ryu-oki will copilot to keep him under control :-)

Chris Taylor

did you see it fly ? if not watch the videos. Coast time after motor burn out was virtually zero. Motor Out Climb is done. DRAG limited.

Longer burn = more altitude.

You will see when I fly on the MUCH longer burn I90 next year. the I90 is sooo nice a motor on paper. Long burn AND higher thrust thoughtout the whole burn. perfect for my glider.

I was very happy with the flight speed of the Grr Glider. I would like next years flight to be a repeat of my first flights just higher. but alas with the longer burn comes higher thrust.

I know the glider can handle it I just want to make sure i can handle it. then again it was so docile and hands off (never touched the sticks once on either boost till apogee) that I do not think I will have a problem.

Chris Taylor

both flights 1 and 2 got nearly 200 feet (I am guessing 150-175ft for first flight and 175-200ft for second flight)

more than high enough.

the model was first WAY overbuilt. I seriously overestimate the power of the motors involved. and it was SO draggy that under freefall it simply never got fast enough during a crash to cause any real damage (I consider that a feature not a downfall)

I have a feeling you only saw the last flight where only the H128 lit. even the SIMS said it would not clear 75ft on that motor alone.

true for effeciency but I love the vertical boost. the thrust I got last year was fine. I was very pleased with teh boost speed. I just needed twice the length of burn really :-)

I just love straight up boosts and then a conversion to level flight. more elegant to me and more a challenge.

alas with that model your flight profile would not have helped all that much. the burn time was simply not enough.

although it might have saved the first flight BUT then I might never have discovered my two problems.

Jr"

a very freaking huge large piece of commercial insulation.

How much of a freaking airfoil do you want. I sure as hell and now going to try to build an airfoil that stinking big. Flat plate is just fine by me.