Estes Saturn V uses Engine-Driven Gas-Dynamic Stabilization!

[eduncan911]

I just realized something. I think the Estes Saturn V kits are designed to use finless redirection of air to stabilize a rocket with such small fins in flight, known as "Engine-DrivenGas-Dynamic Stabilization."

More about this concept here:

https://www.apogeerockets.com/education/downloads/Newsletter379.pdf

The concept from my vintage kit, and the newer 2157 kits, all call for the motor mount to be installed about 3 3/8" up inside of the tube. With the stock 2.25" spacer for thrust ring, or a D motor hook, you are looking at a nozzle around 2.9" from the rear end.

There are concerns about the Krushnik Effect. According to Estes, this happens when the nozzle is inset more than the width of the body tube.

The Apogee PDF states a nozzle can be inset a little more than 1/2 the body tube width without issue.

The SatV bt101 tube is 4".

So basically, you wouldn't want to the nozzle any more than 2.25" from the base of the rocket.

2.9", if following the stock directions, seems a bit too much and may have been hurting these kits all these years. Or maybe it was just the right amount with combined with a low powered D motor (I plan on flying with E and F-sized power).

--

I've been researching stabilizing the Saturn V the last year or so before I tackle my build with my daughter. And reading up how to stabilize a rocket without fins hit me to how the Estes Sat V is constructed.

Here I was planning on a cluster of 5 motors sticking out the back through the original F1 engine plastic. That would have required clear plastic fins to stabilize the rocket.

But now this all makes sense as to stabilizing a rocket with little fins - inserting the nozzle to use redirection of air flow. I'll have to rethink my plans now (really want a cluster!).

 

[Flyfalcons]

Or you could just add nose weight to compensate for the extra weight at the rear.

 

[eduncan911]

Or you could just add nose weight to compensate for the extra weight at the rear.

Per the rational in my OP, that wouldn't solve the Krushnik Effect mentioned though.

I just think it's cool that the Sat V rocket was designed with this concept in mind. Makes me think about a number of custom builds.

 

[samb]

Interesting article, very rocket sciencey. I always thought the motor placement of the Estes Saturn V was a balancing consideration only. Wouldn't the stability advantage mentioned require those holes around the air frame circumference ?

 

[eduncan911]

Wouldn't the stability advantage mentioned require those holes around the air frame circumference ?

Not required, no.

The first method mentioned uses "Air Inlet at or Near the Exhaust" which the Estes Saturn V kit does by placing the motor a little more than 1/2 of the width of the body tube from the rear of the rocket. The air inlet at the rear of the body tube is what stabilizes the rocket during yaw and pitch, at the expense of loosing a little thrust and at the risk of subscribing to the Krushnik effect if placed too far up into the tube.

Alternatively to gain back the thrust amount that is lost, and to eliminate the possibility of the Krushnik effect, one could use holes drilled right at the nozzle location in the tube - as long as the holes are large enough to feed it.

If you look closely at the rocket below, you can see the air pockets drilled into the black painted areas.

This rocket actually flies with no launch rod either!

 

[Daddyisabar]

One look at Gas Dynamic Stabilization and all my model rocket fin-less stability fears just fade away. Fins are over rated on rockets, who needs them! Didn't really need them on the Saturns in real life!

 

[rstaff3]

I prefer to stabilize my rockets with gas by putting the motor over half way up in the body tube. Vents required. Just like the rocket shown in post #5. If you do this you can enter the current design contest.

 

[Nytrunner]

Slowly, Gas Dynamics' following spreads!

Other than the fact that neat odd-rockets can be built, I think this could help draw people into researching more of how rockets and stability really work.

Now I've got to get back to repairing the GBI mk 18

 

[Flyfalcons]

Per the rational in my OP, that wouldn't solve the Krushnik Effect mentioned though.

I just think it's cool that the Sat V rocket was designed with this concept in mind. Makes me think about a number of custom builds.

Sounds awesome, for when the motor is firing. For the rest of the flight, good luck I guess.

 

[eduncan911]

Sounds awesome, for when the motor is firing. For the rest of the flight, good luck I guess.

Right. I've only been eye balling motors with long burn times, like 2.5 seconds or greater.

Would rather have low initial impulse for a slow lift off, and then a long burn time. Also would help not destroy the upper stages from too much initial impulse.

 

[Flyfalcons]

You can see how well gas dynamic stabilization has been working with the Estes Saturn......

[video=youtube;ItDIjxoJBB8]https://www.youtube.com/watch?v=ItDIjxoJBB8[/video]

[video=youtube;7-QvHyEB3Ws]https://www.youtube.com/watch?v=7-QvHyEB3Ws[/video]

[video=youtube;kLnnkTlYkL0]https://www.youtube.com/watch?v=kLnnkTlYkL0[/video]

 

[Daddyisabar]

Power, nose weight and good weather are your friends! They must all be there at your Saturn V launch.

 

[rstaff3]

You can see how well gas dynamic stabilization has been working with the Estes Saturn......

[video=youtube;ItDIjxoJBB8]https://www.youtube.com/watch?v=ItDIjxoJBB8[/video]

[video=youtube;7-QvHyEB3Ws]https://www.youtube.com/watch?v=7-QvHyEB3Ws[/video]

[video=youtube;kLnnkTlYkL0]https://www.youtube.com/watch?v=kLnnkTlYkL0[/video]

Yeah, the only GSD that counts is ....

 

[Nytrunner]

Yeah, the only GSD that counts is ....View attachment 317365

Man, I need to get a good video of one of my attempts.....
Too bad I have to violate at least three of those guideline to retain the scale of a ground-based interceptor.

Brought it with me to the HARA table at the NASA Student Launch rocket fair. Got a few funny looks from some of the NAR team

 

[rstaff3]

Man, I need to get a good video of one of my attempts.....
Too bad I have to violate at least three of those guideline to retain the scale of a ground-based interceptor.

Brought it with me to the HARA table at the NASA Student Launch rocket fair. Got a few funny looks from some of the NAR team

Said diagram obviously doesn't provide any rigorous rules that will ensure success. My first attempt followed the rules on the earlier version of that diagram and it failed. I adjusted a few things, it still followed the guidelines, and it worked. My latter attempts followed the new guidelines and I am 3 for 3. (One of those also took some rework to get it to fly nicely.... shhhh) The other think I found is that the higher the average impulse the better. None of this says your version will or won't fly well.

 

[Nytrunner]

Average thrust or average impulse? My strategy for the future (wider range of thrust curves at bigger diameters) was to try and prolong the active thrust time using lower avg thrust motors of sufficient force.
Fun thing about my last flight was that it spiralled up like a staircase, but definitely didn't weathercock like everything else at the field .

Now I'm curious about whether the recessed motor of the Saturn V was an intentional measure to assist stability on Estes' part.
We need JumpJet to chime in.

 

[rstaff3]

Average thrust or average impulse? My strategy for the future (wider range of thrust curves at bigger diameters) was to try and prolong the active thrust time using lower avg thrust motors of sufficient force.
Fun thing about my last flight was that it spiralled up like a staircase, but definitely didn't weathercock like everything else at the field .

Now I'm curious about whether the recessed motor of the Saturn V was an intentional measure to assist stability on Estes' part.
We need JumpJet to chime in.

I meant average thrust. My working theory as the faster you get the exhaust through the induction tube, the higher the restorative force. This is based on one data point. Two F motors that each should have been more that enough had the rocket been 3FNC. The lower thrust motor made for a very wobbly flight but the higher went straight.

BTW, despite the large air gaps in these things, they do not perform as well as rockets with the motor in the butt end.

 

[rstaff3]

Now I'm curious about whether the recessed motor of the Saturn V was an intentional measure to assist stability on Estes' part.
We need JumpJet to chime in.

I bet not. The rationale is that the mass of the motor is further forward and thus helps the CG ever so slightly.

 

[Daddyisabar]

I bet not. The rationale is that the mass of the motor is further forward and thus helps the CG ever so slightly.

Getting those motors up front to help the CG is what it is all about!!

 

[jadebox]

I bet not. The rationale is that the mass of the motor is further forward and thus helps the CG ever so slightly.

But, I wonder if it also reduces the base drag by filling in some of the vacuum created behind the wide body tube. If so, that could move the CP forward, reducing stability (negating some of the benefit of moving the CG forward).

BTW ... here's a link to a list of your blog posts on "induction stability":

https://www.rocketreviews.com/induction-stability-2550.html

I found them quite interesting. It's intriguing to think that we might be able to make our Saturn V models more stable by punching holes in the sides of them!

-- Roger

 

[rstaff3]

But, I wonder if it also reduces the base drag by filling in some of the vacuum created behind the wide body tube. If so, that could move the CP forward, reducing stability (negating some of the benefit of moving the CG forward).

BTW ... here's a link to a list of your blog posts on "induction stability":

https://www.rocketreviews.com/induction-stability-2550.html

I found them quite interesting. It's intriguing to think that we might be able to make our Saturn V models more stable by punching holes in the sides of them!

-- Roger

I hadn't thought that the base drag could be reduced by recessing the motors. There have been many kits whose designers thought the recess would help not hurt.

 

[jadebox]

I hadn't thought that the base drag could be reduced by recessing the motors. There have been many kits whose designers thought the recess would help not hurt.

I don't know that it is, but it seems possible. Might be a good subject for a research project!

-- Roger

 

[rstaff3]

I don't know that it is, but it seems possible. Might be a good subject for a research project!

-- Roger

For sure.

 

[Nytrunner]

But, I wonder if it also reduces the base drag by filling in some of the vacuum created behind the wide body tube.

Brilliant suggestion: Avoid flow separation BY MAKING MORE FLOW!

Get a patent on that, stat.

 

[ManofSteele]

The Saturn V wasn't designed with Gas Dynamic Stabilization in mind. It was designed that way for Center of Gravity considerations and to allow for placement of the display nozzles.

I discussed this with Mike Dorffler when we were getting ready to re-release the Saturn V in 1998.

BTW, GDS has a significant performance decrease that overwhelms any gains from eliminating fins, and it doesn't provide the roll control that fins do.

Matt

 

[Nytrunner]

The Saturn V wasn't designed with Gas Dynamic Stabilization in mind. It was designed that way for Center of Gravity considerations and to allow for placement of the display nozzles.

I discussed this with Mike Dorffler when we were getting ready to re-release the Saturn V in 1998.

BTW, GDS has a significant performance decrease that overwhelms any gains from eliminating fins, and it doesn't provide the roll control that fins do.

Matt

Well, now we have it straight from a horse's mouth. No fluids fun for the saturn 5.

Speaking as someone that likes messing with this technique, I seriously doubt dabblers are looking for any performance boost by using it. Its just a way to do something different and create an unusual rocket profile. If the hobby was all about performance, everyone would build minimum diameter 3 fin jobs and really wouldn't be as interesting.

 

[JStarStar]

"Performance" for the Sat V consists of flying straight and high enough to get ejection while still off the ground.

Low-and-slow liftoffs which would be considered a serious problem with most rockets are acceptable, even preferred, with the Sat V because of the realism factor. Anybody who ever saw a video of a real Saturn V launch knows they did not zip up off the pad. A Saturn V model only really looks like a Saturn V if it rumbles, rumbles, rumbles up off the pad.

I've seen several Sat V flights which culminated with ejection 10-20 feet off the ground.

AS long as the chutes open and the body tube doesn't get crunched, the flight is a "success."

Busting off one of the fins or the LES -- something that can be fixed with a few drops of CA or plastic cement -- is considered "trivial/incidental" damage, normal wear and tear for a flying model.

 

[rstaff3]

Well, now we have it straight from a horse's mouth. No fluids fun for the saturn 5.

Speaking as someone that likes messing with this technique, I seriously doubt dabblers are looking for any performance boost by using it. Its just a way to do something different and create an unusual rocket profile. If the hobby was all about performance, everyone would build minimum diameter 3 fin jobs and really wouldn't be as interesting.

I agree!

 

[Screaminhelo]

Well, now we have it straight from a horse's mouth. No fluids fun for the saturn 5.

Speaking as someone that likes messing with this technique, I seriously doubt dabblers are looking for any performance boost by using it. Its just a way to do something different and create an unusual rocket profile. If the hobby was all about performance, everyone would build minimum diameter 3 fin jobs and really wouldn't be as interesting.

I wholly agree. I was able to build a rocket that would have been unstable with the motor at the back. Once I moved it forward and added the weight of custom intakes, the CG was in a much more comfortable place.

I still want to build an upscale of my DGS test mule. The idea was to have adjustable nose weight and smallish fins (the Punisher was my inspiration) so that I could get some idea of how much DGS actually added to stability.

 

[Mushtang]

Anybody who ever saw a video of a real Saturn V launch knows they did not zip up off the pad.

I don't recall if I've ever seen video of the Saturn V that wasn't in slow motion. Anyone have a link to normal speed liftoff?