Discus Launched Glider Rocket Boost Help

Hello,

I'm new to the forum. I don't have any real experience with with rockets other than online reading so I have a few questions about my new project. I have a Libelle Discus Launched Glider. It flies great, however, I am rarely able to hand launch the glider high enough to catch any significant ridge or thermal lift. So I'm working on a rocket boost system based on Estes 1/4 A3-2T motors. I could just get another model but that would be as much fun.......:eyeroll:

I have molded a "pod" out of thin fiberglass as well as a rocket motor tube that weighs in @ 1oz. Due to its shape the pod is very strong and will easily survive firings and landings. It will be mounted to the fuselage by two dowels and a nylon screw. The rocket motor tube will be "capped". The model requires a little over an ounce of weight in the nose so the weight of the pod/rocket motor should not require much if any counter weight...

I put a cotton wad over the ejection charge and caped the front end of the 1/4 A3-2T motor closed with hot glue. Hand launching the glider results in approximately a 2g acceleration so I don't think that this motor will over-stress the airframe in a stable flight attitude as the models flying weight is approximately 11oz.

My questions are:

1. What is the best angle to mount the motor in the pod? I'm guessing about a 10 to 20 degree down angle to keep the exhaust gasses away from the airframe. Can anyone comment on this?

2. Since my onboard battery is 5v and limited current I don't think that will be enough to fire the igniter. Has anyone done anything with an application like this? Weight is a concern so additional batteries aren't really an option. I haven't experimented with it yet, but I thinking of creating a small circuit to use the discharge of a capacitor to fire the igniter triggered by an ultra micro servo as the switch.

Here are some picture of the project so far. Thanks all input is appreciated.

Your intent is to air start this after hand launch? an 1/4A won't do much for an 11 ounce glider with a .2 second burn, even an A-34T with a 1 second burn won't do a whole lot.
You also want to be careful using heat on the end of the motor, even though it is a clay cap, there might be some residual bp that might heat ignite, I think epoxy is safer.
For the igniter, they don't take a lot of current, but using the same battery as the receiver battery might cause a small brownout. I know a 2s lipo, even small will fire them no problem.
There are small e-switches you can buy as well to do the ignition...you need to be careful with servos on powerup that they don't glitch and fire the igniter early. Best to mix the igniter switch to two channels so that you have less chance of a glitch firing.

The18mm reloadable D2.3 at this weight class is a good choice for the motor, it weighs about an ounce loaded however, but has a very long burn time.
BP motors in the C class don't weigh a lot less.

https://www.nar.org/wp-content/uploads/2014/08/D2.3.pdf

I'm sure Brian Guzek will chime in, this is more in his area of expertise than mine.

Frank

As Frank said, a 1/4A will do almost nothing to a glider this size. I had a glider that weighed 155g (including motor) that barely made 50m on a C11. An Estes D12, E9, or E12 would probably be more appropriate for something this size. I'm pretty sure it would even stand up to the new F15, if you wanted to try that.

As far as pods and thrust angles, I would recommend an over-the-wing pod for a Libelle. It doesn't change the balance of the glider and keeps the excess mass near the CG. Placing mass away from the CG on a DLG degrades the performance of the discus launch. You can put a high thrust angle in, but the exhaust will be entrained in the free-stream flow, so it may still strike the tail. When we fly high-performance rocket-boosted gliders, both the horizontal and vertical tails are kept below the boom and out of the rocket exhaust. The European style is the inverse, rocket below, tails above; this has the same effect of keeping the tails out of the exhaust, but it has some aerodynamic negatives such as exacerbating the tendency of an aircraft to pitch up as speed increases.

Ultimately, you would be best served by choosing either the discus launch or the rocket boost, but it is possible to maintain both at some penalties on both fronts.

Also, don't forget to follow: https://www.nar.org/safety-information/radio-control-rocket-glider-safety-code/

burkefj said:

Your intent is to air start this after hand launch? an 1/4A won't do much for an 11 ounce glider with a .2 second burn, even an A-34T with a 1 second burn won't do a whole lot.
You also want to be careful using heat on the end of the motor, even though it is a clay cap, there might be some residual bp that might heat ignite, I think epoxy is safer.
For the igniter, they don't take a lot of current, but using the same battery as the receiver battery might cause a small brownout. I know a 2s lipo, even small will fire them no problem.
There are small e-switches you can buy as well to do the ignition...you need to be careful with servos on powerup that they don't glitch and fire the igniter early. Best to mix the igniter switch to two channels so that you have less chance of a glitch firing.

The18mm reloadable D2.3 at this weight class is a good choice for the motor, it weighs about an ounce loaded however, but has a very long burn time.
BP motors in the C class don't weigh a lot less.

https://www.nar.org/wp-content/uploads/2014/08/D2.3.pdf

I'm sure Brian Guzek will chime in, this is more in his area of expertise than mine.

Frank

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Yes, I really don't need much. The discus launch gets me up to 40'-50' and I'm only really looking for a total of 80' to 100' to catch ridge lift of tree rows and thermals. I've already capped the motors I have. I used cotton to insulate the cap before I used the hot glue, no issues whatsoever. I was thinking the current draw for the igniter could be an issue. I only get an hour per charge as it is and I really don't want to drop that too much. I'll do som research on those motors. Thanks.

Thanks for the replies. A 2g discus launch with this glider is about standard and good for about 40 to 45 mph or approximately 40' level off height and I'm really only looking for another 30' to 40'. I have a few of the smaller motors so I will try them just because once I finish the pod and design an ignition circuit. The A10-P3 with a .8s duration and just under three pounds of thrust should give me at least 2g's of acceleration and approximately a 40+ mph speed gain and give me what I'm looking for and may be worth a try.

I used part of a cotton ball to insulate the top of the motors and the hot glue and it worked with no issue.

The elevator and rudder are above the fuselage and IMO would be in more danger with a top mounted motor. I'm hoping the aerodynamic shape of the pod and it's light weight will result in minimal drag and aerodynamic penalties.

Using the ships battery is a concern for brown out and longevity reasons. I'm open for light weight suggestions.

I'll be in and out of town with work the next few weeks and I will report on its progress as I can . Thanks for your input.

Probably a high-start would be simpler than a rocket motor unless you have a special interest in rockets. https://members.optusnet.com.au/~rlearmont/HiStart.html

mikec said:

Probably a high-start would be simpler than a rocket motor unless you have a special interest in rockets. https://members.optusnet.com.au/~rlearmont/HiStart.html

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Thanks, that looks interesting. None of my locations have the required space for a high-start though.

MachTuck said:

Thanks for the replies. A 2g discus launch with this glider is about standard and good for about 40 to 45 mph or approximately 40' level off height and I'm really only looking for another 30' to 40'. I have a few of the smaller motors so I will try them just because once I finish the pod and design an ignition circuit. The A10-PT with a .8s duration and just under three pounds of thrust should give me at least 2g's of acceleration and approximately a 40+ mph speed gain and give me what I'm looking for and may be worth a try.

I used part of a cotton ball to insulate the top of the motors and the hot glue and it worked with no issue.

The elevator and rudder are above the fuselage and IMO would be in more danger with a top mounted motor. I'm hoping the aerodynamic shape of the pod and it's light weight will result in minimal drag and aerodynamic penalties.

Using the ships battery is a concern for brown out and longevity reasons. I'm open for light weight suggestions.

I'll be in and out of town with work the next few weeks and I will report on its progress as I can . Thanks for your input.

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I'm not trying to discourage you, but I'm not sure you understand the physics behind the situation. Ideally, the impulse of a motor is equal to the change in momentum, that is:

J=m*v

where J is impulse, m is mass, and v is velocity. In reality, the change in velocity is lower due to gravity and drag losses. Your glider masses about 313g (11oz). A 1/4A3-3T has 0.59 N-s of impulse. Ideally, this translates to 1.9m/s or 4.2MPH. The change in velocity is, in reality, even lower. To get 40MPH (18m/s), you need a minimum of 5.6N-s of impulse, or a baby C motor. Again, drag and gravity will increase this requirement significantly. Based on my experience launching this type of glider, a D12 or an E9 is the minimum amount of motor for a glider of this size.

For safety's sake, you would probably be best served by a ground-started rocket motor. Please note that the NAR RCRG safety code requires at least 100' of altitude for an airstart, or that the motor be airstarted in a vertical orientation as on launch. You have clearly stated that you only get about 40' out of a discus launch, which means that you will either be too close to the model to safely and reliably airstart the motor or you will be lower than 100' in gliding flight.

Again, I don't want to discourage you from pursuing a rocket-boosted glider, but please reconsider the safety aspects of the flight profile you are considering.

Here is an example of the launcher for an RCRG from the 2012 WSMC. This is Kevin Kooch's Pushit3 flying out of his purpose built glider tower.

Hi. I did a quick sim in the latest OR using the simple rocket and adding 200gm for an all up weight of 254gm. [about 9 oz] Using the 1/4 A3 it did .9Gs for a max altitude of about 2 feet. I think the Libelle really does need a lot more motor. That said, go ahead with your plans it will be good experience at the least and provide some visual effects too.

BTW I fly RC gliders and rocket gliders. Since practice with S8Ep gliders can be expensive I use a fishing pole launcher when a high start is not available at the club field. Works well enough.


Richard

What Brian said while I was typing c:

Thanks for the input guys. The math says it all. The weight penalty of the motor required renders this a non starter.

As per the OP yes but it can be done with a better motor and a little reconfiguration of the tail feathers. Should the tail ever be damaged then the replacement could be a removeable pair one of which would serve as a rocket glider tail and the other for DLG. The motor pod could be removeable also so no penalty there.

S8Ep gliders are about that size, though a little lighter and fly on Ds and Es. There are plenty of RCRG flyers here to help you out if you want it.


Richard

TheTellurian said:

As per the OP yes but it can be done with a better motor and a little reconfiguration of the tail feathers. Should the tail ever be damaged then the replacement could be a removeable pair one of which would serve as a rocket glider tail and the other for DLG. The motor pod could be removeable also so no penalty there.

S8Ep gliders are about that size, though a little lighter and fly on Ds and Es. There are plenty of RCRG flyers here to help you out if you want it.


Richard

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The larger motors pose a significant weight penalty. My plan is to complete the "Pod" and make some flights at varying pod weights to see what the aerodynamic penalties are. Basically, I'm trying to do something with a glider that it wasn't designed for. I'm home for a few days and back on the road. Don't think I'll have time to work on it this week. I will post progress/test flight info when I'm able to.

Thanks for all the replies.

This is kind of an advanced topic for rocketeers...the problem is that getting stable flight for both boost and glide requires a significant shift in the CG, a change in configuration, or both. Check out Stine ' S Handbook of Model Rocketry to get a better idea of what you're getting yourself into.

Maybe what you need is a motor glider with electric or small - glow - plug nitro power?

Actually one can make that work to your advantage if the rocket glider is designed to have the motor ahead of the CP. All S8Eps are made that way.

Put a slimer motor on my RCRG! uke: :rofl:


Richard