2-stage separation

[tim cubbedge]

So I finally flew my WM Jr. 2-stage yesterday. I used a PET2+ timer for separation & booster recovery & an RRC3 for sustainer ignition & deployment. Also had motor back-up for recovery on both booster & sustainer. Few it on an I303 to an H123. The interstage coupler was very loose when fitted in the sustainer. If I picked up the sustainer the booster would slide right out so the separation charge was more of a back-up. Out of the box, the PET2 will fire Event #1 at MECO + 0.7 seconds. Only change I made to PET2 timer was to change time base for Event #2 to extend the timer for recovery. The RRC3 was set to light off sustainer at 3.9 seconds into flight as long as I reached 1200 feet. Booster motor burnout was approximately 2.0 seconds. Sustainer was lit at 4.65 seconds into flight obtained from RRC3 data.

The flight was sucessful for the most part. Only problem that was encountered was at booster burnout, I didn't get separation nor did the PET2 timer have enough "oompf". It did separate but it looked like it was more of a result of sustainer ignition. Still waiting for video of flight to see if there was separation before ignition but it appears it wasn't based on soot on booster. Ground tested the separation charge with 0.3 grams but I guess I'm puzzled why it didn't separate from decelleration alone with such a loose fit. Anyone seen this before?

 

[crossfire]

You need to talk to Jason or Steve who fly with QCRS. They fly 2 stages a lot with very good results.

 

[ColumbiaNX01]

So I finally flew my WM Jr. 2-stage yesterday. I used a PET2+ timer for separation & booster recovery & an RRC3 for sustainer ignition & deployment. Also had motor back-up for recovery on both booster & sustainer. Few it on an I303 to an H123. The interstage coupler was very loose when fitted in the sustainer. If I picked up the sustainer the booster would slide right out so the separation charge was more of a back-up. Out of the box, the PET2 will fire Event #1 at MECO + 0.7 seconds. Only change I made to PET2 timer was to change time base for Event #2 to extend the timer for recovery. The RRC3 was set to light off sustainer at 3.9 seconds into flight as long as I reached 1200 feet. Booster motor burnout was approximately 2.0 seconds. Sustainer was lit at 4.65 seconds into flight obtained from RRC3 data.

The flight was sucessful for the most part. Only problem that was encountered was at booster burnout, I didn't get separation nor did the PET2 timer have enough "oompf". It did separate but it looked like it was more of a result of sustainer ignition. Still waiting for video of flight to see if there was separation before ignition but it appears it wasn't based on soot on booster. Ground tested the separation charge with 0.3 grams but I guess I'm puzzled why it didn't separate from decelleration alone with such a loose fit. Anyone seen this before?

This is helpful for me I am doing a 2 stage. How big are your fins on the booster. Is there enough drag? My fins on my booster are fins to be used on a 4 inch but are on my 3 inch so the idea is there is more drag to separate for drag separation.

 

[tim cubbedge]

This is helpful for me I am doing a 2 stage. How big are your fins on the booster. Is there enough drag? My fins on my booster are fins to be used on a 4 inch but are on my 3 inch so the idea is there is more drag to separate for drag separation.

Fins are identical.

 

[ColumbiaNX01]

Maybe the fins were it. Maybe the booster fins should of been 10-20% bigger. Maybe the booster should of been higher thrust, so that when at motor burrnout the rocket would be going faster at staging. IDk just my guess.

With my 2 stage my booster motor will be supper high thrust I am hoping for 16/1 thrust to wight ratio. I will be relying on drag separation to separate the two.

What was your thrust to weight ratio?

 

[JimJarvis50]

I guess I'm puzzled why it didn't separate from decelleration alone with such a loose fit.

The easiest way to look at the tendency to drag separate is to just look at your simulation results. At the point where you separate the stages, if the acceleration goes up, the pieces will want to drag separate. For example, if the acceleration of stack is -3 G's and the acceleration goes up to -2 G's at separation, then the parts will want to separate, and vice versa. This is a bit of an approximation as the drag of the sustainer when it's with the booster is a bit less than after separation, but you can get a pretty good idea if the parts want to separate or not. You can also vary the separation time to look at the tendency for separation at motor burnout and then after some coast.

Jim

 

[ColumbiaNX01]

The easiest way to look at the tendency to drag separate is to just look at your simulation results. At the point where you separate the stages, if the acceleration goes up, the pieces will want to drag separate. For example, if the acceleration of stack is -3 G's and the acceleration goes up to -2 G's at separation, then the parts will want to separate, and vice versa. This is a bit of an approximation as the drag of the sustainer when it's with the booster is a bit less than after separation, but you can get a pretty good idea if the parts want to separate or not. You can also vary the separation time to look at the tendency for separation at motor burnout and then after some coast.

Jim

Once the booster burns out then the sustainer is heavier so wont it automatically want to separate? The heavier object wants to continue forward. Then the booster is lighter so it drops off.

 

[DrewD]

I have the Wildman darkstar jr 2 stage setup with the PET 2 event timer. I use anywhere from .4g to .5g of FFFF. Makes for a beautiful and powerful separation. And it gives a nice report back too. I also smear the inter stage coupler with some baby powder just to ensure it doesn't hang up.

 

[Rob702Martinez]

Fins are identical.
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Wow, what colors are you using on this bird?

 

[JimJarvis50]

Once the booster burns out then the sustainer is heavier so wont it automatically want to separate? The heavier object wants to continue forward. Then the booster is lighter so it drops off.

Usually, but not always. Boosters that don't have larger fins and perhaps have a larger motor (and heavier motor tube) might not want to drag separate.

Jim

 

[OverTheTop]

Once the booster burns out then the sustainer is heavier so wont it automatically want to separate? The heavier object wants to continue forward. Then the booster is lighter so it drops off.

Depends. The are basically in freefall after burnout, with the addition of aerodynamic drag. The heavier stage has more inertia, but the sustainer has the drag from the frontal exposure to the airstream, and the booster has the base drag from the tail. If there is enough difference they will separate.

 

[tim cubbedge]

What was your thrust to weight ratio?

12.5:1 at ignition, more than enough to get it up off the pad.

 

[tim cubbedge]

The easiest way to look at the tendency to drag separate is to just look at your simulation results. At the point where you separate the stages, if the acceleration goes up, the pieces will want to drag separate. For example, if the acceleration of stack is -3 G's and the acceleration goes up to -2 G's at separation, then the parts will want to separate, and vice versa. This is a bit of an approximation as the drag of the sustainer when it's with the booster is a bit less than after separation, but you can get a pretty good idea if the parts want to separate or not. You can also vary the separation time to look at the tendency for separation at motor burnout and then after some coast.

Jim

Thanks Jim - I will look at this and get back to you if I have any questions.

 

[tim cubbedge]

I have the Wildman darkstar jr 2 stage setup with the PET 2 event timer. I use anywhere from .4g to .5g of FFFF. Makes for a beautiful and powerful separation. And it gives a nice report back too. I also smear the inter stage coupler with some baby powder just to ensure it doesn't hang up.

I talked to a 2-stage flyer at my club and he said he always uses 0.5 grams so I'll probably try that next time. Baby powder probably can't hurt either. What motors have you used for boost?

 

[tim cubbedge]

Wow, what colors are you using on this bird?

Rustoleum Utra Cover 2X Key Lime & Brilliant Blue

 

[tim cubbedge]

Once the booster burns out then the sustainer is heavier so wont it automatically want to separate? The heavier object wants to continue forward. Then the booster is lighter so it drops off.

Usually, but not always. Boosters that don't have larger fins and perhaps have a larger motor (and heavier motor tube) might not want to drag separate.

Jim

In my case, fins are same size. Sustainer weighed 66 oz. loaded & booster weighed ~42 oz at burnout.

 

[cerving]

So, is it better to have the fins staggered or in-line if you're doing drag separation? I would think that staggered would be better, since they're getting more air than they'd get if they were in-line.

 

[claytonbirchenough]

So, is it better to have the fins staggered or in-line if you're doing drag separation? I would think that staggered would be better, since they're getting more air than they'd get if they were in-line.

I wouldn't think it'd really matter that much when they're that far apart... Interested to see what others think though.

 

[jeff2space]

So, is it better to have the fins staggered or in-line if you're doing drag separation? I would think that staggered would be better, since they're getting more air than they'd get if they were in-line.

I too think the fins are far enough apart not to have any effect on drag. Plus you couldn't separate (rotate) them too much without one set or both interfering with launch rail.

My question is if you don't want to rely on drag separation, how do you separate the stages? BP charges? Is charge on interstage pointed towards sustainer or the other way around?

 

[tim cubbedge]

My question is if you don't want to rely on drag separation, how do you separate the stages? BP charges? Is charge on interstage pointed towards sustainer or the other way around?

Well it looked like mine was separated from sustainer ignition (not planned though). I had a BP charge in the interstage coupler bulk plate which shold have been enough to separate (at least it did on the ground) but it's been suggested that I increase my BP charge from 0.3 grams to 0.5 grams. The charge is pointed toward the sustainer.

 

[DrewD]

My question is if you don't want to rely on drag separation, how do you separate the stages? BP charges? Is charge on interstage pointed towards sustainer or the other way around?

For me, I just took a small plastic baggie, stuck an ematch through the corner, poured .5g bp and taped it up to a very flat 1" by 1" square and stuck it right under the motor nozzle cap. Works like a charm!

 

[DrewD]

I talked to a 2-stage flyer at my club and he said he always uses 0.5 grams so I'll probably try that next time. Baby powder probably can't hurt either. What motors have you used for boost?

I have used the Aerotech H550 SBT, Aerotech I245G, and CTI I800 VMAX. The H550 was by far the best. Long blue flame that got the rocket off the pad quick, and kept it low enough that everyone on the ground could actually see the separation and the two halfs drifting apart, and even see the sustainer ignite. Booster altimeter reported an apogee of 600'. Separation was probably around 400-500'

 

[DrewD]

Here's a video of mine on a AT H550 to a CTI H110

https://youtu.be/A9GFK03kJF8

 

[tim cubbedge]

The easiest way to look at the tendency to drag separate is to just look at your simulation results. At the point where you separate the stages, if the acceleration goes up, the pieces will want to drag separate. For example, if the acceleration of stack is -3 G's and the acceleration goes up to -2 G's at separation, then the parts will want to separate, and vice versa. This is a bit of an approximation as the drag of the sustainer when it's with the booster is a bit less than after separation, but you can get a pretty good idea if the parts want to separate or not. You can also vary the separation time to look at the tendency for separation at motor burnout and then after some coast.

Jim,

Here is a plot from open rocket with the actual setting for launch. Basically a 2 second delay from booster burnout to sustainer ignition. If I am reading the data correctly, the accelleration went up from -2.1 G's to approximately -1.6 G's. Is that not enough to separate? Maybe a different motor for boost?

 

[JimJarvis50]

Jim,

Here is a plot from open rocket with the actual setting for launch. Basically a 2 second delay from booster burnout to sustainer ignition. If I am reading the data correctly, the accelleration went up from -2.1 G's to approximately -1.6 G's. Is that not enough to separate? Maybe a different motor for boost?

View attachment 307227

What you're looking for is the difference just before and after the stage separation (not the difference between burnout and the separation). In your simulation, you need to separate the times for stage separation and then motor ignition so that you have a bit of time where the sustainer is coasting (i.e., add some ignition delay). Then, you look at the difference in the acceleration between the stack and the sustainer at the separation point. Then, if you want, you can change the separation time to some other value (followed by a little coast time) to see the tendency to drag separate at different speeds).

Jim

 

[tim cubbedge]

What you're looking for is the difference just before and after the stage separation (not the difference between burnout and the separation). In your simulation, you need to separate the times for stage separation and then motor ignition so that you have a bit of time where the sustainer is coasting (i.e., add some ignition delay). Then, you look at the difference in the acceleration between the stack and the sustainer at the separation point. Then, if you want, you can change the separation time to some other value (followed by a little coast time) to see the tendency to drag separate at different speeds).

Understand now. I didn't realize I could change separation time in OR

So if I plot separation as you say, what I am seeing at the expected separation time is the booster appears to go from -2 Gs to about -3.5 Gs (opposite of what you said earlier??) and the sustainers is rather constant. The booster is slowing down more than the sustainer, correct? If so I should have had separation? Am I reading this right?

 

[JimJarvis50]

Understand now. I didn't realize I could change separation time in OR

So if I plot separation as you say, what I am seeing at the expected separation time is the booster appears to go from -2 Gs to about -3.5 Gs (opposite of what you said earlier??) and the sustainers is rather constant. The booster is slowing down more than the sustainer, correct? If so I should have had separation? Am I reading this right?

View attachment 307273

It looks to me like the change in acceleration is from -2 G's just before separation to about -2.1 G's just after. This is comparing the acceleration of the stack (before separation) to the sustainer only (after separation). The acceleration shown is just another way to say whether the rocket is slowing down or speeding up. In your simulation, the deceleration is a bit more (at -2.1) for the sustainer than for the stack. So, the simulation is saying that the sustainer, after separation, will slow down a bit faster than the stack prior to separation. At first blush, that would say that the parts won't separate.

The problem with this approach is that you want to know what the sustainer drag is while it is still with the booster rather than after it separates (which is what you get with the simulation). The difference in the two cases is the base drag of the sustainer, which is not present in the stack configuration, but is present after separation. I don't really know if it's particularly rigorous, but the way I account for this is to determine the percentage of the total drag on the sustainer that is base drag and then adjust the sustainer deceleration to account for it. Rocksim gives the drag coefficients associated with the nose/body, the fins, and the base drag. I don't know if open rocket does this or not. But let's say the base drag is 10% of the total for the sustainer at the separation velocity of interest. If you have -2.1 G's total for the sustainer, then -1 G is due to gravity and -1.1 G's is due to drag. If base drag is removed, then the acceleration due to drag drops to 0.9 x -1.1 = -0.99 g's, or a total deceleration of -1.99 G's. So, you have -2G's for the stack and -1.99 G's for the sustainer when it's part of the stack. Clear as mud? With that result, you might conclude that there is essentially no force between the parts, and they might separate or they might not. Sort of consistent with your flight results.

By the way, if anyone out there thinks I'm wrong with the above approach, please speak up. I need to do this calculation from time to time, and if I'm off base, I'd like to know.

Jim

 

[tim cubbedge]

Jim - quite clear and I believe OR will provide drag coefficients for components as well. I'll dive into that next! As I think I stated earlier, this was my first 2 stage and was quite a bit of fun and a bit more challenging than my other projects. Thanks again for all your help!

 

[BDB]

Jim - quite clear and I believe OR will provide drag coefficients for components as well. I'll dive into that next! As I think I stated earlier, this was my first 2 stage and was quite a bit of fun and a bit more challenging than my other projects. Thanks again for all your help!

I just wanted to say thanks for posting this conversation. I learned a ton by reading this today.

 

[tim cubbedge]

Just an update. Made some minor changes to ISC. Increased separation charge to 0.5 grams and put some baby powder on ISC. Launched it again yesterday and separation clearly occured before sustainer ignition!