Discussion in 'Oddrocs' started by KenECoyote, Jan 18, 2016.
What was that movie? 'Life is like a box of chocolates' or maybe we should just call you Astronaut Farmer?
I would follow the original kit idea "Make a rocket of any size you want" and make as much of this interchangeable as possible. Such sacrilege... you sliced the fin can all apart, you barbarian! :shock:
You guys are churning out posts like no tomorrow, I have to catch up with it all.
Yes, there will be some back pressure or air resistance because of the blocked tube. I would think the placement of the shroud in relation to the nose tip is critical. The bow shock from the tip will divert air away from or into the shroud. Ken, I think you need to look at the nose cone shape (ogive, VK, etc) and determine where the bow shock would be. If you can place the shroud so the bow shock is inside the shroud you will get more airflow out of your slots. I'll look around and see if I can find some aerodynamics sites for you.
Too bad you don't have access to a wind tunnel, and no, don't attempt to build one. I know the wheels are already spinning trying to figure out how to make one! It is harder than it looks, we built one in school (Aviation High School) and it was an epic fail the first time one of the test samples let loose. :facepalm:
Glen, we both said the same thing to Ken. The area of the inlet has to be equal to the area of the slots +/- a couple of percent. Great minds think alike. Bob
better than Forward Air Rearward Throughput) : Sorry I would have left it as FART just for parochial giggles. Have you thought about rail buttons so you can put this WAY out on the HPR pads? At least for the first few Christmas Tree launches.
Well, the 10:1 is the magical ratio where base drag is s'posed to help. Looked longer to me and I forgot if that was mentioned earlier.
Yay, I hope that works out!
I made an earlier comment about the flow rate in/out business. I guess it's not so much that I don't believe that generally but I still think there will be a limit on how much air actually goes in. It won't flow unimpeded IMO.
Sure looks like weather will not be permitting this Saturday. :eyepop:
LOL - I double-dog dare you!
Honestly though, looking at my first iteration, I don't think there is much to worry about. It's close in resemblence to the original rocket and only the fin can area is different. Even if it was a solid tube on the back end, it should fly upwards.
Are you nuts?
As noted earlier, I don't think that some back pressure is necessarily a bad thing, but it will take a bunch of testing to see.
You have an interesting point about the bow shock; however that is present in most rockets and even in the original rocket I'm using as a testbed; however it doesn't make the fins ineffective. In addition, I think the farther away the nose cone is from the shroud, the less there is of this (I think). However in the 2 of 2 example, the bow shock may actually help to send a higher pressure wave into the shroud (I think).
Wind tunnel...hmmm....maybe I can make one using a leaf blower, Sonotube and plexiglas...add a delay grain for smoke... NAH! Swing test is much easier.:wink:
I can bring one of the long rods out to the HP pads and sink it into the ground. Geez...what fraidy cats. Do what I do and get in shape with running so you can dodge oncoming test rockets! :wink:
Yes! I do recall that from the Apogee newsletter. I think the point I'm trying to say is that a wider nose cone should provide more stability than a narrow one, so if the front of the rocket is wide/draggy, then it has some additional stability (much like the flat plate effect or saucer rockets with wide frontal areas). I'm not saying this is a noticeable effect, but clearly it's present and as the front gets wider in comparison to the length, the CP moves rearward (I think ).
We'll see...winds are howling like mad today and there's a snow storm on the way...not ideal conditions for a rocket launch.
Yes, agreed. This design will always have some air impeded since it redirects the flow of the oncoming air. This may also aid stability by moving the CP back; however there is the complexity of multiple factors involved here. A short shroud with wide outlets on a long rocket should make the rear act similar to a cone base; however a very wide and short shroud with small outlets on a short rocket should make it act more like a squat rocket or even saucer at the extreme. #mindsimblown
I agree, I think. It'll get more interesting as you start making it shorter.
What? 5-8" of snow and 21mph winds and you guys quit? I thought you guys were Radical Rocketeers! :wink:
I actually heard that forecast last night; however for me as a weather watcher, I take long range forecasts as being 50/50 propositions...meaning it's a flip of a coin. I rely more on the forecast a day or two before to be more accurate/closer.
In that case I would like to update my protective gear:
See, I am a Radical Rocketeer of the first order! (or First Order)
Love it. Every post, included quoted responses, should have tags supported. What is this, the 2000's?
LOL! I don't know...ST armor hasn't been proven to provide any real protection. :wink:
Wasn't there a whole thread about that?
in the interests of launch safety, you might consider adding a base disc or cone at the bottom of the rocket of greater diameter than the shroud to ensure a stable flight. Then test it with progressively smaller ones to see if the air fins are becoming effective. You could tell if it starts to get squirrelly as you decrease the size of the disc or cone. Then, hopefully it would be stable with the device removed. Just an idea. or just put on some regular fins, and cut them down smaller after each successful flight.
This one may be of some help.
No worries...all appropriate safety precautions will be taken!
For a split sec I was like :facepalm: someone else thought of this last month?!? Then I saw the fins and realized that this is for GDS (or GAS as it is listed). Gas Dynamic Stabilization is a different thing than what I'm attempting here and uses the exhaust gases to form a base cone for some stability (in fact, in that example I don't quite understand the goal since there are fins present). I may later use GDS to augment stabilization during the boost phase, but for now I want to test the RAIS design on it's own.
*Edit: Re-reading the thread (which is quite amusing), it is more gas induction rather than necessarily GDS; however why it is done isn't clearly stated. By the design I would assume that it is to get more boost and maybe for the different sound.
I bet the ducting in the 'Augie' rockets mostly just allows the motor to be recessed forward and thus help CG.
So I honestly see this design working quite well. Even without the slots at the bottom of tube, although at that point it would probably act more like a wind sail. Anyway I'll just be watching this neat design. And I apologize if this has been answered, I just skimmed, when are you flying it?
That is possible; however if you're adding an additional shroud extending out the back, the weight of the shroud is also bringing the CG back which kind of kills some of that benefit and it also adds a lot of drag.
FWIW - I just did a search and saw someone post on YORF that the Augie design is for "thrust augmentation" and another person on Rocket Reviews said the ducted design was another method for staging rockets (http://www.rocketryforum.com/showth...Induction-Stabilization&p=1541410#post1541410).
Thanks for the vote of confidence! I'd like to fly it this weekend; however the weather may not cooperate. Even if I get it launched successfully, there are still a lot of variables in design involved so if I succeed, I'll be testing different versions successively (and hopefully). We should know by Spring!
The goal may have been thrust augmentation but I don't believe this can be realized on modrocs. This has been discussed numerous time but nobody ever convinced me. I added a thrust augmentation tube to my most recent GDS Saturn V. I saw where the Jetex motor's augmentation tubes were proven to work via actual measurements. However, someone quickly pointed out the error of my ways. It flew with the tube but the thrust was reduced instead of being augmented, And it was toast afterwards as a bonus.
Yeah, I'm with you...I just don't see it. Also making it more complex and adding weight and drag seems counterproductive to that. I would think that allowing more air to flow to the sides of the burning motor would augment the exhaust, which sounds more like minimum diameter or possibly a tailcone. From what I can tell, GDS is more inefficient since some of the thrust is used to pull the air in from the sides (I think ).
I spent some time yesterday and today re-reading some aerodynamics papers, blah, blah, blah. Anyway, what I had originally meant about nose cone shape and relation to entrance of tube is called the compressability of dynamic airflow. An hour later, yada yada,
So I think your safe with whatever you do regarding nose cone shape, tube size, etc. There has to be some mathematical correlation between outer tube size and exhaust port size/shape. What that relationship is is what you are attempting to determine by experiment. https://spaceflightsystems.grc.nasa.gov/education/rocket/shortr.html is one of the NASA homepages I stumbled across. Flip through the rocket aerodynamics page, do not let the first few pages fool you because it then turns into this https://spaceflightsystems.grc.nasa.gov/education/rocket/wcora.html :y:. And if you really want your socks blown off hunt down their section on rocket nozzle design or isenotropic airflow, which cover supersonic airflow through a tube.
Back in the day, or when I was a boy, or other notable quotes from older gentlemen, this was a whole semester in school!
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