Do sustainer rocket segments have to be stable in single stage configuration?

[BABAR]

this came up on the Scratch forum, I figured the expertise is over here.

For a single stage rocket, need enough effective fin area or base drag so the rocket achieves stable flight parameters when it reaches the velocity at the end of the rail. Many if not most rockets are continuing to accelerate after they leave the rail, but for Typical model rockets the only “guidance” forces for straight flight are the fins once they leave the rail.

For MultiStage rockets, like single stage rockets, the full STACK has to be stable when it leaves the rail. At separation it seems like many if not most multistage rockets are at a much higher velocity at staging than when they leave the rail.

So can sustainer rocket segments get by with smaller fins (less fin surface area) when flown SOLELY in multistage configuration? Figuring the fins will be much more effective since they are already at a higher velocity at staging then they would be in single stage configuration at the end of the rail.

 

[Chad]

that makes sense but keep in mind, if you're doing a high altitude design, when the sustainer ignites the air will be significantly less dense and so you would need larger fins to compensate. In the "use some guidance" thread Jim talks about trying to get the stack as vertical as possible when the sustainer ignites. I suspect that whatever direction the sustainer is pointed in is where it is going to go once ignited, the fins may not have a huge effect on the sustainer at high altitude. Having said all of that, i'm just trying to think it through and don't have first hand experience... yet.

I'm not sure at what altitude fins become meaningless. There's discussions on this forum where rocket sensor data shows tumbling end-over-end at very high speed (i think even > M1) without the airframe coming apart so that altitude is certainly within HPR reach.

 

[BABAR]

that makes sense but keep in mind, if you're doing a high altitude design, when the sustainer ignites the air will be significantly less dense and so you would need larger fins to compensate. In the "use some guidance" thread Jim talks about trying to get the stack as vertical as possible when the sustainer ignites. I suspect that whatever direction the sustainer is pointed in is where it is going to go once ignited, the fins may not have a huge effect on the sustainer at high altitude. Having said all of that, i'm just trying to think it through and don't have first hand experience... yet.

I'm not sure at what altitude fins become meaningless. There's discussions on this forum where rocket sensor data shows tumbling end-over-end at very high speed (i think even > M1) without the airframe coming apart so that altitude is certainly within HPR reach.

I am definitely thinking of a low power or mid power rocket here, so not thinking in terms of reaching altitudes where air density becomes an issue.

Speaking of which, however, IS THERE ANY CONSENSUS about what altitude sim programs such as RockSim and OpenRocket start to break down? Does stability calculation only matter during the “thrust” phase? Presumably every rocket as it approaches apogee after coasting approaches zero velocity where fins are completely ornamental.

 

[Reinhard]

The sustainer needs to be stable on its own. Otherwise it will skywrite or tumble. It doesn't make a difference if the sustainer was previously guided by a launch pad or a booster with big fins. Speed wont help here either because more speed will _not_ fix an unstable rocket. *)

Reinhard

*) Speed influences stability, but usually a rocket becomes less stable at higher supersonic speeds.

 

[Chad]

I am definitely thinking of a low power or mid power rocket here, so not thinking in terms of reaching altitudes where air density becomes an issue.

Speaking of which, however, IS THERE ANY CONSENSUS about what altitude sim programs such as RockSim and OpenRocket start to break down? Does stability calculation only matter during the “thrust” phase? Presumably every rocket as it approaches apogee after coasting approaches zero velocity where fins are completely ornamental.

1) not that i'm aware of when it comes to altitude (assuming subsonic speeds)

2) after motor burnout a rocket has its momentum (p=mv) and becomes an arrow basically. Arrows are balanced with vanes to maintain their trajectory after leaving the bow so, yeah, I would say it matters but it's going to matter less and less as the rocket slows.

3) As a rule of thumb, I consider 40-50 ft/sec the minimum velocity to safely leave the rail. Turning it around you could say once a rocket in coast phase slows to under 50 ft/sec the fins won't have much effect.

 

[cerving]

You can probably get away with smaller fins than you would need to have if you were launching it in single-stage configuration, since it's going to have some significant velocity when the motor lights. OR or Rocsim should be able to show you that. Another bonus is that the smaller fins on the sustainer will move the CP back a little bit for the whole stack... always a good thing.

 

[neil_w]

I had a PM discussion about this with @BABAR a while back. Here's a copy/paste of one of the messages from that thread. I do not claim this is authoritative, but it's what OR gave me, for better or worse.
----------------------------------------------------------------------------------------------------------------------------------------------------
It turned out to be a really quick experiment.

First I made a single-stage rocket, and shrank the fins until it was just unstable (static margin = .126). Simulation plot looks like this:

That is a mess. When I enlarged the fins enough to make it stable, it made about 800 ft.

Then I added a second stage, made it nice and stable, and got this:

So, in the two-stage scenario, the exact same sustainer flew fine given it got a 100 mph head start. In fact, I was able to get it to simulate with a sustainer with .062 static margin. Not much change to the 2-stage plot, but the single-stage plot got worse.

So that would seem to answer your question, if I have everything covered here properly. Sustainer does not need to have as much margin because it is going much faster when motor is started. That said, I'm not sure I would trust these simulations sufficiently to validate a sustainer with such a tiny static margin.

 

[Reinhard]

On paper, a rocket with a stability margin of 0.001 flies just fine. The 1 caliber rule (or even better: 10-15% of length, like some pros do) ensures that all the little things that are different between "on paper" and "in real life" will not topple such a house of cards.

Yes, more speed increases the stabilizing lift that the fins will generate. But the same also applies to the other parts of the rocket, those that generate destabilizing lift or drag.

Reinhard

 

[dhbarr]

I'm lazy so I'd just design a 4 fin booster and a 3 fin sustainer. You can just cut 8 fins and throw away the worst one.

 

[BABAR]

Yes, more speed increases the stabilizing lift that the fins will generate. But the same also applies to the other parts of the rocket, those that generate destabilizing lift or drag.
Reinhard

You may be correct. I was thinking in terms of thrustcurve.org, where it seems like the main parameter is “speed at end of launch guide“

Engines that fail do so because they don’t reach adequate velocity before they are released from the forcibly guided (rod or rail) portion of the flight, before the fins “kick in.”

Logically, while I agree that “the same also applies to the other parts of the rocket” it seems likely that these would be proportional to the surface area, thus logically the faster the rocket, the MORE stable it should be.

Sometimes “logic” doesn’t always work, and I guess it is possible that complexities of fluid dynamics, Mach speed, and for the bigger rockets where separation occurs waaaaaaay up there where the air is rare (a situation that doesn’t occur with the black powder staging I do ) the rules may not apply.

By the same token, however, if you ARE correct and the rules don’t necessarily apply, then just because the sustainer IS stable as flown in single stage configuration, it may NOT be stable in multistage. I suspect however that in the low power range the “rules” probably apply consistently. Probably not too many low power or mid power multistage rockets are going to stage at anything close to Mach velocities (although now that I think about it, the SUSTAINER might exceed Mach AFTER staging, given it already has a pretty good head start initial velocity.....)

 

[Nytrunner]

Loving this discussion. Fascinating topic seeing how the ground launcheck physics differ from sustainer-away behavior.

3) As a rule of thumb, I consider 40-50 ft/sec the minimum velocity to safely leave the rail.

Thrustcurve fails a motor under 50 and thats a safe conservative number.

I have successfully ignored that recommendation and will fly with as low as 45 ft/s (L1), but won't descend below that unless the wind is near non-existant.

 

[Ez2cDave]

Reinhard

*) Speed influences stability, but usually a rocket becomes less stable at higher supersonic speeds.

Reinhard,

That is an excellent observation . . . The CP "shifts around" above Mach 1 and continues to be variable as Mach Number increases. This can result in instability, as a result.

Dave F.

 

[jlabrasca]

1
2) after motor burnout a rocket has its momentum (p=mv) and becomes an arrow basically. Arrows are balanced with vanes to maintain their trajectory after leaving the bow so, yeah, I would say it matters but it's going to matter less and less as the rocket slows.

"after motor burnout a rocket still has ... momentum (p=mv)..."

3) As a rule of thumb, I consider 40-50 ft/sec the minimum velocity to safely leave the rail. Turning it around you could say once a rocket in coast phase slows to under 50 ft/sec the fins won't have much effect.

In the coast phase the only torques on the rocket will be due to aerodynamic forces. Assuming the sustainer CP is aft of its CM, as long as the nose is pointed up, those torques will tend to keep the nose pointed up.

I can turn the OP's question around. I built a two stage from an Estes Red Nova. The sustainer flew arrow straight as a single stage. It went squirrelly after separation EVERY time we tried it as a two stage. Nice straight boost phase on C11 or D12, then it was a sky-writer on the sustainer on everything from a B6 to a C11.

Thinking the speed at separation might a problem, we sent it up on an E12 in the booster, D12 in the sustainer. It appeared to be moving pretty fast (faster than on a D12) at separation. It still got all aerobatic on the sustainer. That time the booster came down ballistically and got irreparably crunched -- so there will be no further experiments (plus which people were starting to notice our repeated attempts to fly an unstable rocket)

 

[BABAR]

" The sustainer flew arrow straight as a single stage. It went squirrelly after separation EVERY time we tried it as a two stage. Nice straight boost phase on C11 or D12, then it was a sky-writer on the sustainer on everything from a B6 to a C11.
)

Interesting. So you think it went supersonic and this decreased fin effectiveness?

 

[jlabrasca]

Interesting. So you think it went supersonic and this decreased fin effectiveness?

>smile< No. With that stepped nosecone (packed with clay), and squared-off fins, I don't guess it broke mach on a C11-0/B6-6.

 

[BABAR]

>smile< No. With that stepped nosecone (packed with clay), and squared-off fins, I don't guess it broke mach on a C11-0/B6-6.
View attachment 398812

Agreed. I’d be interested in alternative interpretations.

 

[Bruiser]

How was the separation on the Red Nova? Was the booster a tight fit into the sustainer? I'm just thinking that if the booster was getting hung up somehow, maybe it was inducing some oscillation. It's really not much different than my Tempest build and I did not have a problem like that. I don't see any vent holes in the booster. Did you have any?



-Bob

 

[jlabrasca]

How was the separation on the Red Nova? Was the booster a tight fit into the sustainer? I'm just thinking that if the booster was getting hung up somehow, maybe it was inducing some oscillation.

Its a good question. I kind of thought of that, and pulled the engine hook out of the sustainer after the first wild ride. Didn't help. When I built it, I sanded the coupler down -- took all the red paper off the surface. The weight of the booster alone wasn't enough to make it fall apart, but it was easy enough to shake it apart.

It's really not much different than my Tempest build and I did not have a problem like that. I don't see any vent holes in the booster. Did you have any?

View attachment 398827

-Bob

Yeah, its vented. Interesting story -- it was originally going to vent out the back

but I broke the aft fenestrated centering ring wile pressing everything together -- after the epoxy went in. I only had a few minutes of open time, so I ended up replacing it with the kit card-stock ring and cutting a vent in the side of the airframe.

The vent in the outer tube is offset from the vents in the motor tube -- so I don't think it was acting like a thruster to topple the rocket.

 

[neil_w]

If you only have a single vent, you’re going to get a net lateral force on the rocket regardless of internal construction. Could definitely explain the observed behavior.

I say add a second port at 180 degrees and send it up again.

 

[BABAR]

If you only have a single vent, you’re going to get a net lateral force on the rocket regardless of internal construction. Could definitely explain the observed behavior.

I say add a second port at 180 degrees and send it up again.

Possible. I routinely do two or three vents equally space for the same reason.

This is actually more important on motor eject gliders like the Falcon/ Hawk. Reports of broken booms I think may be due to unilateral vent forces

 

[jlabrasca]

If you only have a single vent, you’re going to get a net lateral force on the rocket regardless of internal construction. Could definitely explain the observed behavior.

FWIW, the rocket passed before the eyes of a few different RSOs on the way to the pads for its various failed attempts. The single vent only once drew comment, and that was on account of its size (I cut it large to reduce the exhaust speed of the exiting gas).

Still, you might be right -- while I've flown gap stage rockets with a single vent hole successfully, none of them were much like this particular combination of odd shapes.

I say add a second port at 180 degrees and send it up again.

It would be worth a try, except that the damage to the booster (accordioned coupler and a broken fin) is more than my limited time and interest will allow me to repair. The sustainer isn't in great shape either. At the end of its last triple lindy it harpooned its chute and came down hard in the sagebrush. If I were to rebuild it I'd go back to the original plan to have it vent to the rear, and I'd try to fix whatever it was that made the booster lawn-dart when loaded with a 24 x 95mm motor.

EDIT: Because my bench is uncharacteristically clean this morning -- I pulled it down. The damage to the fins doesn't look as bad as it is. The leading edges on the booster fins are all chipped and/or chewed up. The coupler is the probably-could-but-I-really-don't-want-to-fix-it part.

 

[neil_w]

Ooh, that is pretty beat up, although it doesn't look too far from being flyable. Without reflying that one, the hypothesis of asymmetric venting will remain unproven, but it still seems like the most likely culprit.

In the future I would just say either vent out the back (as you were planning) or multiple symmetric vent holes at the front.

 

[BABAR]

In the future I would just say either vent out the back (as you were planning).

Backward venting for NON-minimum diameter tumble recovery gap stage boosters is actually amazingly easy to do.

don’t use centering rings.

if you are NOT using a motor hook, don’t put the engine block in until AFTER you glue the mount in place, then you can test fit the motor before placing the block.

Cut 4 (or 5....) 1/8 inch balsa strips (you can go with the grain, for low power If it is less than 1/4 inch, over 1/2 I would cut the length AGAINST the grain for strength) the width of the centering ring.

Put them equidistant around the booster motor mount, you want the motor mount forward end to stop about 1/4 inch from the nozzle of the sustainer when joined. this guides the hot flame to the nozzle, gas will then go laterally and back around the motor mount and body of the booster.

After gluing the strips to the motor mount tube, smear a touch of epoxy or wood glue around the forward ends of the balsa strips if they will be exposed to the exhaust from sustainer.

Test fit the motor mount with strips a few times into the booster AND then test fit into the SUSTAINER with real motors a few times before you glue the mount to the booster (don’t ask me how I know this......). Make sure the forward edge of the mount will be in the right position relative to sustainer.

after glue is dry, put the engine block in the appropriate depth.

 

[jlabrasca]

Ooh, that is pretty beat up, although it doesn't look too far from being flyable. Without reflying that one, the hypothesis of asymmetric venting will remain unproven, but it still seems like the most likely culprit.

In the future I would just say either vent out the back (as you were planning) or multiple symmetric vent holes at the front.

Assume mass flow out of the vent hole produced sufficient thrust to topple the rocket. I've seen rockets go sideways after staging, but they went sideways, they didn't -- that I can recall -- do arabesques. This rocket is enough of a puzzle that I will probably patch up the sustainer and build it a new booster

Backward venting for NON-minimum diameter tumble recovery gap stage boosters is actually amazingly easy to do.

You can also do it in a needlessly complicated way with CNC milled and 3D printed parts -- which is the way I think it ought to be done.

 

[neil_w]

You can also do it in a needlessly complicated way with CNC milled and 3D printed parts -- which is the way I think it ought to be done.

I am always up for a good overly-complicated solution to something. Are you thinking something different from your previous solution here?

 

[BDB]

....
Yeah, its vented. Interesting story -- it was originally going to vent out the back

but I broke the aft fenestrated centering ring wile pressing everything together -- after the epoxy went in. I only had a few minutes of open time, so I ended up replacing it with the kit card-stock ring and cutting a vent in the side of the airframe.

The vent in the outer tube is offset from the vents in the motor tube -- so I don't think it was acting like a thruster to topple the rocket.

Is that a cork borer? That brings back horrible memories of freshman chem lab!

 

[jlabrasca]

I am always up for a good overly-complicated solution to something. Are you thinking something different from your previous solution here

No. I am not thinking of anything right now. The rocket is back on the shelf and my bench is full of bits of salvage lumber heading for the thickness planer on their way to becoming a hall tree.

Is that a cork borer?

It isn't. I couldn't find my cork borers, so I made one out a of scrap of brass tubing.

I have an embarrassingly large collection of K & S scraps.

And with that, the thread-jack is complete.