# HP 2 stage question

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#### wyo17

##### Well-Known Member
Looking for some advice, have built and flown a couple of HP 2 stage rockets, with pretty good success. The 1st was a 4 Dominator, 13 tall, boosting on a CTI K-2045 to a CTI K-445. [video=youtube;jMa_eijr4EY]https://www.youtube.com/watch?v=jMa_eijr4EY[/video]
And have since flown it on boosting on a CTI L-1720 to a L-3200.
Got 5 flights with it each time lighting the sustainer from a Raven 3rd channel in the sustainer.
The inter-stage coupler is built to withstand sustainer ignition separation, hence coated with a very thick application of West System epoxy.
The L to L flight showed a little bit of wobble with the inter-stage coupler after the boost which provided a little off vertical axis by about 5 degrees. Landed about a mile away.
Looking at doing an M to M at Balls this year to about 40k. Thinking of boosting on an M-1101 to an M-840. Get the long burn on both the bottom and top. 4.7s boost to a 9s sustainer.
Currently the inter-stage coupler has an Adept 22 solely for recovery. Ignition of the sustainer is from the Raven in the second stage.
Questions:
1. Should I change the altimeter in the inter-stage coupler to provide a separation charge and then deployment charge of recovery stage of booster? If so should I put shear pins in the coupler?
2. How close should the separation of the booster and sustainer ignition should it be?
3. Thinking of controlling ignition of the sustainer from the ground, in the event it is off axis. The M-1101 is enough to get the sustainer to an altitude to say a 1500-2000 main deployment.

Thanks

Pat

#### Kip_Daugirdas

##### Well-Known Member
Looking for some advice, have built and flown a couple of HP 2 stage rockets, with pretty good success. The 1st was a 4 Dominator, 13 tall, boosting on a CTI K-2045 to a CTI K-445. [video=youtube;jMa_eijr4EY]https://www.youtube.com/watch?v=jMa_eijr4EY[/video]
And have since flown it on boosting on a CTI L-1720 to a L-3200.
Got 5 flights with it each time lighting the sustainer from a Raven 3rd channel in the sustainer.
The inter-stage coupler is built to withstand sustainer ignition separation, hence coated with a very thick application of West System epoxy.
The L to L flight showed a little bit of wobble with the inter-stage coupler after the boost which provided a little off vertical axis by about 5 degrees. Landed about a mile away.
Looking at doing an M to M at Balls this year to about 40k. Thinking of boosting on an M-1101 to an M-840. Get the long burn on both the bottom and top. 4.7s boost to a 9s sustainer.
Currently the inter-stage coupler has an Adept 22 solely for recovery. Ignition of the sustainer is from the Raven in the second stage.
Questions:
1.Should I change the altimeter in the inter-stage coupler to provide a separation charge and then deployment charge of recovery stage of booster? If so should I put shear pins in the coupler?
2.How close should the separation of the booster and sustainer ignition should it be?
3.Thinking of controlling ignition of the sustainer from the ground, in the event it is off axis. The M-1101 is enough to get the sustainer to an altitude to say a 1500-2000 main deployment.

Thanks

Pat
1.) I would separate after boost. Let the stages drag separate and have a backup charge if they don't a second after burnout. The reduction in drag here will help your altitude and you'll still have plenty of inertia with the loaded motor in the sustainer.

2.) I shot for ignition around 750 ft/s on my high altitude two stager. That was about a 9 second coast. Everyone has a different opinion here. Take into account the upper level winds. The higher the wind speed at the altitude you will stage at, the faster your rocket should be traveling. As a general rule I would avoid anything slower than 600 ft/s. Also assume it takes your motor a couple seconds to come up to pressure after popping the igniter.

Short coast: higher sustainer velocity, lower altitude. But a better chance of a vertical flight.

Long coast: Lower sustainer velocity, higher altitude. Greater chance of gravity turning, weathercocking, off vertical ignition.

3.) You can use your Raven for off axis logic (altitude/time). Your velocity data from any avionics will likely not match your simulation so I would avoid using it for staging criteria. Or use Altus Metrum products which have tilt sensing.

For two-stagers a good sim is critical. Know your flight weights before setting up your staging avionics.

Sounds like a fun project! Good luck!!!!

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#### FredT

##### Well-Known Member
I recommend a high thrust, short burn motor in the booster. Getting off the pad fast and straight gives the best chance for a vertical trajectory, especially with the typical ground winds.

#### Eat, sleep, and Fly

##### Well-Known Member
I would absolutley use a seperation charge. I prefer to seperate the booster a couple seconds after motor burnout, as it's weight and drag are initially less than it's inertia, menaing that is it contributing to overall altitude. If you really wanted to you could calculate out the inertia vs. drag and find the ideal seperation time. This being said, the ideal seperation comes from drag, though drag seperation cannot be relied on by it's self. If you want drag seperation (and even if you dont) do not put shear pins in the interstage coupler. If the seperation charge and drag seperation fail, you run the risk of firing a motor into the isc and pushing out the contents of the isc. I had this occur last year. Not only was the coupler used in the isc ruined, the electronics in the booster were too, leading to a lawn dart. Hope this helps.

#### FredA

##### Well-Known Member
I prefer to seperate the booster a couple seconds after motor burnout

Why wait?
The ideal separation time is right at booster burn out.
Dump the drag of the booster and transfer as much energy to the sustainer as possible.
It's all "downhill" once the motor is done.

The only wait should be to sustainer ignition.
That should be as long as possible with the caveat that you need to watch velocity, tilt and not to go transonic (negatively)

#### JimJarvis50

##### Well-Known Member
I prefer to seperate the booster a couple seconds after motor burnout

Why wait?
The ideal separation time is right at booster burn out.
Dump the drag of the booster and transfer as much energy to the sustainer as possible.
It's all "downhill" once the motor is done.

The only wait should be to sustainer ignition.
That should be as long as possible with the caveat that you need to watch velocity, tilt and not to go transonic (negatively)
I often wait a little to separate just because I like my boosters and don't want to see them shredded. Setting them free at Mach 1.8 doesn't seem like a good idea to me. They can survive Mach 1.2 based on my experience, and it is not always the case that hanging on to the booster will cause a significant reduction in performance. This can be easily simulated.

I have never done a two/three stage flight that didn't go transonic (above Mach to below before staging). Among other things, that keeps the top speed down.

My approach is for composite rockets. Metal rockets can be flown differently.

Jim

#### soopirV

##### Well-Known Member
Nice flight! No experience yet with multistage HP, but my one piece of advice is get a tripod!

#### Eat, sleep, and Fly

##### Well-Known Member
I prefer to seperate the booster a couple seconds after motor burnout

Why wait?
The ideal separation time is right at booster burn out.
Dump the drag of the booster and transfer as much energy to the sustainer as possible.
It's all "downhill" once the motor is done.

The only wait should be to sustainer ignition.
That should be as long as possible with the caveat that you need to watch velocity, tilt and not to go transonic (negatively)
Just different approaches. My logical reasoning (and what i have found through simulation) is that by holding on to the booster for a few seconds, or more like a fraction of a second, the energy carried by the mass of the booster will increase altitude. This is how I do it, but it is just my opinion and is not fact. My apologies for presenting it as such in my original post. It should be noted that I tend to fly heavier rockets with vmax booster motors, certainly not the same situation as the OP. The other, and far more important reason I do it is that otherwise the booster would be recovering from a few hundred feet, it's much saver to recover from 700'-1000'. Once again, very different from what the OP needs to worry about. If the goal is to maximize altitude then it is worth running the sims for your individual rocket/ motors. If not then my answer would be to separate sometime between booster burnout and sustainer ignition, that is all that is really required for a safe flight. All this being said, the real point I wanted to get across in my original post was that shear-pinning the isc to the sustainer provides additional risk with no real advantage.

Also, I would love to hear what the OP's plans are about lighting the sustainer from the ground. If it is a remote control to actually light it that sounds risky. Stress filled split second decisions don't always go well. I would be more intrigued by a "no go" button, or something of the sort. This way it would be mostly up to the altimeters or tiltometer to decide not to light the sustainer, with an extra layer of redundancy added for some other unforeseen event. That being said I cannot think of what might occur that a tiltometer and well programmed altimeter wouldn't catch, but that is not to say that it couldn't happen. I would NOT rely on this hypothetical "no go" button as the only means of sustainer inhibition. Unless it is there for the purposes of experimentation, you should be just fine with a combination of an altimeter to make sure it is high enough and moving fast enough, as well as a tiltometer to ensure it is on axis. Altus Metrum makes some electronics that could do all of this, though I have no personal experience with them.

#### FredA

##### Well-Known Member
Sheer pinning a ISC will control the separation until you command separation.
There won't be any separation on a chuff -- ask JimJ about that....

It will also lock the spin rates for better or worse.

I'd like to see somebody make a stab at a real physics explanation for any increased sustainer performance achieved by any separation delay....I say it isn't possible. The stack's velocity is peak at booster burn out and the drag on the booster must be eliminated ASAP.

Now if you want a slower, stunted flight [or whatever] do what you want. But for peak performance you need to separate right away.

#### JimJarvis50

##### Well-Known Member
Sheer pinning a ISC will control the separation until you command separation.
There won't be any separation on a chuff -- ask JimJ about that....

It will also lock the spin rates for better or worse.

I'd like to see somebody make a stab at a real physics explanation for any increased sustainer performance achieved by any separation delay....I say it isn't possible. The stack's velocity is peak at booster burn out and the drag on the booster must be eliminated ASAP.

Now if you want a slower, stunted flight [or whatever] do what you want. But for peak performance you need to separate right away.
I can certainly say it's necessary to shear pin the ISC to the booster air frame. It may seem obvious, but I used to not do it on one of my two stagers until this flight:

https://youtu.be/XqvR8xslvHI

Instead of drag separating above the coupler, the drag separation happened below it. Lucky to make it through that flight, and lesson learned.

As far as pinning the coupler to the sustainer, I generally do it because if I'm using stabilization or if I have wires going to the bottom of the sustainer, I can't allow the booster to turn relative to the sustainer. In the absence of something like that, I'd say pinning to the sustainer is optional, but I think it would be a good idea to do it if the design permitted. I like to control the timing of the flight events.

In most cases, getting rid of the booster will improve performance. However, there are real designs where this is not the case. These show up when the booster fins are relatively small or where the booster remains "heavy" after motor separation or where the sustainer motor is small, etc. I'd guestimate this might cover 10% of real designs, but it can happen. It is easy to do a simulation to determine this. You simply look at the change in the acceleration of the sustainer just before and after separation of the booster (with a delayed sustainer ignition obviously). If the acceleration becomes less negative, then it is better to separate, and vice versa. In many cases, there isn't much change, and then you can choose to separate or not separate based on other factors. This is the most common situation with my rockets.

Jim

#### jeff2space

##### Well-Known Member
Now if you want a slower, stunted flight [or whatever] do what you want. But for peak performance you need to separate right away.
You're talking about peak performance when it comes to velocity. Peak performance in altitude would require second stage to ignite when first stage is near apogee. I say near because you still want everything vertical when second stage ignites.

#### FredA

##### Well-Known Member
Peak performance in altitude would require second stage to ignite when first stage is near apogee. I say near because you still want everything vertical when second stage ignites.

Don't think so.
The combined stack will have much higher drag and apogee will be lowered significantly.
Think boosted dart...then add a motor to the dart and wait as long as possible for sustainer ignition with "as long as possible" be defined as not tilting too much.

Now - if you somehow built a booster without drag would be able to retain higher momentum, but for stability reasons booster fins are usually pretty big and cause substantial drag.

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#### JimJarvis50

##### Well-Known Member
The combined stack will have much higher drag and apogee will be lowered significantly.
Fred, I don't think you can generalize about the effect of the booster without at least looking at the simulation. The pic shows the acceleration plot for my two-stage Balls project. It was a P7240 to an O3400. At the separation point (the arrow) the acceleration goes up (less negative) after separation, which means the sustainer will more effectively maintain its speed with separation than with the booster attached. However, if I run the same rocket with a K300 in the sustainer, the acceleration goes down after separation, and for this scenario, it would be better for final altitude to keep the booster attached.

Obviously, it wouldn't make much sense to run my Balls rocket on a P to a K. At the same time, though, I'm not picking an exaggerated rocket design to make this happen. Granted, in most cases it will make sense to drop the booster, but often, there isn't much change in the acceleration with or without separation, and you can choose when to do the separation based on other factors.

Jim

#### markkoelsch

##### Well-Known Member
You're talking about peak performance when it comes to velocity. Peak performance in altitude would require second stage to ignite when first stage is near apogee. I say near because you still want everything vertical when second stage ignites.
I believe you are wrong. If you want peak altitude or velocity you separate right away. When you ignite the second stage is the question. If you want max velocity you hit it right away after first stage burnout ( or sooner if the booster burn hits a point were it slows under burn- motor tail off). If you want max altitude you need to figure out a time after separation. Hitting anywhere near apogee is very likely to result in a trajectory far from vertical.

Rockets do not fly straight up. They fly in an arc. Figuring this stuff out takes a fair amount of simulation. To really optimize it you probably need active vertical seeking guidance like Jim Jarvis and others are working on.

#### JimJarvis50

##### Well-Known Member
If you want peak altitude or velocity you separate right away.
No. Separating right away doesn't always give peak altitude. Look at the graph.

Jim

#### dhbarr

##### Amateur Professional
This is a repeat of the high-vs-fast problem: there's a magical triangle of mass, speed, and drag.

When your booster mass is low but your speed is high, separate as soon as possible since you haven't much inertia in the can.

#### Handeman

##### Well-Known Member
TRF Supporter
This is a repeat of the high-vs-fast problem: there's a magical triangle of mass, speed, and drag.

When your booster mass is low but your speed is high, separate as soon as possible since you haven't much inertia in the can.
that would seem to me that you want to separate when the g forces on the sustainer go negative relative to the booster. In other words, when the booster stops pushing the sustainer, separate. So a drag separation should work, but is probably slightly late to be optimum.

#### SMR

##### Entropy Demonstrator
Questions:
1. Should I change the altimeter in the inter-stage coupler to provide a separation charge and then deployment charge of recovery stage of booster? If so should I put shear pins in the coupler?
2. How close should the separation of the booster and sustainer ignition should it be?
3. Thinking of controlling ignition of the sustainer from the ground, in the event it is off axis. The M-1101 is enough to get the sustainer to an altitude to say a 1500-2000&#8217; main deployment.

Thanks

Pat
My 2 cents worth on your questions. There are a lot of variables that impact them, my opinions are general opinions and not the definitive word by any means.

Q1 - I do shear pin the ISC to the booster, as in any other flight where drag separation is a possibility (draggy bottom, heavy top). That way, if it does drag separate, it is the sustainer staging prematurely which won't be a terribly bad thing. IMHO, I believe the staging charge should be bigger than necessary just to push off the sustainer. I want to transfer a lot the energy of the booster to the sustainer. In a perfect world, if you could slingshot the sustainer hard enough to STOP the booster, 100% of its kinetic energy would pass to the sustainer. Obviously you can't do that with the charge alone, but compare it to not having a charge at all, where friction from sliding off the coupler takes energy AWAY from the sustainer (and adds it to the booster). Residual thrust from the booster (especially with a dual thrust motor) can cause it to catch up to the sustainer before its motor can come up to pressure. IIRC, this was an issue with one of the Saturn V launches.

Q2 - A lot of variables come to play in that. Sims are your friend.

Q3 - My eyes aren't what they used to be... I would have a hard time judging attitude of the sustainer within a limited acceptable range for sustainer ignition, especially in the very small time allotted. Here is where a Tiltometer or smart altimeter (with a programmable channel) pays for itself. Some great projects under development now with Arduino boards that can help here, too.

Sounds like an ambitious project. Please keep us posted.

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#### djs

##### Well-Known Member
IMHO, I believe the staging charge should be bigger than necessary just to push off the sustainer. I want to transfer a lot the energy of the booster to the sustainer. In a perfect world, if you could slingshot the sustainer hard enough to STOP the booster, 100% of its kinetic energy would pass to the sustainer. Obviously you can't do that with the charge alone, but compare it to not having a charge at all, where sliding off the coupler takes energy AWAY from the sustainer.
Sather and other smart people on this thread- have you ever had a concern about the staging charge pushing the sustainer off axis so it doesn't go straight up? or does the coupler keep it going straight enough so that this won't happen?

#### Eat, sleep, and Fly

##### Well-Known Member
Sather and other smart people on this thread- have you ever had a concern about the staging charge pushing the sustainer off axis so it doesn't go straight up? or does the coupler keep it going straight enough so that this won't happen?
I'm not Sather and I'm certainly not that smart, but I'll take a stab at this one anyway.

A separation charge should act quite uniformly on the bottom of a sustainer. By the time the rocket has had time to slide up the coupler, pressure should have equalized, and should push on the sustainer evenly, meaning that it would not change the path of the rocket. The key word there is should. There could be some weird situation where pressure wouldn't have equalized. In this situation intuition would say that it could, but in reality I doubt that a stable sustainer moving at high (ish) speed would be affected any significant amount by an off center separation charge.

As a hypothetical situation, lets say we are flying a 4" rocket with a 54mm motor. It's aft CR has an area of about 9". Say an absurdly large separation charge, causing 50psi, were used, and it acted on exactly half of the CR. This would create a 225lb force on the sustainer, pointing vertically less than 1" of the center axis. While this force alone seems huge, it would occur just for a moment (the instant the sustainer separated from the isc), on a rocket that is already flying strait (or at least in the same direction of the booster), and is stabilized by fins. For reference an average K motor produces more maximum thrust than this, sustained for multiple seconds. Not to mention that this situation, or any Un-uniformity, would not occur in any normal ISC, and that 50psi is an absurd pressure for a separation charge. In this situation I would be much more concerned that the torque caused by the un-uniform pressure would cause the coupler to get stuck. This would once again force my most feared situation, firing a motor into the ISC and into the booster electronics. I did that once (actually twice), it is not pretty.

Long story short, I wouldn't worry about it, but maybe some people would. Of all of the things that can go wrong on a two staged flight, this is very low on the list.

#### SMR

##### Entropy Demonstrator
Sather and other smart people on this thread- have you ever had a concern about the staging charge pushing the sustainer off axis so it doesn't go straight up? or does the coupler keep it going straight enough so that this won't happen?

So, the quick and dumb response, with no data to back me up, is "hadn't really thought about that". If one assumes a relatively snug fit commercial coupler with a typically good tolerance, I would guess, even if the charge is off center, that the resulting pressure would quickly equalize through the volume that you are pressurizing and push relatively evenly on the symmetrical a end of the sustainer with a good quick smack and send it on its way. And any small side load should be resisted by the inherent stability of the sustainer itself, which you are adding energy to which should improve its stability. This would be a good argument for staging early while everybody still has good wind over the fins. I think the staging charge may have a minor contribution to the instability of the booster by pushing backwards on it and slowing it down, which helps it arc over and avoid getting torched by the sustainer's soon-to-be-happy motor.

On a macroscopic scale, a bullet fired down a barrel pretty much goes where it is pointed. (Yeah, I know, the coupler isn't rifled, but IMHO that is really to give the otherwise unstable bullet some stability 'cause it doesn't have fins and who knows where the Cp/Cg relationship is?) I do have some pretty good video of Jayhawk nose cone separations at apogee, and as unstable as a nose cone by itself is, they fly pretty much in a straight line until hitting full extension of the shock cord.

Wide open here for counter arguments. Anybody have an issue with the sustainer cocking due to the staging charge?

#### wyo17

##### Well-Known Member
All,
Thank you for your feedback. Woe be it for me to request help on TRF and then not check back for days. I appreciate your comments, feedback and banter. With the 5 2-stage flights I have under my belt with this rocket here is my current plan, starting from the bottom.
Looking to boost on an M-1101 and sustain on an M-840, if available. If not then an M-3700.
Change the altimeter in the ISC from an Adept22 to an RRC3 to provide the 3rd channel for booster separation 2-4 seconds after detection of boost shut down. Looking at 3 2-54 shear pins to hopefully will mitigate the drop of boost wiggle. Also will allow for the deployment of ISC with a small chute, 18-24&#8221;, with a Tender Descender for main deployment of the main at a lower altitude. Have had one for about 4 years and haven&#8217;t used it yet. Looking at 7-10k on boost based on past performance.
Sustainer will change to a primary altimeter of an AltusMetrum Telemega being responsible for not only ignition of the sustainer, with a 10 degree off vertical setting, to prevent off axis ignition of the sustainer, also controlling both apogee and main deployments. Back up will be a Raven for apogee and main charges.
Tracking will be very redundant. The booster will be with a WM tracker, very effective on the PLAYA. The Sustainer with the Altus Metrum as well as another WM tracker and probably a BRB as another backup.
Jim Jarvis, I was pointed to you by someone in AK.
To all of you, I treasure you input, as I begin on this journey above 16k.
Please keep feeding your input and I will tweek as I go along and continue to seek guidance..
Thanks,
Pat

#### JimJarvis50

##### Well-Known Member
I was pointed to you by someone in AK.
I understand it's high season for rocketry in the arctic. Who knew? Feel free to ask questions.

Jim

#### Samsulajojo

##### Member
I am also working on a two stage right now. It's 4" diameter with 75mm mother mounts in booster and sustainer. 14' talk over all two 60" long booster and sustainer air frames. I'm using the telemega in the sustainer which will take care of the motor ignition. As far as the isc separation charge should that be run off the telemega on a second channel or should it be from the altimeter in the isc. I saw above that that some one is using rrc3 on the auxiliary Chanel what settings should be used I want to get rid of the booster fairly fast after motor burn out mabie Waite max 2 seconds

Thanks