Help me gas dynamic stabilization, you are my only hope!

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Metallic aluminum bellmouths to replaceable blue tube augmentation with multiport vent holes. Spar supported lugs with hidden black "anti" Krushnic vents. Telescoping display nozzle is a real Q Ship feature. Fiber glassed, core reinforced fairings for a bulletproof, air inducted flying machine.

Yeah, you gotta talk the talk with GDS.
 
Outstanding! I have long been a fan of that tail-sitting aircraft and had seen the Jetex plan. However, it hadn't bubbled back up into my consciousness. That may be the build motivation I was looking for!

There was an old modroc design that looked similar. Don't remember if it was a kit or plan. I saw one try to fly at NARAM 50 but, IIRC, there was no augmentor tube involved and the ringtail may have been a glider.
 
The idea of thrust augmentation, if that is what we are now talking about, is very different from gas dynamic stabilization. Separately, the augmentation of jetex motors and our current solid rockets motors is also very different. The thrust augmentation of jetex motors was documented in the jetex literature and would have some meaning for such low thrust, long-burning motors. Our solid rockets motors burn very short and the time for heating incoming air is short, too. Many or all of the jetex motors consisted of a pin-hole nozzle exit, which also acted as the throat, meaning that the motors were highly under-expanded. This could mean that there was a lot of external aft-end surface area on the jetex motor for incoming air to mix, be heated, and exert a positive forward thrust. These conditions are not as abundant in our solid rocket motors.
 
The idea of thrust augmentation, if that is what we are now talking about, is very different from gas dynamic stabilization. Separately, the augmentation of jetex motors and our current solid rockets motors is also very different. The thrust augmentation of jetex motors was documented in the jetex literature and would have some meaning for such low thrust, long-burning motors. Our solid rockets motors burn very short and the time for heating incoming air is short, too. Many or all of the jetex motors consisted of a pin-hole nozzle exit, which also acted as the throat, meaning that the motors were highly under-expanded. This could mean that there was a lot of external aft-end surface area on the jetex motor for incoming air to mix, be heated, and exert a positive forward thrust. These conditions are not as abundant in our solid rocket motors.

Thanks for the keen assessment of the Jetex design. While I am aware these concepts (GDS and thrust augmentation) are apples and oranges, I have to admit I am mixing the two concepts in by latest kludge (the Saturn V-like thing). I also feel like I should have known about air heating and under-expanded nozzles but also have to admit I didn't consider them. I really wasn't expecting augmentation on my Saturn V and was more just wondering how such a tube affected its flight profile. Similarly, the Saturn doesn't prove, or even require GDS.

I'm really enjoying my hijack of daddy's thread :D
 
Thanks for the keen assessment of the Jetex design. While I am aware these concepts (GDS and thrust augmentation) are apples and oranges, I have to admit I am mixing the two concepts in by latest kludge (the Saturn V-like thing). I also feel like I should have known about air heating and under-expanded nozzles but also have to admit I didn't consider them. I really wasn't expecting augmentation on my Saturn V and was more just wondering how such a tube affected its flight profile. Similarly, the Saturn doesn't prove, or even require GDS.

Thanks, Dick, for the encouragement!

After going back and looking at Dean's Apogee paper and a similar paper he sent me, I think that I finally understand the general concept of Dean's GDS. Since Dean does talk about external inflow induced by the motor in the boost phase, I'm going to revisit the Jetex Augmenter tube. You may need to rotate the attached pdf file in order to get the correct perspective. A perfectly straight augmenter tube at zero angle of attack cannot generate any forward thrust itself, because there is no forward facing surface. Remember that pressure is force per unit area (normal stress tensor) and must act on a surface to generate a force. (This is clear in a fluid dynamics control volume analysis, but let's not go there. A tricorder will be of no help.) In the diagram only the surface area in Region 1 and Region 2 can produce a forward thrust for different reasons. In Region 1 there is the opportunity of the under-expanded pin-hole throat (nozzle) to have locally high pressure for forward thrust on the base of the Jetex motor. In Region 2 ambient air is sucked in from the outside, accelerated, and produces a locally low pressure on the inside curved surface of the augmenter tube. The higher ambient static pressure on the outside external curved augmenter tube surface creates a net forward thrust.

The basic idea of an augmenter tube is the fact that rocket engines are basically very inefficient devices, because there is a huge amount of untapped thermal energy in the rocket exhaust. Jet engines are more efficient because (1) they carry their own oxygen, and (2) the unconsumed nitrogen in the air is used as a working fluid to expand and increase thrust. Thus, a jet engine exhaust is much cooler than a rocket engine exhaust and more energy has been extracted. Estes issued a plan several decades ago called Lil Augie with a straight augmenter tube. There is a thread somewhere on TRF discussing this about 5 to 10 years ago. IMHO I don't think that this model produced any additional thrust. Years ago, Thiokol looked at augmenter tubes in-house to no avail. I remember there was an AIAA paper that showed augmentation tubes would work theoretically, but myself and fellow engineers concluded that this paper implicitly assumed that all the exhaust thermal energy went into heating the incoming ambient air, but the author never justified how this thorough mixing occurred.

On Dean's GDS device I conceptualize that some portion (perhaps, a high portion) of incoming gas momentum through the vents is deflected inside the airframe tube to the axial direction. Thus, the net change in the gas momentum vector exerts a force on the internal tube in the direction of turning the tube back so there is a zero angle of attack. I can draw the diagram for this concept and will present it later.

View attachment Jetex Augmenter tube.pdf
 
Last edited:
Very interesting, thanks. I formally give up on thrust augmentation. Plus, I have a hunch that my Saturn will experience thrust degradation. Hopefully GDS will still augment, not degrade its stability.
 
GDS launch #3 today. Excited for the new GDS modifications, great conditions, a successful launch and recovery of a D-C-C-3 stage Comanche, all is going well. Top, Highly Certified Men present, and this happens:

[video]https://youtu.be/msUCada32wk[/video]

Tough crowd.

GDS CATO Damage.jpgGDS CATO Damage 2.jpg

Tough Rocket.
 
Holy holey motor tube! Pulse detonation GDS isn't the future. Looks very fixable :)
 
Daddy, was this a 24 mm BP motor like an E9 or E12?

That was the last of the pack of Hobby Lobby E9 4s and it had a surprise waiting. Seems like there are more E9 Catos these days. It blew the motor back along the guides but the tape seized up and kept it in the rocket. Now the whole thing has charring but can fly again with a knot to retie the shock cord and some CA to the motor tube. The dog barf did its job. The inside of the casing was severely delaminated along three distinct ridges, the nozzle came clean out, the top of the motor case really burned. Nice big fireball so there must have been a big void full of lovely air.
 
That was the last of the pack of Hobby Lobby E9 4s and it had a surprise waiting. Seems like there are more E9 Catos these days. It blew the motor back along the guides but the tape seized up and kept it in the rocket. Now the whole thing has charring but can fly again with a knot to retie the shock cord and some CA to the motor tube. The dog barf did its job. The inside of the casing was severely delaminated along three distinct ridges, the nozzle came clean out, the top of the motor case really burned. Nice big fireball so there must have been a big void full of lovely air.

Do you know the lot number? The two bad lot numbers I have for E9's are 06 28 11 and 08 18 11. The bad lot numbers I have for E12's are A 08 18 11 and 06 14 01.
 
Of course he knows...he filled out the MESS form immediately. :confused:

I have the number - it was a new pack from '14. So many have blown I don't fill out reports, just swallow hard and take the damage like a man, then go cry in the car. They must get banged around a lot in the store. The other 2 motors in the pack were fine.
 
I will just go ahead and jinx the heck out of myself and state I have had only one E9 go boom and I happen to know it was likely user error. 'Nuff said. Have had a few E12's that were from the suspect batch. Later E12s have been OK.
 
I have read 3 of Dean Black's papers, one of which is the Apogee paper, which has been mentioned previously on this thread. Basically, all 3 papers cover the same subject. Dean talks about 2 types of Gas Dynamic Stabilization concepts: (1) Psuedo-fins, where the motor has been recessed a short distance from the aft end, (2) Induction tube stabilization, where the motor is mounted internally near the center of the model rocket. I have attached a drawing that has helped me understand the first concept. Perhaps, it may help others, but it is still a long ways from deriving design equations, a useful goal that Dick has mentioned in this thread. We should keep in mind that although the cut-out method is a simple tool for fin designs, more complicated and accurate methods like Barrowman's exist. The nice thing about the fin "CP" concept along with cut-out method (The cut-out method is over-simplified in that it implies that the static pressure distribution around the entire rocket surface is uniform. My take is that Dean strongly objects to this assumption of uniform static pressure, which he calls transverse pressure.) is that it is easy to visualize how fins stabilize a model rocket.

I did not strictly draw a control volume (C.V.) analysis for Fig.1, but Fig. 2 has some similarity to a C.V. analysis. A C.V. analysis is used in fluid dynamics along with the accompanying integral equations to sort out the forces and momentum fluxes acting on a body, a sort of Geordi's VISOR for theoreticians. The usual C.V. analysis uses linear momentum rather angular momentum, but linear momentum can be easily converted to angular momentum. Essentially, my Fig. 1 agrees with Dean's psuedo-fin drawing. In the reference frame of the rocket the external air has velocity vector V relative to the rocket and angular momentum at the aft end relative to the c.g. This external air flow is pulled into the aft end of the rocket by the aspirating affect of the rocket motor (or the low pressure region created there). The influx of external air mixes and then is expelled. Neither the influx or the expelled air has any angular momentum relative to the c.g., because they are essentially in the axial direction and the moment arm to them is zero. This is a concept from physics that the moment arm is defined as the perpendicular distance from the pivot point or c.g. to the line of action of the force or velocity. A perpendicular distance does exist for the moment arm to the external aft-end velocity as shown in the Fig. 1. This means that there is a net loss of angular momentum for the external flow at the aft end. The rocket body exerts a torque on the aft end flow that causes this loss. In turn the external flow exerts an equal and opposite torque on the rocket body. It it this torque that moves the rocket back to the null condition of zero angle of attack.

In Fig. 2 rather than talk about angular momentum I show the static pressure distribution externally and internally. (As an aside one can draw more than one C.V. in order to describe a problem. Classically, this is often done for a rocket motor.) When the model is at zero angle of attack, the static pressure externally and internally is uniform around the circumference. In such a case, even though the internal pressure is lower than the external, the net force is zero. At a angle of attack the pressure around the circumference can no longer be uniform. In such a situation the higher external pressure on the outside opposing the lower internal pressure on the inside will create a net force pushing the rocket back to the zero angle of attack.

View attachment GDS Psuedo Fins.pdf
 
Last edited:
I have a hard time believing that a rocket based solely on the psuedo-fin concept will actually be stable. I don't doubt there is some restorative force, but will it be enough? Since this is proportional to the moment arm, would a very long thin rocket be the best test case? However, I don't doubt that it could help with scale models that use what would be considered undersized fins. This seems to be what Dean did on the Polaris that he presented.

What does everyone think?
 
I have a hard time believing that a rocket based solely on the psuedo-fin concept will actually be stable. I don't doubt there is some restorative force, but will it be enough? Since this is proportional to the moment arm, would a very long thin rocket be the best test case? However, I don't doubt that it could help with scale models that use what would be considered undersized fins. This seems to be what Dean did on the Polaris that he presented.

What does everyone think?

In one of Dean's paper he shows a picture of David Hall's 3-inch diameter model that flew on a 1.75" recessed CTI Pro-29 F-59 motor. He states that this was successful. I believe that you, Dean, and William Cook have flown the Induction Tube concept successfully. I agree that the Polaris model as shown looks like a candidate for psuedo-fin concept. I would be skeptical of both concepts, if they hadn't flown. I hold that the proof is in the flying. (I disagree with Dean's equations for the Induction Tube concept. I will post a drawing of how I would begin to formulate equations for that concept.)
 
Last edited:
I had forgotten about that but after you mentioned it, I remembered where I had tucked his paper away. FAIW, Bill's induction tube rocket flew as well as mine on its maiden flight :rolleyes: He may have reworked it and flown it without my knowledge, but at the time Dean wrote the paper is was not operational. Bill? You there?

I too believe the proof is in the flying. My Saturn should have been more stable than my Inductor. I used the same design rules and the small fins did in theory make it statically stable. But...
 
Last edited:
I have attached my formulation for Dean's second GDS category or Inductor. In Dean's derivation I don't know specifically which thrust (F), velocity, or tangential velocity he is referencing. I have decided to go with a steady-state C.V. analysis, which is better defined here than my previous drawing for the pseudo-fin concept. I have drawn a relatively small C.V. (dotted lines) for the purpose of capturing the main concepts for understanding. In principle a C.V. involves at the very least surface integrals, but we won't go there. I am assuming that the external velocity comes in on the windward side (some external flow probably comes in from around the full circumference), and is turned inside the air-frame and may be accelerated down the tube. If I assume that the mass flow rate (mdot) for the incoming air is constant as turns it and goes down the tube and that the air density does not change, then I can draw the velocity vector diagram on the right hand side. Note that the force perpendicular to the tube does not depend how fast the internal air velocity goes down the tube. Even if the internal air velocity inside the tube went very fast, it would not change the Delta V vector perpendicular to the tube. However, a high internal air velocity would increase the force along the centerline, which we call Thrust. Note that the body tube exerts a force on the incoming air in order to turn the air parallel to the rocket body center-line. In turn the air exerts an equal and opposite force perpendicular to the tube. This is the corrective force for stabilization.

I think that with some further development it should be possible to develop equations to describe GDS. I have never looked at the derivation of the Barrowman equations, but I wonder if there might be a description there of how fins develop forces to stabilize a rocket. If so, that method might give insight on developing GDS equations. Perhaps, there might be some way to have GDS described in terms of equivalent fins. I don't know if anyone has ever done a NARAM Research report on this stuff that we have discussed. If not, I would think that this subject would be ripe for a good NARAM Research report.

View attachment GDS Inductor.pdf
 
It would be nice to have a factor that you could use along with a Barrowman representation of the nose cone (and other applicable components). Kind of like how Bruce Levinson described how to represent base drag in short stubby rockets.
 
My Saturn V, SA 667 flew nicely today! The altitude was even lower than its predecessor, due to it being a little larger and having a performance-robbing skinny induction tube. I have some photos and a video in my ESL-207 album. The video is lacking unfortunately. I removed the induction tube and have a few shots of its fried insides.

I may fly it again on a G but with a full sized liner in the 3" tube. But what to do with that cute bell-mouth?
 
My Saturn V, SA 667 flew nicely today! The altitude was even lower than its predecessor, due to it being a little larger and having a performance-robbing skinny induction tube. I have some photos and a video in my ESL-207 album. The video is lacking unfortunately. I removed the induction tube and have a few shots of its fried insides.

I may fly it again on a G but with a full sized liner in the 3" tube. But what to do with that cute bell-mouth?

Spittoon? Hearing Aid? Cod Piece? Mongolian Style Helmet top?
 
The GDSM III Flew Saturday after quick and dirty repairs to the blow hole and broken elastic shock cord.. The 10 cent Chinese igniters just pop on our system, so I had to give in and use a precious long Q2G2. With all the hassle I did not get a video but it flew well. completely stable with just a slight wobble into the wind. The lowest vent hole shows some burn through as it was located in the danger zone. I just don't know if I can take the burn. In the end all we can hope for its that a little gas dynamic stabilization seeped into our lives. There is a proper induction tube and the gas runs through it.

GDS A1.jpgGDS A2.jpgGDS A3.jpg
 
Oustanding. GDS or not it appears a finless rocket with a forward mounted motor can be stable. All the barbecued parts give them character.
 
Back
Top