Lakeroadster's X-Wing Alpha Build Thread

[kuririn]

• X-Wing has 2 ft of Kevlar, then,
• a swivel that the nose cone attaches too, and then,
• 18" of 1/8" elastic, and then,
• a swivel that the parachute attaches to.

If you read the leader line text shown in the drawing at the right it defines the components.

Since the nose cone has it's own swivel it will be easy to enough to hook it directly to the chute chords.

I thought the ejection charge pushed the entire contents of the recovery bay out, not just the nose cone?

Might be another good reason for a static test?

View attachment 358169

I see. I saw the text but it looked like the nose cone and swivel might have been attached to the end of a doubled length of elastic from above, and hanging down. Would you consider the more conventional, and proven, attachment method? I would go Kevlar leader tied to a swivel, elastic tied to a swivel to allow swapping out a Nomex blanket, parachute with swivel attached to a loop a third of the way down the elastic, and end of the elastic tied to the nose cone. Not that your way wouldn't work, but wouldn't it be safer to go with a proven method?

 

[lakeroadster]

IWould you consider the more conventional, and proven, attachment method?

Yes... makes sense to me. Thanks.

 

[jlabrasca]

The kevlar is inelastic (it won't stretch). The nosecone has more mass than the parachute. Use the elastic part of the shock cord to stop the nose cone.

 

[lakeroadster]

I see. I saw the text but it looked like the nose cone and swivel might have been attached to the end of a doubled length of elastic from above, and hanging down. Would you consider the more conventional, and proven, attachment method? I would go Kevlar leader tied to a swivel, elastic tied to a swivel to allow swapping out a Nomex blanket, parachute with swivel attached to a loop a third of the way down the elastic, and end of the elastic tied to the nose cone. Not that your way wouldn't work, but wouldn't it be safer to go with a proven method?

View attachment 358171

Sounds like a plan.

Here is my reasoning of the recovery components used on this small LPR... as it pertains to this rocket: On many competition rockets they simply use kevlar and streamers, no shock chord at all. The elastic shock chord is used due to the chute... not the nose cone. The rocket is decelerating as it reaches apogee, when the engine ejection charge deploys, yet the nose cone still has wind forces on it, unlike the static test video's shown above. On a small rocket with a lightweight nose cone, using a 50# kevlar cord, seems like having elastic between the cone and the body isn't a requirement.

Is the Nomex used instead of wadding, or in conjunction with wadding. I've never liked the concept of puking out wadding (a.k.a. littering).

 

[kuririn]

Some builders like to use elastic in conjunction with the Kevlar because of it's shock absorbing properties. The theory is that when the chute pulls open the force on the shock cord is dampened by the elastic. Having said that I just built a mid power kit by Apogee that uses only Kevlar for the shock cord, no elastic. But the Kevlar shock cord is super long. Nomex or Aramid is the same material that is used in race car drivers' jumpsuits. Fire resistant, replaces wadding, reusable. There are also Nomex sleeves that can go over the shock cord. Generally used more in MPR than LPR. Cheers.

 

[neil_w]

I've never liked the concept of puking out wadding (a.k.a. littering).

That's one of the benefits of cellulose insulation aka dog barf. It decomposes quickly and doesn't really present a litter hazard. Regular paper wadding, which is basically toilet paper, should degrade also, but not as quickly as the shredded-up barf.

 

[jlabrasca]

Sounds like a plan.

Here is my reasoning of the recovery components used on this small LPR... as it pertains to this rocket: On many competition rockets they simply use kevlar and streamers, no shock chord at all. The elastic shock chord is used due to the chute... not the nose cone. The rocket is decelerating as it reaches apogee, when the engine ejection charge deploys, yet the nose cone still has wind forces on it, unlike the static test video's shown above. On a small rocket with a lightweight nose cone, using a 50# kevlar cord, seems like having elastic between the cone and the body isn't a requirement.

Is the Nomex used instead of wadding, or in conjunction with wadding. I've never liked the concept of puking out wadding (a.k.a. littering).

Lots of stuff to unpack in your post. Your rocket is low mass and the X-wings (and all the decoration) mean a lot of drag. A high speed deployment seems less likely than a separation -- or other damage -- due to ejection charge. However, there is no reason not to harden the design against BOTH of these issues.

FWIW, a rocket of comparable dimensions to your X-wing with 6 flights on the recovery harness.


That is 1/16" round sewing elastic attached to 50Lb Kevlar kite string. The kevlar leader is attached to the top of the motor mount, and doesn't extend as far as the top of the body tube, so I had to fish it out through the motor mount for this shot. (The elastic is getting a little toasted, I will probably take the opportunity of showing this to you to replace it.)

Clipping the parachute swivel over the elastic so that it slides makes it easier to pack (for my clumsy fingers, anyway).

Bonus, you get to see a parachute made from misprinted balloon mylar (American Science and Surplus sells it in 10 yard strips), Dacron kite string, and Avery labels.

 

[lakeroadster]

FWIW, a rocket of comparable dimensions to your X-wing with 6 flights on the recovery harness.
........

I appreciate your input, but how is your rocket similar? The nose cone on my X-Wing weighs 4 grams (less ballast)... your rocket has a long nose, a payload section and body tube with a coupler, thats a lot of weight to slow down. I agree, that indeed needs an elastic shock chord.

If we step back and review, on a case by case basis, recovery requirements vary.

 

[jlabrasca]

I appreciate your input, but how is your rocket similar?

It is BT50-based. I'd guess that the volume available for the packing a chute and harness isn't too much larger than in your X-wing.

... a long nose, a payload section and body tube with a coupler, thats a lot of weight to slow down.

Indeed. Altogether, with nothing in the payload bay, it is about 30 grams. The elastic has (so far) been equal the task.

The nose cone on my X-Wing weighs 4 grams (less ballast)...

What is the mass of the nose cone with ballast?

I understand that you doubt the benefit of attaching the nose cone to the rocket body with elastic, but I wonder what you are seeing as the cost? What are you trying to avoid, or what are you trying to gain, by using kevlar to attach the nosecone to the rocket body?

 

[lakeroadster]

What are you trying to avoid, or what are you trying to gain, by using kevlar to attach the nosecone to the rocket body?

We've already covered that I'll be changing the design, per kurin's suggestion. Recap for you below.

Would you consider the more conventional, and proven, attachment method? I would go Kevlar leader tied to a swivel, elastic tied to a swivel to allow swapping out a Nomex blanket, parachute with swivel attached to a loop a third of the way down the elastic, and end of the elastic tied to the nose cone. Not that your way wouldn't work, but wouldn't it be safer to go with a proven method?...

Yes... makes sense to me. Thanks.

Edit...........

What is the mass of the nose cone with ballast?

15 grams, total. Based on the added mass Open Rocket recommends, with the new shortened nose cone.

 

[jlabrasca]

We've already covered that I'll be changing the design, per kurin's suggestion. Recap for you below.

Good day, sir! I said good day! <smile>

Actually, that recap does not answer the question what you were trying to do but, yes, I understand that "that makes sense to me" means that you are awarding kuririn the win. When do you think you might get this rocket in the air?

 

[kuririn]

Good day, sir! I said good day! <smile>

Actually, that recap does not answer the question what you were trying to do but, yes, I understand that "that makes sense to me" means that you are awarding kuririn the win. When do you think you might get this rocket in the air?

The win? Since when was this a competition? Just helping out a friend and fellow rocketeer.

 

[lakeroadster]

When do you think you might get this rocket in the air?

I'm retired, this is a hobby, no deadlines, no worries.

The win? Since when was this a competition? Just helping out a friend and fellow rocketeer.

Indeed... and thanks again.

 

[lakeroadster]

From another thread... https://www.rocketryforum.com/threa...e-came-w-a-52-shock-cord.145690/#post-1781408

BEC gives an explanation of shock chords that reinforces what others have instructed here also.

... .

Shock cord length is one of those things that you almost can't have too much of (as long as it doesn't shorten itself with tangling). You may recall from your first turn at rockets how you'd get dents in nose cones or the tops of body tubes after flights. This is what happens when the shock cord stretches and snaps back, bringing the parts back together quickly. You may see references in discussions to an "Estes dent" or a "smile dent" in a nose cone - and that's a pejorative for what happens when the shock cord isn't long enough to actually absorb the shock of the ejection charge and leads to parts slamming back together again. Having a segment of a circle imprinted in a balsa nose cone by the top of the body tube is what gives a "smile" dent.

Going longer is the easiest way to avoid this damage as long as it doesn't tangle up in the process (and you have room in the model for it). Ejection charges for some motors in particular seem to be rather stronger than they were 40 years ago, too, which exacerbates this problem. So does a heavy nose cone or payload section since, of course, it has more momentum than a light blow-molded plastic or balsa nose cone alone. So if both parts which are attached to the shock cord are relatively massive, longer is again better.

Many of us are happy to see shock cords that are actually long enough to avoid this included in recent Estes kits. It's a small but nice change that has happened in the past few years.

If that new scratch build's nose cone is very light, you might be OK.....but I'd untie the shock cord from the nose cone and put another section of rubber or elastic or even Kevlar in there to make it at least a couple of feet long if at all possible. It's a pretty little model, by the way.

 

[neil_w]

At the risk of prolonging this already overlong discussion of shock cords... I think that excessively long shock cords on LPR models are... excessive, if you're using elastic. I don't usually go beyond 4 or 5 feet, with 1/8" or 1/4" elastic (depending on rocket size). Too much shock cord becomes a pain to stuff into small body tubes, and adds needless weight.

When using non-elastic cords (i.e. straight Kevlar) then longer is better.

 

[lakeroadster]

At the risk of prolonging this already overlong discussion of shock cords... I think that excessively long shock cords on LPR models are... excessive, if you're using elastic. I don't usually go beyond 4 or 5 feet, with 1/8" or 1/4" elastic (depending on rocket size). Too much shock cord becomes a pain to stuff into small body tubes, and adds needless weight.

When using non-elastic cords (i.e. straight Kevlar) then longer is better.

Seems logical.... And I can stuff a whole lot of kevlar in a small body tube... where elastic would normally go, due to the size differential.

I am thinking some calculations based on rocket mass, parachute size, chord elasticity (or lack thereof) and rocket velocity at deployment would help bring clarity to all of this?

Probably should also include recovery bay volume vs engine size... logic would dictate a D engine has more ejection force than B/C's?

Rocket science

That being said... when an anomaly happens it would be nice to have the shock absorbing qualities in the mix regardless of what the number crunching indicates.

If we step back and review, on a case by case basis, recovery requirements vary.

 

[Nytrunner]

I am thinking some calculations based on rocket mass, parachute size, chord elasticity (or lack thereof) and rocket velocity at deployment would help bring clarity to all of this?
...

That being said... when an anomaly happens it would be nice to have the shock absorbing qualities in the mix regardless of what the number crunching indicates.

Can't agree with that statement more. Unfortunately the proper material data is woefully lacking for shock analysis most of the time, So we are currently doomed to the rule of thumbs it seems.

 

[GlenP]

you can't have a shock cord too long, and you can't have a too-long discussion about shock cords.

There is really absolutely nothing wrong with your original plan, your Kevlar is plenty long enough and it should work fine as is. Only suggestion, which sounds like you are considering now, is putting the nose cone in the middle of the elastic shock cord or somewhere closer to the chute end, instead of at the Kevlar end.

 

[lakeroadster]

Can't agree with that statement more. Unfortunately the proper material data is woefully lacking for shock analysis most of the time, So we are currently doomed to the rule of thumbs it seems.

Seems like we could do some testing of our own. What kind of data would be needed in regard to the elasticity? Various weights vs stretch verses rebound?

 

[Nytrunner]

Yep! Mass1 gives Deflection1, mass2 gives Def2, etc.... Do a few of those and you can determine "k"

The cord compliance (aka spring constant "k") is very important for shock determination. For elastics, it's easier to get deflection measurements, but kevlar and nylon are a lot harder since their deflection is smaller.

I went through several calculations for my L2 project, but couldn't get any measurable deflection in my 5/8 nylon cords (my apartment isn't set up as a testing lab lol)

Eventually I got stuck making assumptions.

If you know the ejection charge mass (BP grams) and tube volume to be pressurized, there are calculators online that can give data (Chuck Pierce's spreadsheet is good).

Pressure on the tube area gives you a force. That force acting on the nose cone mass gives you an ejection velocity. Velocity, object mass, and cord compliance give the shock force. That's where your "Gs" come from.

 

[lakeroadster]

Some interesting discussion here related to shock chord sizing: https://www.rocketryforum.com/threads/calculating-shock-cord-size-and-lengths.48638/

 

[BABAR]

Interesting conceptual question to NON-elastic shock cord length.

The theory is that a longer shock cord allows the components to “slow down” before the cord becomes taut. For a NON-elastic cord this means that air resistance over the “flight path” of the components is slowing them down. Does this make sense?

I guess there are at least two components of deployment here (double that for black powder dual deploy.).

First is the ejection force separating the nose section (or for dual deploy, forward section) of the rocket from the body section of the rocket (which has the mass of the expended motor in it.). Each section has mass and velocity, so energy is 1/2 * m* (V squared.) for low power, does lengthening the cord from say 1 foot to 5 feet allow a significant time/distance for air resistance to slow down the V? Does the chute or drogue or streamer unfurl enough in the time span it takes to extend say 60” of NON-elastic shock cord (versus 18”) to have an effect?

Second component is when recovery device unfurls enough to bring line taut. This will be dependent on the speed of the rocket at time of deployment. Ideally at apogee deployment this is zero, but for early or late deployment may be much higher. Also for dual deploy, drogue may slow rocket down but still traveling at a good clip when main deploys (otherwise why bother with second chute.). Again, does a longer NON-elastic shock cord (50 feet vs 15 feet in high power) allow the body of the rocket to “slow” down that much?

I have seen that some High Power rockets have the shock cord looped in breakable masking tape, which makes sense as it would dissipate some of the kinetic energy, but Discounting this effect for purpose of discussion. I do wonder whether there is likely “some” stretch even in classically consider “NON-elastic” high power shock cord segments which accounts for the improved outcome of longer versus shorter cords.

I am not questioning the proven experience that “longer is better.” Just trying to understand the physics behind it. Does kinetic energy drop due to significant velocity drop due to air resistance/drag due to longer travel before line goes taut?

Logically a combination of both makes sense. Elastic allows some dissipation of the kinetic energy as the cord stretches. Problems with elastic include bulkiness (takes up space), less resistant to heat from ejection, and the “recoil” may bring the two parts together with destructive consequences (classic “Estes dent.”)

NON-elastic at the least provides more space between components, so even if combined with elastic makes it less likely will get recoil with actual impact of the components.

 

[kuririn]

Let's make it simple. Would you rather jump off a bridge with a bungee cord made of elastic or Kevlar?

 

[lakeroadster]

The theory is that a longer shock cord allows the components to “slow down” before the cord becomes taut. For a NON-elastic cord this means that air resistance over the “flight path” of the components is slowing them down. Does this make sense?

Yes and No.

Yes when recovery deployment doesn't occurs at apogee, in which case the aerodynamic rocket becomes less aerodynamic.. in some cases very non aerodynamic.

No when recovery occurs at apogee... seems like shorter would be better and component sizing would only need to be based on the rockets weight.

so energy is 1/2 * m* (V squared.) for low power, does lengthening the cord from say 1 foot to 5 feet allow a significant time/distance for air resistance to slow down the V? Does the chute or drogue or streamer unfurl enough in the time span it takes to extend say 60” of NON-elastic shock cord (versus 18”) to have an effect?

Wouldn't Force = Mass x Acceleration be the better equation here? Thus you can select the strength of the chord and other recovery components, based on rocket weight and the G's being experienced.

The question is what G force should be used? Max G, in the event of an anomaly where the recover deploys unexpectedly, or using the G forces at Apogee, which should be negligible if the correct delay motor is used?

The Open Rocket simulation for my X-Wing shows a maximum of 30 g's with a D12-5. Assuming an anomaly and recovery deployment at that time it equates to about 6-1/2 lbs of force on the recovery components. But if I look at the G forces at recovery deployment, that force is now more like 1 lb.

Let's make it simple. Would you rather jump off a bridge with a bungee cord made of elastic or Kevlar?

Bungee chord... but the flaw in that comparison is the bridge is a fixed immovable object.

Recovery devices attach to the chute and the rocket, both of which are moving.

 

[BABAR]

Let's make it simple. Would you rather jump off a bridge with a bungee cord made of elastic or Kevlar?

Lol, definitely a bungee.

But there is a significant difference between a rocket in freefall with kinetic energy imparted by the motor ejection versus me jumping off a bridge with the energy imparted with gravity. Unless I actively force myself down there is no chance that the bungee is going to pull me back up and slammed me into the span of the bridge (although slamming into a support post is certainly possible :wink

Still going back to question of length of Kevlar, something that is NOT elastic, just thinking would help is we really understood WHY more is better. Until we have a concept of why, hard to come up with logical reason to quantify “how much more” is enough.

John/LakeRoadster, this is a great build thread and you have invited review of lots of basic concepts which also make this a neat thread. Sorry if I derail it from time to time.

 

[BABAR]

The question is what G force should be used? Max G, in the event of an anomaly where the recover deploys unexpectedly, or using the G forces at Apogee, which should be negligible if the correct delay motor is used?.

Since “stuff” happens, I think the answer is that a responsible rocket hobbyist should be prepared for any reasonable possibility. You can’t anticipate EVERY possibility, but early and late deployments are not “Acts of God”, and recovery system should be reinforced appropriately. That said, an ounce of prevention (making sure packing is “loose”, nose cone not too tight, appropriate delay, tolerable winds, aim away from crowd.....) is worth a pound of Kevlar and elastic.

 

[lakeroadster]

John/LakeRoadster, this is a great build thread and you have invited review of lots of basic concepts which also make this a neat thread. Sorry if I derail it from time to time.

No worries.. I appreciate the correspondence.

Like I've previously commented, working through the build, questioning things and working through them, is what makes this hobby fun for me... much more so than launching. I guess that's why I'm a scratch builder and not a kit builder.

The bridge jumping comment also got me to thinking.. the bridge is fixed and anchored.. just how much of an anchor affect can you really expect from the parachute? There will only be as much force on the recovery materials as the chute can anchor.

And if you look at chute deployment at maximum vertical velocity, when you run the numbers..... most chutes will shred. Therefore the shock chord needs only to be as strong as the weakest link in the parachute construction.

 

[jlabrasca]

The question of how long/what kind of shock cord will depend on what problem you are trying to solve.

What you want to happen: The shock cord and associated parts of the recovery harness keep the parts of the rocket connected to each other and individually intact during the ejection and deployment of the recovery device.

What you don't want to happen: Anything else.

Somewhere upthread, Lakeroadster observed that recovery requirements will vary from rocket to rocket. There are also MANY different ways to meet those differing requirements. Lots of different ways to address potential failure modes of the recovery gear.

BABAR's long rumination in #202 gets it right. The job of every part of the recovery harness is to dissipate mechanical energy.

Some of that energy will be due to the motion of the rocket after motor burn-out but before deployment. Some of that energy will be due to the work done on the rocket parts by expanding gasses expelled by the ejection charge.

The relationship between the energy developed and the forces acting on various parts of the recovery harness is stated most simply as the work energy equivalence theorem: The change in the kinetic energy of an object is equal to the net work done on the object. The work done is the scalar product of the force acting on the body and the displacement of (distance moved by) that body while the force acts on it.

The mathematical description of this starts here

https://hyperphysics.phy-astr.gsu.edu/hbase/enecon.html

This is a lot more physics but it all adds up to the same thing that Random Flying Object said in the thread that Lakeroadster linked in post #201. It is going to be a lot of math, and there is no part of this that does not require calculus (at a minimum) to explain.

How deep do you want to go? We can talk about fluid drag, and elastic hysteresis, and the dependence of the elastic coefficient of restitution on strain rate, etc. Its long walk in very tall weeds, and it is no good asking for a simplified, summarized, or condensed version -- handy for comparing different solutions -- until you have a specific problem to solve.

The bridge jumping comment also got me to thinking.. the bridge is fixed and anchored.. just how much of an anchor affect can you really expect from the parachute? There will only be as much force on the recovery materials as the chute can anchor.

The bungee cord comment was on point, but not for the reasons you suggest. The tension (force) on the recovery harness that will be exerted after the parachute deploys is likely/hopefully going to be small compared to the force exerted between the separable rocket parts by the expanding gas of the ejection charge.

But there is more. It isn't just force that concerns us. As Nytrunner indicated in #197: it is shock, which has to do with the rate at which the force increases (in our case) at some point along the recovery harness.

This video demonstrates the difference in behavior on a system undergoing a large shock loading (rapidly increasing force) and a quasi-static load (slowly increasing force).



Again -- there is a lot more to this. There are other reasons that jumpers, falling at terminal speed, will have very different outcomes if that fall is arrested by an elastic bungee cord rather than a steel cable of equivalent length and mass*. I offer the video only as an illustration that strain rate matters, and is something that can be controlled by using an elastic (or otherwise extensible) tether to attach the rocket parts.

* Edit: In case anybody has read this far, a better explanation than invoking shock would be to note that the work done by the steel cable is the same as the work done by the bungee cord, but that the elasticity of the bungee cord means that a smaller force is exerted over a longer distance to effect the same change in kinetic energy. With the steel cable, it is likely that the larger acceleration due to the larger force will result in stresses internal to the jumper's body which will exceed the yield strength of something the jumper needs to not be dead. Same with the recovery harness: The elastic means a smaller force over a longer distance and -- correspondingly -- smaller stresses applied to the communicating parts of the harness.

 

[lakeroadster]

Revised recovery components drawing by increasing length of the elastic shock chord and relocating the nose cone from the kevlar to mid span of the elastic.

 

[BABAR]

i will try to be less ruminant

I like the nose cone attachment in the middle of the elastic. Covers the force of ejection on one side and the force of parachute deployment on the other.