Rocket Challenge on Discover

Less. Them teeny rockets is like pointy sticks.

Hope you have your banana handy.

Reported Isp is ~245 for the shuttle SRBs. Some commercial propellants are in the 210s (actually the CTI O motor is 230). I've measured Isp up to about 240 on some of my EX propellants. So I don't think they are all "much less" than the SRBs. Of course there are some real dogs being sold out there too, like Black Jack and Smokey Sam, which are about 140.

- Jeff Taylor

kaplow snipped-for-privacy@encompasserve.org.TRABoD (Bob Kaplow) wrote in news: snipped-for-privacy@eisner.encompasserve.org:

You mean it's not obvious?

"He's the man who never returned."

By the way, it's been MBTA for as long as I've lived in the area, in belated acknowledgment that there's a lot of the state west of Worcester that doesn't get any service.

len.

Jeff Taylor wrote in news:%Xrub.5722$y27.2249585 @news4.srv.hcvlny.cv.net:

All true, but O motors and EX are, if you'll pardon the expression, the fringes of sport rocketry. The ATF's meddling will have the most impact on the H through K impulse categories.

len.

Don't forget Firestarter! Although ironically it may actually be higher performance than errortech blackjack.

I mean the distance from the CG to the CP divided by the diameter or length. 10% of length is good static margin, but more may be needed for flexible rockets at high dynamic pressure. If you know the static margin and dampimg ratio, you have a very good idea of what the transieient dymnamics are go to be like.

It gives you very little idea of the restoring torque without knowing the reference area, angle of attack, pitch/yaw rate, and dynamic pressure.

I did not say that it does.

K is what??? Regardless, you seem to fixating on natural frequency when that is the least significant factor for safety and transient dymamics.

I guess we just dissagree. I find "slow" transient response to be comforting and not at all dreadfull. I find short stubby rockets that can turn on a dime coming off the laucher to be more dreadful. Inertia is agood thing, especialy at low speeds.

You are the one tallking about gain margins, when sport rockets do not have control systems with gains. (I do know what you are trying to say when to say gain margin.) Again, stability is paramount, dynamic response is important, and rapid response is of much less importance (unless you need to intercept a maneuvering target).

Bail out quick before you crash and burn! ;)

Alan

This should be in the FAQ.

as soon as I read the post all I could hear was my son's 3 year old voice singing it loudly and mostly on-key. trains were the big thing then and he learned every train song we could find. Jimmie Rogers still lives!

Cliff Sojourner wrote in news:yovub.40834 $Dw6.183773@attbi_s02:

Kingston Trio.

Distance from CG/CP divided by the diameter tells very, very little about the dynamics without making a bunch of assumptions that don't apply in a general case. It was a nice approximation for a rule of thumb of a lot of more complicated analysis back in 1973 - but it's not really answering the question asked. Unless you assume that the body provide a larger fraction of the damping forces than I do.

Additionally, for a flex body, the key is to keep the flex frequency (let's take only the first symmetric mode) away from the aerodynamic natural frequency. You can't even guess at this without taking the intertia into account - yet it's vital to both sides of the equation, doesn't cancel out (unless you make a bunch of assumptions) and totally unaccounted for in terms of "calibers of static margin".

K being the effective "spring constant", given that we model it as a second-order system. K in case of a spring is the rate of compression force per distance compressed. In the case of a rocket, it's the rate of restoring torque per angle displaced (kudos to you for finding a brain fart). sqrt(K/I) gives the natural frequency. It's only reasonably true at low AoA, of course - which was sort of the point to caring about the bandwidth.

If you want to discuss higher-order systems, we can.

The point of the caring about the dynamic response, in this context, is to improve the knowledge of the trajectory - not to prevent weathercocking. You can easily predict weathercocking if you have a good idea about the transient response - because it makes the simple approximation of nosing into the wind more accurate by reducing the angles of attack to where simple analyses are valid. A superroc leaving the rail at 30 fps into a 20 fps wind experiences a very large angle of attack for a long time - and none of the information we have at hand is particularly valid at large AoA. A stubby model get the same AoA but it doesn't stay there long - in fact, pretty quickly goes right around to the relative wind.

There's a valid argument to be made which factor results in more trajectory dispersion - second-to-second wind variation and turbulence VS. errors due to invalid approximations in the analysis. I do guess that it's better with a reasonable response bandwith, but I will admit that there could easily be an upper limit, at least at low velocities.

Brett

But they likely apply in the specific case of a cylindrical 3 or 4 or 5 or 6 finned rocket with a stability margin over 0.75 and under 3.0 and an L/D over 12 and under 24.

You can if you assume a range of typical inertias and mass distribution.

Obviously it is easy to get bogged down by the math and lose connection with common sense or typical variables. Tail wags the dog by fixating on extreme cases to declare the model invalid.

2nd and 3rd order effects are EXPONENTIALLY LESS RELEVENT.

Let's waste even more time and fixate on 4th and 5th and 6th order effects!!! rmr has no bandwidth limits and you obviously have plenty of time on your hands.

Harmonics means it corrects BOTH WAYS and thus DAMPS THE FINAL RESULT toa very small variance from a PERFECTLY BALLISTIC FLIGHT.

I am assuming at least the rocket is at a stabilizing velocity when it leaves the rod. if not, then goto line 040

040 REM Obviously it is easy to get bogged down by the math and lose connection with common sense or typical variables. Tail wags the dog by fixating on extreme cases to declare the model invalid.

GET A LIFE!

Deep Tech Jerry

Sounds like determining if these things are actually significant issues for typical HPR models might be an interesting "NAR research and development" entry... I suppose one could fly a telemetry package containing rate-gyros (to measure dynamic attitude response) and strain-gages (to detect airframe flex behavior).

-dave w

No, he never returned.

.... And his fate is still unlearned he may ride forever through the streets of Boston, he's a man who'll never return.

It's been a very long time but I think it was the Kingston Trio. circa 1962. I was 6.

Randy

so, when do I get to fly the rocket?

- iz

Brett Buck wrote:

The flex behavior, or at least the frequency and damping, should be pretty easy to determine on the ground with accelerometers. Hang it up with a rope, turn on the accelerometers, and hit it with a hammer. That's how they do it on, for instance, Trident missiles. Of course, they are just using it to validate a finite-element model.

A lot of other information might be gleaned from flying multiple accelerometers in several axes.

There's a ton of projects along these lines that could be done for R&D (if they haven't already, that is - I don't have a list). I still think that R&D probably shouldn't be a points event, but there are many worthy projects that I could think of.

As long as you're at it, the best one that I can I think of, that almost certainly has not been done, is an analysis of the effects of oxidizer slosh in hybrids.

Brett

Well, I expect Alan had another shot, but I'm pretty much done. I still think that something along the lines of what I suggested would be a wise course, but it's clear it's not going anywhere, and I'm not a crusader.

Brett

====== MTA The Kingston Trio Words and Music by Bess Hawes and Jacqueline Steiner

(peak Billboard position #15 in 1959)

Well, let me tell you of the story of a man named Charlie On a tragic and fateful day He put ten cents in his pocket, kissed his wife and family Went to ride on the MTA

Well did he ever return, no he never returned And his fate is still unlearned (what a pity) He may ride forever 'neath the streets of Boston He's the man who never returned

Charlie handed in his dime at the Kendall Square station And he changed for Jamaica Plain When he got there, the conductor told him "one more nickel" Charlie couldn't get off of that train.

But did he ever return, no he never returned And his fate is still unlearned (poor old Charlie) He may ride forever 'neath the streets of Boston He's the man who never returned

Now all night long Charlie rides through the station Crying "what will become of me?" "How can I afford to see my sister in Chelsea?" "Or my cousin in Roxbury?"

But did he ever return, no he never returned And his fate is still unlearned (shame and scandal) He may ride forever 'neath the streets of Boston He's the man who never returned

Charlie's wife goes down to the Scollay Square station Every day at quarter past two And through the open window she hands Charlie a sandwich As the train comes rumblin' through.

Well did he ever return, no he never returned And his fate is still unlearned (he may ride forever) He may ride forever 'neath the streets of Boston He's the man who never returned (pick it, Davey)

SPOKEN

Now you citizens of Boston dontcha think it's a scandal How the people have to pay and pay Fight the fare increase, vote for *George* O'Brian!! Get poor Charlie off the MTA!!

Or else he'll never return, no he'll never return And his fate is still unlearned (just like always) He may ride forever 'neath the streets of Boston He's the man who never returned He's the man who never returned He's the man who never returned

Transcribed by Robin Hood

*TRANSCRIBER'S NOTE* : from ON THE MTA SOURCE: Bob Pfeffer SOURCE'S SOURCE: Will Holt COMMENTS: Written by Jacqueline Berman (now Steiner) and Bess Hawes as a campaign song for Walter A. O'Brian, the Progressive Party candidate in Boston's mayoral election. When Will Holt recorded the number as a pop song for Coral, the record company was astounded by a deluge of protests from Boston because the song made a hero out of a local "radical". The record was hastily withdrawn an a new version recorded which eliminated O'Brien's claim to musical fame. In the later Kingston Trio release, Walter A. was changed to George to avoid advertising Commies on the air.

Sorry for drifting back ON-TOPIC.