Quantcast

1/20 Scale SR-71 Blackbird

The Rocketry Forum

Help Support The Rocketry Forum:

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
Sooo, I am currently at the end of the design process of a 1/20 or 1/22 (haven't decided yet) scale of the SR-71 Blackbird I plan to launch at NERRF 6.

I have made it on RocSim and it works well. The margin is 1.5ish without the motor(s) and .5ish with the motor(s). That is without any nc weight so I'm planning to add a few ounces of clay to the nc. It probably doesn't matter now but my plan is to launch it twice:
1) Cluster; central G77R and 2x F52T
2) I345-WT

Since the wingspan of the Blackbird is so large, about 32 in including the engines, I plan to fiberglass the fins with 2 oz fiberglass. I'm having determining what material to use for fins. I'm trying to keep it lightweight, 1/4in preferably, and I’m trying to steer away from G10 fins. I've already ruled out Birch plywood because of the weight. I was looking at Poplar but I think its pretty close in weight to the Birch. 1/4 aircraft plywood would be the best option right now since its lightweight and durable but cost wise its a little more and I can't seem to find it in big enough sheets that I need. Is there anything I can do to 1/4in (maybe even 1/2in) Balsa to make it stronger before glassing it? I was thinking paper layering it first but I don't know how the fiberglass would hold to the paper. Maybe just glassing balsa would be enough? But I'm not sure. Anyone have any suggestions or feedback on material?

Would any sort of lightweight but dense plastic do the trick if the surface were roughed up? Would the fiberglass grip to plastic?

I will also be using lots of foam and bondo to make the various curved parts and details for this rocket.

Just giving a heads up, I will most likely have more questions along the way....I hope people don't mind :D

Thanks!

View attachment SR-71 Blackbird internet pictures 1.bmp

View attachment SR-71 Blackbird internet pictures 2.bmp
 

powderburner

Well-Known Member
Joined
Jan 18, 2009
Messages
7,356
Reaction score
4
Please be assured that I am not criticizing or picking on you by these comments, just trying to make sure you have considered everything.

Do you have a lot of experience with getting composite motors started up, and reliably, consistently, and (most important) quickly? I think you already understand some of the possible problems with motors located far away from the vehicle centerline (at least, that's what I read into your reference to a large wingspan). All three of your motors are going to have to fire up just about exactly at the same time to get a good flight. If the motor on one side starts even a fraction of a second sooner than the other side, your rocket may end up doing cartwheels at the top of the launcher. And if the center motor does not light (an easily possible failure mode) will the rocket still fly safely to a decent altitude for safe deployment?

I know the SR is a cool-looking airplane, and the rocket kit kinda begs for more motors/smoke/noise, but this stuff makes me nervous. I am not good at all at trying to get composites to ignite reliably, and if I tried this conversion project it would almost certainly come out as a disaster.

Would the single centerline motor be enough to keep you happy?
(That's safer, simpler, cheaper, more reliable, etc)

For construction of the wing, before you plank the whole thing (heavy), have you looked into model airplane-style construction? Leading edge spars, mid spars, ribs, and monocote-type cover material could be a LOT lighter.....
 

davalf

Well-Known Member
Joined
Dec 1, 2009
Messages
152
Reaction score
0
I have read that the old kit got a bad rap but I always enjoyed flying mine. I am looking forward to this build thread. Have fun and good luck.

Dave
 

MaxQ

Tripoli 2747
Joined
Jan 18, 2009
Messages
4,411
Reaction score
58
Location
Central Viginia - USA
Sooo, I am currently at the end of the design process of a 1/20 or 1/22 (haven't decided yet) scale of the SR-71 Blackbird I plan to launch at NERRF 6.

I have made it on RocSim and it works well. The margin is 1.5ish without the motor(s) and .5ish with the motor(s). That is without any nc weight so I'm planning to add a few ounces of clay to the nc. It probably doesn't matter now but my plan is to launch it twice:
1) Cluster; central G77R and 2x F52T
2) I345-WT

Since the wingspan of the Blackbird is so large, about 32 in including the engines, I plan to fiberglass the fins with 2 oz fiberglass. I'm having determining what material to use for fins. I'm trying to keep it lightweight, 1/4in preferably, and I’m trying to steer away from G10 fins. I've already ruled out Birch plywood because of the weight. I was looking at Poplar but I think its pretty close in weight to the Birch. 1/4 aircraft plywood would be the best option right now since its lightweight and durable but cost wise its a little more and I can't seem to find it in big enough sheets that I need. Is there anything I can do to 1/4in (maybe even 1/2in) Balsa to make it stronger before glassing it? I was thinking paper layering it first but I don't know how the fiberglass would hold to the paper. Maybe just glassing balsa would be enough? But I'm not sure. Anyone have any suggestions or feedback on material?

Would any sort of lightweight but dense plastic do the trick if the surface were roughed up? Would the fiberglass grip to plastic?

I will also be using lots of foam and bondo to make the various curved parts and details for this rocket.

Just giving a heads up, I will most likely have more questions along the way....I hope people don't mind :D

Thanks!
Would be a cool project.
I tend to agree with the comments made about lighting outboard motors that are located away from the centerline......

George Gassaway had a lengthy discussion on this Forum recently regarding outboard motor thrust curves and the difficulty compensating for the probable difference in the outboard thrust curves that you could encounter, and the effect on flight.

He was discussing the composite outboard motors relative to a scale Space Shuttle "stack" and some of those points apply here as well...although the stronger central motor you are proposing may help.
I'd launch it with a single central motor first.
I would try and perfect airstarting outboards on a "proof of concept" rocket to avoid dinging the real thing.

Rocsim isn't the greatest tool for accuately portaying overall weight distribution, particularly with a complex scratchbuild using unique materials.

As for weight, foam is indeed light...but all the bondo to effect compound curves will be heavy. It is likely that lead, not a few ounces of clay, will be needed for noseweight if the airframe and clustered motors moves the CG back substantially.

As for the build and the curves....I am working on an upscale of the Estes SRX and I was able to get a light structure loaded with complex curves by putting a LOT of cross section "formers" longitudenly on central motor tube...filling the voids with pink foam, sanding to a rough shape and applying a light wood filler.
Of course I plan on using this to make a buck to do a fiberglass layup....not flying it.

http://www.rocketryforum.com/showthread.php?t=3212

I wouldn't rule out looking into some SR 71 RC airplane plans to see how a frame up of spruce and ply with foam or balsa wood sheeting might be done. I have plans for a pretty nice Lear Fan aircraft that is basically foam blocks shaped on a few wood formers.
Maybe you can avoid the bondo...
 
Last edited:

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,351
Reaction score
29
Professional aerospace engineers work hard to make strong, lightweight structures to maximize performance while minimizing vehicle weight and cost. This leads to the use of materials with high strength to weight ratios, but as these material are expensive. the parts are engineered to use the minimum amount of the material to reduce cost and maximize performance. If you don't minimize the weight of parts made from expensive materials, all you get is an out of spec, under performing, overweight weight vehicle that costs way more than was expected.

The initial design of the current F35 fighter is an example of this. The original central structural frame castings were made from a titanium alloy. While the strength to weight ratio of titanium is very high and structures made from these alloys are very stiff, titanium alloys are 50% heavier than aluminum, and much more expensive because it is difficult to process and machine. As the prototype was significantly overweight, the engineers went back to the drawing board, and found that they could shed 3,000 pounds by changing the casting to a 7000 series aluminum. A rather impressive amount as the aluminum castings weight 6,000 pounds!

Using "high strength" FG composites in a rocket does not necessarily made a rocket either better or stronger, but it definitely will make it heavier if you don't use them sparingly.

Powderburner mentioned you really should look at how the R/C guys build airplanes, and I have found a nice 1/8 scale SR-71 R/C project on the web. http://www.mmrca.org/lance/index.html and specifically starting at http://www.mmrca.org/lance/sledframe.html This 1/8 scale SR-71 Blackbird, AKA the Sled, will be 13 feet long, and with a 82 inches wingspan and a design final dry weight of 45 lbs. If you used the same wood rib/foam/FG design technique, your 1/20 scale version would come in at less than 7 pounds without motors if you used the same material thickness used in the 1/8 scale article and would be plenty strong. (Approximate weight scaling factor is scale ratios squared. (45 pounds * (8/20)^2 ~ 7.2 pounds)

Bob
 

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
I am looking into the framing idea, looks like it would work really well. I took a look at the 1/8 scale R/C plane and I read through his procedure pages and get the basic jist of what he's doing. Does anybody know where I can get some scaled renderings or plans for the SR so I can get some more measurements?
 

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
Instead of trying to find books with accurate measurements, I am going to use/have started to use my 1/72 scale model kit to get the measurements.

Hopefully, these should be my final thoughts on the materials:
I have decided to go with my own little idea for the many panels that will form the general shape of the Blackbird. I am using 1in thick pine boards (that are actually 3/4in thick) and cutting them in half to make the different panels.
For the skinny wing part in between the main body and the engines, which will end up being a little less then 1in thick at its smallest point, I am going to extend the panels out and reinforce them with a layer of fiberglass to keep them from snapping. I am also going to run either 1/4 or 3/8in dowels through the wing section to give more stability and also to make the foam stay on easier.
The main section of the airframe should be strong enough where I would not have to fiberglass the entirety of the panels. I'm trying not to fiberglass the entire panels, just to keep the weight down.
For the foam, I'm going to use blue insulation foam to fill in the areas in between the panels.
For interior tubes, I am going to use a 2.14in tube to a 2.56in tube. The bigger tube will be the upper body tube in order to fit a larger parachute, most likely a 56in parachute.

Going into the motors category:

I will most likely be upping the motor, because as it looks like right now, I am going to up the scale to 1/17 or 1/18 instead of 1/20 (yes, it makes a big difference). I will have to up the motor from the 3-grain I345 to a 4-grain I470. I need the extra oomph to get the 5:1 weight to thrust ratio that I’ll need.

If anyone has any thoughts, please let me know! I’m open to any suggestions! :D

The pictures:

The first one is my 1/72 model kit.
The second one is a test panel that I cut from the 1in thick pine boards to test its strength and weight.

Thanks!

IMG_3029.jpg


IMG_3031.jpg
 

delta22

Well-Known Member
Joined
Jan 21, 2009
Messages
1,015
Reaction score
3
This looks like a very cool project.

Someone at LDRS 28 flew an SR-71 about the size of what you plan. Had a high thrust central motor (Red flame) and 2x small (Smokey) outboards for an awesome liftoff effect.

The pine boards seem extremely heavy for the size of the project. Also because they are not ply they will be weaker accross the grain.

If I were you, I would use 3/8" basswood and rely on a thin fiberglass shell to provide primary strength. Could build it very much like the Estes kit, but with FG to harden and strengthen.

Similar concept to what I am doing on a 2x upscale FlisKits Thunderbird build in progress: http://www.rocketryforum.com/showthread.php?t=8817
 
Last edited:

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
Someone at LDRS 28 flew an SR-71 about the size of what you plan. Had a high thrust central motor (Red flame) and 2x small (Smokey) outboards for an awesome liftoff effect.
My original intentions were to make it flyable as both a cluster and non-cluster. I am still making the engines of the Blackbird into motor mount tubes so if I wanted to I could send it up on a 3 motor cluster. They will probably end up being 38mm tubes and I'll use adapters to bring it down to 29mm if I needed/wanted to. By the way (I think delta22 is Boris) I still have your motor adapter. Sorry :bang:

I will most likely fly it on only the central big motor before trying the cluster to make sure the thing flies safely first. Then for a possibly second flight I'll send it up on a 3 cluster.

This should be a really fun build and a good learning experience for me. This will be my first time fiberglassing so I'll do test runs before I do it on the real thing. Hopefully it turns out the way I want it to.

I'm still taking measurments of the entire thing, figuring out the panel sizes mostly. I should be done with measurements in a week or two then once I get some meterials I'll start posting some pictures.

I'll take a look into the basswood. When you say "thin fiberglass shell" do you mean around the panel or around the entire rocket like I intend to do???
 

redsox15

Well-Known Member
Joined
Jan 28, 2009
Messages
547
Reaction score
0
just a heads up bro,

the fiberglass we have at home is the 6oz stuff...might be a tad heavy for your rocket so use it only where it is needed...6oz fiberglass adds 6oz/square foot so dont go crazy. You and I probably wont be fiberglassing our projects until it gets warmer and we can go outside so it is still a little ways off...

Anyways good start on the project and good luck

your older bro,
matthew
 

dixontj93060

Well-Known Member
Joined
Feb 20, 2009
Messages
12,882
Reaction score
19
Ummm wrong. 6oz. fiberglass weighs 6oz./sq. yard. 9 times lighter than you state.

just a heads up bro,

the fiberglass we have at home is the 6oz stuff...might be a tad heavy for your rocket so use it only where it is needed...6oz fiberglass adds 6oz/square foot so dont go crazy. You and I probably wont be fiberglassing our projects until it gets warmer and we can go outside so it is still a little ways off...

Anyways good start on the project and good luck

your older bro,
matthew
 

redsox15

Well-Known Member
Joined
Jan 28, 2009
Messages
547
Reaction score
0
oops that is correct my mistake got my units messed up...thank you for correcting me


Matt
 

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
also, just as a heads up, as the building and stuff gets underway, I will most likely be moving the build thread over to the "Scale" section of the website as it better suits the project. :cyclops:
 

delta22

Well-Known Member
Joined
Jan 21, 2009
Messages
1,015
Reaction score
3
By "thin fiberglass shell" I suggest 1 to 3 oz FG over the entire exterior for surface hardness and slight strength boost. This is only if you use light, soft underlying materials like balsa or basswood.

Only go with thicker FG if a key area is much too flexible.

If you use thick, or heavy wood, or lots of FG, weight will be a very big problem with this design. The wing area is huge by rocket standards and a heavy build will end up being a flying plank, which could get scary fast in any kind of wind.
 

Handeman

Well-Known Member
TRF Supporter
Joined
Jan 18, 2009
Messages
7,728
Reaction score
324
Location
Stafford, VA
Ummm wrong. 6oz. fiberglass weighs 6oz./sq. yard. 9 times lighter than you state.
oops that is correct my mistake got my units messed up...thank you for correcting me
Matt
Actually, you both may be right. The 6 oz is per square yard is correct, but that is just for the cloth. By the time you add the epoxy resin, it could end up at 6 oz per square foot.

Probably closer to 2 or 3 oz per square foot, but.... who's to say :confused2:
 

dixontj93060

Well-Known Member
Joined
Feb 20, 2009
Messages
12,882
Reaction score
19
A layup, if done correctly, should add no more weight than the cloth. A good rule of thumb is that the final weight is 2x the cloth--in this case 12oz/sq. yd. A 3oz veil layer for example, would end up adding 6oz/sq. yd.

Actually, you both may be right. The 6 oz is per square yard is correct, but that is just for the cloth. By the time you add the epoxy resin, it could end up at 6 oz per square foot.

Probably closer to 2 or 3 oz per square foot, but.... who's to say :confused2:
 

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
alright, all of my measurements are done and now I'm starting a parts list so I can order what I need within the next few days. The rocket will end up being slightly less then 6 feet tall and will have a wingspan of approx. 3 feet. I am adding two extra fins to each of the engines for added stability. This is what Estes did on their smaller model.

Anybody have any suggestions on what size parachute I should go with? I was thinking 36 at first but now I'm thinking that'll be to small. Does 50in with a 5 to 7 inch spill hole seem about right? Also, do you think a 50in parachute will fit into a 2.56in/63mm body tube and be able to pop out?
 

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
another thing, should I go with small or large rail buttons? what are the advantages/disadvantages between the small and large rail buttons?
 

redsox15

Well-Known Member
Joined
Jan 28, 2009
Messages
547
Reaction score
0
small rail buttons bro,

the small rail buttons can handle a rocket weighing up to 20+ pounds and I dont think that your rocket will be weighing that much. If you are ordering rail buttons get me a couple sets of small rail buttons too cuz I need some for my crayon rocket :)
Thanks and good luck with the build

Matt
 
Last edited:

delta22

Well-Known Member
Joined
Jan 21, 2009
Messages
1,015
Reaction score
3
The size chute depends on the weight. One chart here:
http://spherachutes.com/chart.asp

The smaller rail is much more common, so using the smaller rail buttons is more convenient if your rocket is under 20lbs.

Test how your chute is packed and deployed on the ground before flight. It will be a challenge to eject a large enough chute from a small tube.

Another reason to build light, smaller chute required. I would not buy the chute until the rocket structure is completed and you know your flight weight.
 

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
I've got a build question that I was hoping someone could help me out with:

I am using insulation foam boards as filler to make the general shape of the SR-71. Do I have to seal the foam with a layer of anything before I can apply a fiberglass and resin coating? It seems like the insulation foam would just soak up the resin and it would not adhere to anything properly.

If it does have to be sealed, what's the best stuff to use?
 

ben_ullman

Well-Known Member
Joined
Jan 18, 2009
Messages
1,509
Reaction score
0
I've got a build question that I was hoping someone could help me out with:

I am using insulation foam boards as filler to make the general shape of the SR-71. Do I have to seal the foam with a layer of anything before I can apply a fiberglass and resin coating? It seems like the insulation foam would just soak up the resin and it would not adhere to anything properly.

If it does have to be sealed, what's the best stuff to use?
NOPE! Fiberglass right over it. The foam wont really "soak" it up but it will bond to it fairly well.

Ben
 

avets8

Well-Known Member
Joined
Jul 29, 2009
Messages
56
Reaction score
0
NOPE! Fiberglass right over it. The foam wont really "soak" it up but it will bond to it fairly well.

Ben
Sounds good, thanks.

JUST A HEADS UP!!!! If you want to keep following the build, I'm going to be switching the build thread over the "Scale" section of the website, it fits the project a little better. The build thread will be called "1/18 Scale SR-71 Blackbird" since I upped the scale since last time. I have some pictures of the frames that I have built so far that I will be posting over there today. Thanks.
 
Top