What Would Happen If ...

[ezgb59a]

This stems from a comment on another thread.

What would be the result if one would build a rocket approximately 15 inches tall, tube just big enough to hold a D engine (or larger), and using a light weight nose cone? Basically - majority of all the weight is in the rear of the rocket.

What kind of problems could arise?

I'm not building anything like this but kind of curious what happens or what could happen.


Even a bit more interesting - I'm sure someone here has tired this. What would happen if you attached fins to a D engine with a nose cone on top and launch lug on side? What are the results?

Thanks
ez

[kapton]

For a stable flight, a rockets center of pressure must be behind of its center of gravity. So in the case of your first question, the fins would need to be large enough so that the CP ends up behind the CG. If the CP is not behind the CG, the rocket will be unstable in flight and most likely cartwheel.

Attaching fins and a nosecone directly to a motor is against the safety code. First, it would not make a reusable aerovehicle, and secondly glueing directly to the motor is a no-no. Other than that, it would fly fine.

[STRMan]

Attaching fins and a nosecone directly to a motor is against the safety code. First, it would not make a reusable aerovehicle, and secondly glueing directly to the motor is a no-no. Other than that, it would fly fine.

I agree in this instance that this would be in violation of the model rocket safety code, because there is no recovery system. The odds are the engine with nose and fins attached would be just as stable and come in ballistic, thus failing paragraph ten requiring a recovery system.

I don't see where is says gluing anything to a motor is a no-no. Gluing a fin to a motor is no different than wrapping it with tape as long as it is being used for it's intended purpose, meeting the requirement so f paragraph 2. What is tape other than paper (which is made from wood) and glue?

My friends older brother (who introduced me to rocketry) built a rocket that had a single 18 mm in the center and a couple of drop off 13 mm boosters along side. The way the boosters worked was he basically had a pair of nose cones glued alongside the main body tube that had a wooden dowel hanging from them. The 13 mm engines slid over the dowels. These 13 mm engines had a single sacrificial fin glued right to the side.

While ascending, the fin helped the stability of the rocket. When it was spent and blew off the dowel, the fin caused the casing to tumble back, better than just ejecting the casing IMO. In this instance, gluing something to the engine did not break the model rocket safety code, it in fact helped uphold it.

[Royatl]

This stems from a comment on another thread.

What would be the result if one would build a rocket approximately 15 inches tall, tube just big enough to hold a D engine (or larger), and using a light weight nose cone? Basically - majority of all the weight is in the rear of the rocket.

What kind of problems could arise?

I'm not building anything like this but kind of curious what happens or what could happen.


Even a bit more interesting - I'm sure someone here has tired this. What would happen if you attached fins to a D engine with a nose cone on top and launch lug on side? What are the results?

Thanks
ez


you don't mention if there are any fins on it.

if there aren't, the rocket will likely just go end-over-end. Fins of small sizes will allow the rocket to fly somewhat straight, but won't be strong enough to overcome any misdirection caused by wind or mis-aligned thrust and it too will end up going end-over-end. Fins of larger sizes may allow it to fly straight like you expect a rocket to fly.


your second scenario: if the fins are swept back behind the D-motor, it will probably fly fine.
there are kits (Estes Quark, Swift 220) that are essentially the same concept. However, with fins attached, your rocket will streamline in, possibly damaging or hurting what ever it hit. We always either include a recovery system in the rocket, or have the rocket do something that will slow it down as it comes back down... something that will cause it to go nearly end over end, like your first rocket without fins.

[ghrocketman]

I have glued dowels to D12 and old cato-prone Estes E15 motors ala "bottle rocket style" (ignited by a FUSE, of course) and they flew fine, albeit against the safety code due to "un-recoverability".

[Bob Kaplow]

I have glued dowels to D12 and old cato-prone Estes E15 motors ala "bottle rocket style" (ignited by a FUSE, of course) and they flew fine, albeit against the safety code due to "un-recoverability".

So I probably shouldn't mention anything from the 70s about launching rockets up kite strings...

Or how one year, the neighbors across the street were "shelling" our house with bottle rockets. I went over with some old FSI motors, taped one of their M80s to the top, and sent it so high it didn't sound much different from the bottle rocket reports.

THEN I showed them a 38mm Silver Streak! They never sent anything else towards my house after that :)

[ghrocketman]

I chose not to mention the "M80"/"silver salute" payload atop a D12-0 so as to not give anyone any worse ideas....

[ezgb59a]

Okay - better educate myself. My kid is fairly open minded and has a really good imagination. Sometimes scary to think youngsters can be so smart without knowledge and create things you just want to go wow ... let's not try that right now.

Are there any good reading materials on the CP/CM/other acronyms to get an idea of the dos, don'ts and why nots. (My kids will always ask the why nots).

Also - are the calculations to determine center of mass, center of pressure, etc. fairly straight forward? Been such a long time since I had a physcis class ... tossed the books.

Finally, is there a posting of the rocketer rules on this board? Better get them memorized.



Thanks
ez

[Royatl]

Okay - better educate myself. My kid is fairly open minded and has a really good imagination. Sometimes scary to think youngsters can be so smart without knowledge and create things you just want to go wow ... let's not try that right now.

Are there any good reading materials on the CP/CM/other acronyms to get an idea of the dos, don'ts and why nots. (My kids will always ask the why nots).

Also - are the calculations to determine center of mass, center of pressure, etc. fairly straight forward? Been such a long time since I had a physcis class ... tossed the books.

Finally, is there a posting of the rocketer rules on this board? Better get them memorized.



Thanks
ez


I think others have mentioned:

Handbook of Model Rocketry (http://tinyurl.com/647guf) from amazon,
or from the co-author himself (http://tinyurl.com/5zbof5).

www.nar.org/NARmrsc.html for various things, including safety codes.

[Solomoriah]

You can calculate Center of Gravity, but it's a lot easier and more accurate to build the rocket, put an engine in (the largest you expect to ever use with that rocket) and check the CG directly. Where it balances is the CG.

Calculating CP by hand is generally done using the (legendary) Barrowman equations. I learned the basics from the out-of-print Centuri TIR 30 "Stability of a Model Rocket in Flight" by (tada) Jim Barrowman. You can get a PDF copy of this report from JimZ's site: http://www.dars.org/jimz/pubs.htm

Most modern rocketeers who are "serious" apparently use Rocksim to do this. It works quite well, but is still out of my price range. You can also use VCP, which is free, to calculate the CP.

Honestly, it's not that hard to do by hand for most traditional rockets. "Strange" designs may be beyond figuring by simple methods, and may stymie the programs as well. Sometimes you just have to holler "heads up" and try it.

If you haven't spent some time perusing the contents of Ninfinger's and JimZ's sites, you should.

http://www.ninfinger.org/~sven/rockets/rockets.html
http://www.dars.org/jimz/rp00.htm

[Maniac BAR]

You can also use the "quick and dirty" method to find the CG and see if the rocket is stable at the same time with a long piece of string. Get you rocket all finished, painted and such and put the largest motor you plan to use in it. Get the rocket completely in flight trim, shock cord, parachute, wadding and motor, etc. Make a loop on one end of the string and form a loose noose (hey, that rhimes) to go onto the rocket. When the rocket is balancing on the end of the noose, evenly, you have now found the CG. Keep the string in place with some tape and take the whole thing outside and start to spin it around. As it gets up speed (not to fast) note which way it is traveling. Nose first means it should be balanced for flight. Tail first means you need to add nose weight. I do this by taking some clay and placing a chunk of it right at the nose cone tip. Try the swing again. When you have the proper amount of nose weight the rocket will then start to go nose first.

Take the clay and get it into the nose cone, pack it in good and you are good to go.

[Bob Kaplow]

Ah, the old "string test". I don't trust it. I've seen to many models string test unstable that are in fact stable, and enough string test stable that are anything but stable.

[barone]

Ah, the old "string test". I don't trust it. I've seen to many models string test unstable that are in fact stable, and enough string test stable that are anything but stable.
Hhmmmmm......maybe we're doing it backwards? :rolleyes:

[Peartree]

Neither Barrowman nor Rocksim do well outside the "normal" rocket design of a tube with fins. If you do something more complex like a tube with fins that have attached tubes (outboard pods or outboard engines) or heaven forbid outboard tube fins, they don't work. In that case (and it works for simpler designs as well) you resort to the cardboard cutout method. Trace the silhouette of the rocket onto cardboard and then cut it out. Where the cardboard cutout balances (the CG of the cardboard) is the CP of the rocket.

[Bob Kaplow]

Cardboard cutout is worse than string test.

IMHO I trust the "that looks about right to me" method over the cardboard cutout method.

Try this test: NARAM-51 has D Dual Eggloft Duration. Do a cardboard cutout stability test for a typical model for this event, and tell me how big your fins need to be on a D DELD. Then go fly the model, NOT on my field, and tell me how it worked. There's a very good reason we don't have TARC teams using the cardboard cutout method.

Try the same test with a C payload model design from this years NARAM. When I did this for my first NAR contest back in the mid 70s, it told me I didn't need any fins at all to make the rocket stable. Any one care to guess if this was true or not?

Finally, try cardboard cutout for a C Stupidroc :p

[Peartree]

Cardboard cutout is worse than string test.

IMHO I trust the "that looks about right to me" method over the cardboard cutout method.

Try this test: NARAM-51 has D Dual Eggloft Duration. Do a cardboard cutout stability test for a typical model for this event, and tell me how big your fins need to be on a D DELD. Then go fly the model, NOT on my field, and tell me how it worked. There's a very good reason we don't have TARC teams using the cardboard cutout method.

Try the same test with a C payload model design from this years NARAM. When I did this for my first NAR contest back in the mid 70s, it told me I didn't need any fins at all to make the rocket stable. Any one care to guess if this was true or not?

Finally, try cardboard cutout for a C Stupidroc :p

Black Death (http://www.rocketreviews.com/images2/sta72192.jpg)

I don't have that much experience Bob, but my point was with designs like this one, there aren't many other options. How else do you make some kind of guess (without simply guessing) as to how much noseweight a design will need? Rocksim is fine when it works, but once you play with something more complex than a tube with fins then you need something else. I'd love to know what else is better but so far I haven't seen it.

[ezgb59a]

Sounds like Rocksim might be based on a two dimensional calculation - height of the rockent and the weight. The width of the rocket and where the weight resides I'm sure will move the CG/CM up and down on the heigth.

Also, making program to account for the complexities of drag would be difficult. Though they exist ... wonder if NASA would let one borrow their programs. Imagine walking int NASA ... can I enter my rocket dimensions to see what the results our.

Question on rocksim ... does it ask for the weight and dimensions of the rocket parts?

thanks
ez

[barone]

........
Question on rocksim ... does it ask for the weight and dimensions of the rocket parts?

thanks
ez
Rocksim has a database of parts with the info. It's not all inclusive but you can add items to the database. I think Craig had made up a more comprehensive parts database for RockSim last year????

[Nuke Rocketeer]

I chose not to mention the "M80"/"silver salute" payload atop a D12-0 so as to not give anyone any worse ideas....

:D Or putting one on top of a D12-0/D12-3 stack in sacrificial rocket on July 4th. ;)

[Doug Sams]

Question on rocksim ... does it ask for the weight and dimensions of the rocket parts?No, it calculates the weights based on what it knows of the material properties. For example, given the density of kraft paper (grocery sack paper), it will calculate the weight of a body tube based on the tube dimensions. You may select a tube from the database from which Rocksim gets the inside and outside diameters, or you can enter them manually, say for a custom or oddball tube. You also enter the length.

From these numbers, the actual volume of material can be calculated and hence the mass. Rocksim also adds fudge factors in for such things as the mass of a paint job.

You can also override the calculated mass and CG numbers and instead use the actual values. Every computer with Rocksim installed should have a small scale next to it :D

Rocksim calculates the CP, and estimates the CG. But the rocketeer should find the real CG, with the motor and recovery system installed. Then see how this compares to the CP. That's the true indicator of stability. (At least for the rockets for which Rocksim can calculate the CP - ie, finned rockets with simple tube setups.)

Every time I get ready to fly a new rocket, I enter the prepped values in and re-sim it to see what the best delay is, what the expected deployment velocity will be, and what size chute is best for it.

HTH.

Doug

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