SAAB RB05A/98

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egp

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Now that I’m basically done with the Darkstar, I started some work on the Saab RB05A/98 which I picked up from Wildman 6 months ago. This is a scale model of the Saab air-to-surface missile that was developed in the 1960s. The model has a 75mm motor mount, has 6 centering rings, and is going to need a bunch of weight up in the nosecone, 4-6 pounds is my estimate, although I do need to sim a few more motors.

IMG_4042.jpg

IMG_4034.jpg

I’ll post more photos as I make progress.
 
Not sure why there are 6 CRs.

I would be inclined to ditch at least three of them and possibly five. I would use one at the top, possibly double it up if it was being used as an anchoring point for the harness, that is if you wanted the extra comfort. Then I would use the bottom one as a thrust plate/CR. Don't forget the TTW fins will also act like centering rings for your MMT.

I do like this rocket though.
 
Ditching three CR bad idea. Lot of force by motor. I did ditch one CR on the RB-05A 38mm MMT sport, but it's an L-1. Haven't flown the 98. Guess you'll get a bigger flame. The smaller Wildman SAAB kit needs a pound of lead. And if they say no, they haven't flown it and are talking sh*t.
 
Not sure why there are 6 CRs.

I would be inclined to ditch at least three of them and possibly five. I would use one at the top, possibly double it up if it was being used as an anchoring point for the harness, that is if you wanted the extra comfort. Then I would use the bottom one as a thrust plate/CR. Don't forget the TTW fins will also act like centering rings for your MMT.

I do like this rocket though.

With a 75mm motor mount and a 98mm body, there’s not much room for hardware in the CR for mounting the harness. I’m planning to take the common approach and epoxy it to the motor mount. With the tabs on the fins, the 6 CRs do permit injecting of the internal fillets. So, in my opinion it is not that big of a deal. Should be a fun build.
 
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Ditching three CR bad idea. Lot of force by motor. I did ditch one CR on the RB-05A 38mm MMT sport, but it's an L-1. Haven't flown the 98. Guess you'll get a bigger flame. The smaller Wildman SAAB kit needs a pound of lead. And if they say no, they haven't flown it and are talking sh*t.
Are you suggesting that there will significant lateral forces. I would say with four fins you could use a thrust plate, no CRs and anchor your harness in the forward closure.
 
Ditching three CR bad idea. Lot of force by motor. I did ditch one CR on the RB-05A 38mm MMT sport, but it's an L-1. Haven't flown the 98. Guess you'll get a bigger flame. The smaller Wildman SAAB kit needs a pound of lead. And if they say no, they haven't flown it and are talking sh*t.

Thrust forces are transferred to the airframe through the fins (and in some cases a thrust plate, which some use the bottom CR for), not the CR's themselves.

The amount of nose weight required for stability depends on how the rocket was built. More or less weight may be needed depending on the amount of CR's installed, amount of epoxy used, foam, motor retention, and other items that affect weight. Please don't install 1 lb of nose weight in the smaller SAAB kit because Andrew _ASC said so.

This post is provided courtesy of the Andrew_ASC advice-correcting system.
 
Thrust forces are transferred to the airframe through the fins (and in some cases a thrust plate, which some use the bottom CR for), not the CR's themselves.

The amount of nose weight required for stability depends on how the rocket was built. More or less weight may be needed depending on the amount of CR's installed, amount of epoxy used, foam, motor retention, and other items that affect weight. Please don't install 1 lb of nose weight in the smaller SAAB kit because Andrew _ASC said so.

This post is provided courtesy of the Andrew_ASC advice-correcting system.

[emoji23]
 
having not built this kit, nor anything larger, I make a humble suggestion:

The 6 CRs are probably to: 2 in the traditional places, top & bottom. Then 2 each to straddle the fin tabs, so when you're make the 'injected internal fin fillets' (or foam) they have a dam top & bottom to not ooze out. (this is just a guess, based on the photo provided.. and having build a DS Jr. )

But, as others have suggested.. two should be plenty, with the fin tabs well bonded to the MMT. Maybe an additional one in the middle for good measure..


Don't forget to sand & scratch (not scuff & polish) them tubes & tabs!
 
When that L-2 or L-3 motor goes punching through the top of the airframe blowing the nosecone off, from a CR failure at surface bond on MMT from too much vertical shear forces on epoxy bond joints of reduced quantity, you'll understand why reducing CR's is a bad idea. It's not funny when an L-3 motor takes flight itself without an airframe. I'm just not even going to comment to these people who recommend sacrificing airframe structural integrity by half or more reduction in joint area and CR surface area with large motors. Steven Shannon and others have calculations to determine when it fails. If I was building the 98, I'd use all six CR. Better to be overbuilt than under. That safety factor.
 
When that L-2 or L-3 motor goes punching through the top of the airframe blowing the nosecone off, from a CR failure at surface bond on MMT from too much vertical shear forces on epoxy bond joints of reduced quantity, you'll understand why reducing CR's is a bad idea. It's not funny when an L-3 motor takes flight itself without an airframe. I'm just not even going to comment to these people who recommend sacrificing airframe structural integrity by half or more reduction in joint area and CR surface area with large motors. Steven Shannon and others have calculations to determine when it fails. If I was building the 98, I'd use all six CR. Better to be overbuilt than under. That safety factor.

Overbuilding the rear of this particular airframe is not the best idea, in my opinion and experience....
 
The Wildman said that the 6 are for injecting! So, if injecting, then use one on the leading and trailing edges of each fin and the top and bottom of the motor tubes.

If not, maybe skip a few rings. I used four, and added a hunk of weight to the nose. Flew once on a K740; great flight, but man that's a lot of mass in the top! John Olevich and I were talking about the 4" Saabs over the weekend...he built his with about 25% of the noseweight as mine and his flew okay the first flight, but squirrely the second. Some info about the 4" Saab in the link below, if interested.

https://www.rocketryforum.com/showthread.php?121242-A-Saab-Story

Great rocket; just wish it were more naturally stable. Those true-scale missiles will do that to you!:p
 
Overbuilding the rear of this particular airframe is not the best idea, in my opinion and experience....
I ditched the forward most CR that didn't support any fin tab loadings and also used Cotronics on the CR to MMT to airframe from it being aft CG with negative stability margin as is. I then used rocketpoxy on the outer fillets and also on the fin tabs. I did not fill mine. Mine isn't an L-3 and I don't have the experience on that matter. The rocketpoxy was about 143grams for 16 fillets on the L-1. That was more epoxy mass than tube mass for mine. This model has a lot of aft mass and needs a lot of counterweight at a forward position. Ditching one CR was five grams.

When I added enough lead with rocketpoxy to forward nose, it requires a bigger chute, and heavier recovery gear for motor H to I range. The nose ended up weighing more than the body tube and fins. His problems will be scaled up. These fly nice once balanced. Mines at 1.5-1.7 Stability in the motors I like. It's not stable out of the box and requires careful attention compared to other kits with manuals or better stability ranges. It's not as easy as a min diameter build due to all the counterweight required to fly stable. I wouldn't fill this entire model with epoxy between fins for CG reasons but maybe I'm an idiot. I'm not qualified to comment on the L-3 build. He might want to look at previous build threads for this. I don't think I was overbuilding mine, but I may have used more nose mass than absolutely necessary.
 
I wouldn't fill this entire model with epoxy between fins for CG reasons but maybe I'm an idiot.

Don’t fill the entire model, or any rocket with TTW construction, with epoxy between fins. Nobody has recommended this.

For information on how to inject internal fillets, see the sticky titled “How To Build A “Stock” 3IN DARKSTAR CJ Step By Step” in the High Power Rocketry (HPR) Forum here: https://www.rocketryforum.com/showt...-quot-Stock-quot-3IN-DARKSTAR-CJ-Step-By-Step Items 34-36.

This post is provided courtesy of the Andrew_ASC advice-correcting system.
 
You CAN inject foam between the fin tabs (Wildman is big on foaming...) but you'd have to leave out the two middle centering rings. Personally, I'm a big fan of fin pockets... this seems to be a really good example of a build that would benefit from them. They do add a little bit of weight, but I'd be willing to bet that the big heavy nose cone and whatever other hardware goes with it will more than offset it.
 
I'm cool with building this kit, including balancing the model with 4-5 pounds of lead in the nosecone. I plan to inject my internal fillets, and use all of the CRs. By the way, Andrew_ASC, I do not plan to use it for a certification flight, so don't refer to it as a L3 rocket. Instead, it has a 75mm motor mount, and will fit a 3-grain motor (mid-size L-motor), although I haven't computed the CG with a bunch of candidate motors yet. I suspect that I'll probably fly it mostly on K-motors, but the devil is in the details, so to speak.

As far as I can tell, there is only one "build" thread for this kit: https://www.rocketryforum.com/showthread.php?132761-SAAB-RB-05A-98-Build. There, farsidius used 48 ounces of powdered lead plus the weight of epoxy. He flew a CTI K661 Blue Streak to 8060 ft, and was able to get it down on a 36" drogue and a 58" main parachute.

I've now sanded the CRs and motor mount tube, and tacked the CRs on using CA. As soon as I get the kevlar harness from Wildman, I'll start working on this kit again. In the meantime, I have a Darkstar Ultimate to fly this weekend, including a full kit of missileworks avionics: RRC3, RRC2, and RTx.
 
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