Nomex, Kevlar & Chute Protection

 

 

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Nomex, Kevlar and Chute Protection

 

The Truth about Kevlar vs. Nomex -- Decomposition Temperatures

 

There is some misunderstanding about parachute protection, what material is best, and how much protection it gives.  Let's start with some facts that should make the truth obvious:

 

 

Black Powder Flame Temperature:

3300 F

Nomex Carbonizes at:

800 F

Kevlar Decomposes at:

900 F

Nylon Melting Temperature: 510 F
Nylon Combustion Temperature: 905 F
Polyester Melting Temperature: 490 F
Polyester Combustion Temperature: 910 F
 

 

If you have seen a statement that Kevlar makes a better chute protector than Nomex because of the higher "melting" temperature, looking at the temperature above, how much better do you think it could be.  There is no fabric material that will not be damaged if exposed directly to a black powder ejection charge. 

 

 

Kevlar costs more than twice as much as Nomex and six times as much as Nylon or Polyester.  If it has no better survivability, why would you use it for this application?

 

 

 

Kevlar Thread on Nomex Fabric -- NOT

 

This is another another silly notion and maybe just a marketing ploy.  "We use Kevlar thread to sew the button holes on our Nomex chute protectors" or something like that.  The button holes on chute protectors are for threading your shock cord through so when you eject your parachute, you don't loose your chute protector.  It stays attached to your shock cord which runs through it.  There should be very little force on your chute protector if it is packed properly.  You should have enough shock cord below your chute protector that when it is fully extended, your chute protector is considerably beyond the end of your body tube that it was ejected out of -- at least two or three body tube diameters.  This way, there is plenty of room for the compressed ejection gases to vent and depressurize so that there is no force trying to blow the chute protector off the shock cord.  Even if there was some odd situation that tried to rip the chute protector off the shock cord, think about it.  It is the weakest link scenario.  When a chain is stretched beyond its strength, what gives?  The weakest link.  There is no sense in using thread stronger than the material itself because it would just rip apart the Nomex and leave the Kevlar ring around the shock cord.  Whatever is the weakest is going to fail first.

 

This may lead to the question of what thread should be used around the edges to prevent fraying and around the button hole for heat protection.  You certainly could use Nomex thread, and if you wanted to waste your money, you could use Kevlar but the truth is that it really doesn't matter all that much.  You could use nylon or polyester as far as that goes because if it comes into contact with burning black powder, it is going to burn through regardless of what it is.  Otherwise, it may scorch or melt a little but if it does, it isn't going to hurt anything any more than the fabric itself being scorched anyway.  All the sewing  is going to be near the edges so when it is properly loaded, it is the farthest away from the ejection gases anyway, and first to leave the body tube.

 

I could see an argument (slightly) for more reason to use Nomex thread on the Nomex shock cord protective sleeve because it is closer to the ejection charge, especially the bottom end.  You probably want all the protection you can get there but even in this case, I don't believe you would see that much advantage.  There definitely is no more reason to use Kevlar thread in this case than for the chute protector.

 

 

Exposure Time and Damage

 

The actual black powder burn time is about a tenth of a second so any burning particles or flame can only be in contact with the chute protector for a time less than that.  After ignition of the black powder, the chute protector is out of the rocket body in about a half second so the hot ejection gases are not in contact with anything very long either.  Because of this short duration, the damage is mostly restricted to discoloration and some small holes where a few burning particles or flame actually touch the material.  Below are frames from an ejection charge sized for a 4" diameter by 14" long parachute storage tube.  The frames are 1/15th of a second apart.  You can see, 1) the flame is large and will come into contact with anything next to it in the recovery section of the rocket, and 2) the huge amount of gasses that are obscuring the brick in the 3rd, 4th, and 5th frame and is what ejects the recovery system.

 

 

 

Thickness and permeability

 

The material (Nomex, Kevlar or other material) is not the only consideration.  The thickness and the permeability are also factors.  The weight of the material is a direct indication of the thickness.  4 oz per square yard is fairly light, 7 or 8 oz per square yard is fairly heavy.  The tightness of the weave can also slightly affect the thickness for a given weight, the tighter weave being slightly thinner for the same weight.  You want low permeability in a chute protector also.  Low permeability means that not much gas will pass through it.  You can do a subjective test on a fabric by putting it up against your mouth and blowing through it.  If you can blow through it easily, it has a high permeability and is not a good protection material.  If you can't blow much air through it, it has a low permeability and is more suitable for a protector.  Burning particles cause the most damage but the hot ejection gases and flame can also damage a chute so you don't want those hot gases passing through the protector and coming in contact with the parachute.

 

Strength of Material

 

If Kevlar isn't any better than Nomex, then when should you use it?

 

The main difference between Nomex and Kevlar is that Kevlar is MUCH stronger than Nomex and is difficult to cut.  Just ask someone that has used Kevlar instead of fiberglass over a body tube.  That is why Kevlar is used in "bullet proof vests" (puncture resistant vests) and other high strength applications like your shock cords or recovery harness.  When it comes to strength, there is another factor that is important -- whether the fabric is made from continuous filament or spun.  Spun thread is made from short pieces, 1/2" to 8" is normal, that are twisted together or are wound around a small continuous filament. This is also referred to as staple fibers and the fabric woven from it is called staple fabric (duh!).  Here is an example of the difference in strength:

Strength for size *E threads

  Kevlar, Filament 25 lbs
  Kevlar, Spun 12 lbs
  Nylon, Filament 11 lbs
  Polyester, Filament 11 lbs
  Nomex, Filament 7.5 lbs
  Polypropylene, Filament 6.4 lbs
  Acrylic, Filament 5.3 lbs
  Nomex, Spun 4.5 lbs
  Polyester, Spun 2.9 lbs
  Cotton, Spun 2.9 lbs

* There are three different common thread sizes used in the textile industry.  How these relate are shown in the table below.  Standard multipurpose polyester sewing thread is about a Tex size 40.  Lightweight is considered 23 to 69.  69 is the heaviest you can use on a standard home sewing machine.  Heavier than that requires a commercial sewing machine.

 

Tex Size

24

35

50

70

90

105

Commercial Size

33

46

46

69

92

92

Government Size

A

B

B

E

F

F

Spun fabric is much cheaper than filament woven fabric but it is also possible to get a tighter weave and so lower permeability with filament fabric.  Nevertheless, a tight weave spun thread fabric is adequate for chute protection, especially when used with cellulose wadding.

 

Use Wadding WITH Protective Chute Material

 

Mark Canepa in his book, Modern High-Power Rocketry 2, page 186 and 187, talks about Nomex cloth and  "cellulose-type" material (it is actually treated cellulose, not "cellulose type" material) as being used separately and then says that the "cellulose-type material" is "particularly beneficial when used in conjunction with a Nomex pad."  Actually it makes no sense to NOT use them together unless you use an entirely different method of protection.  If you don't, you will have to replace the Nomex frequently if not after every flight.  Nomex is not that inexpensive.  The cellulose is similar to or the same thing as the cellulose insulation that is blown into attics.

 

 

Cellulose insulation before being ejected

 
   
 

Cellulose insulation after being ejected.  Note the scorching.

 

The wadding is put in as a sacrificial flame retardant material between the ejection charge and the parachute and parachute protection.  Cellulose insulation is treated with a flame retardant chemical so that it will not support a flame in your attic and you need the same as recovery wadding in your rocket so it doesn't come down flaming and setting something on fire.  Notice I said "sacrificial" material.  It is going to be charred by the ejection charge but the amount you would use in each flight is dirt cheap so you don't care.  It also is not going to be recovered.  It will fall to earth and decompose quickly.

 

So why do you need additional chute protection if you use wadding?  If you are talking about a model rocket with a C motor in a 1" body tube, you don't.  If you are talking about a high power rocket, you do.  The higher mass of the recovery equipment will not "get out of the way" of the ejection gases fast enough.  Some will leak around the perimeter of the wadding and can still damage your parachute.  Your parachute needs to be completely wrapped in the protective fabric, or at least bottom and sides, so gases escaping along the inside body tube wall do not come into contact with the parachute, and preferably, not the recovery harness (shock cord).

 

Nomex is expensive, wadding is inexpensive.  Since it is very cheap leave the insulation that is in your attic there and buy some for your rocket launches.  You can usually get it at any home supply store, insulation company, or lumber yard.  It is basically just shredded paper, usually recycled newspapers or newsprint, that has been treated with a fire retardant chemical so that it will not support a flame.  Note that it is of concern to fire departments and agencies concerned with fire.  This stuff can still smolder and it has been argued that in a fire, it could still reignite other flammable material after a house or building fire has been put out.  In our case, it will usually just scorch in spots, not smolder.  However, I would think about this if you are foolish enough to be launching in an extremely high fire danger area and time period.

 

Rock wool (or Rockwool) is another wadding material that can be used and is actually, in my opinion, the best wadding material if you can find it locally.  It is used in the same way as fiberglass insulation with a paper or foil backing and looks very similar to fiberglass insulation except it is dark colored.  It is also used as a blown in insulation but more expensive than cellulose so not as common today.  It is made from  Basalt rock and Chalk. These are melted at 1600 C into a lava which is blown into a large spinning chamber, which pulls the lava into fibers like "cotton candy."  This material absolutely does not burn or support a flame but can melt and discolor in spots.  This material is the favorite medium for hydroponic greenhouses and hobby growers.  It can be obtained from hydroponic suppliers but is definitely more expensive from them than alternatives.

 

Testing Results

 

A torch or hot air gun directed at fabric for a couple seconds is not a good test.  A proper test is actually ejecting a sample in a real situation with a parachute or mockup and a standard ejection charge in a real rocket or mock up body tube.  See below.  Also when reading about a test that is done, note whether the material is the same weight because unless they are the same weight and same permeability, the test is not valid.

 

These pictures show five out of several types of material in their original state before being tested.  The Top Flight material had the highest permeability (lets gases through), the green surplus was the next highest.  The other three had very low permeability.

   

 

Filament woven tight weave HT483-54 Nomex, 6.0 oz/sq yd.

Nomex from a military contract for flight clothes, military green, 4.5 oz/sq yd.

Top Flight Recovery, LLC Firewall Parachute Protector, 4.0 oz/sq yd.

Filament woven tight weave 16082 Polyester, 3.6 oz/sq yd.

Filament woven tight weave
HT10-41 Nomex, 6.5 oz/sq yd.

 

 

The following show the same materials as above after being ejected from an actual 4" diameter rocket, the first four without recovery wadding, the last (the HT10-41 Nomex) was ejected WITH wadding.  The first row of pictues show the ejection side, the second row show the parachute side.  You need to click on each picture to view the large picture version to see clearly what the damage is to each.  Note: the colors look different because the pictures were taken under different light conditions.

 

 

Note:  The purpose of these pictures is NOT to show relative damage or what type was damaged the most.  The point is that ALL fabrics ARE damaged without wadding and every single one tested ended up with one or more actual holes in the fabric, though small.  You can see the holes better in the larger views.  It is interesting that the green light weight Nomex and the polyester actually appear to have the least damage on the chute side.  However, keep in mind that each of these was tested only once and another round of tests might have shown differences so again, the point is that using any fabric chute protection without wadding is not a good idea.  Actually, I think this particular tight weave polyester is even a better choice than a high permeability Nomex when using wadding.  The problem is that people get an idea in their head based on what SEEMS logical and then it is hard to look at actual test results and reality and change that opinion.

 

Check out the last column, the HT10-41 Nomex.  It was ejected WITH cellulose wadding and as can be clearly seen, there is a huge difference.  There is virtually no scorching on the bottom.  The only scorching is in the area that was adjacent to the body tube and that is minimal.  The hot gases were effectively blocked from the bottom but some escape along the sides of the tube and that portion of the protective cloth. What little scorching is present is there.  On the chute side, there is ZERO scorching.  The chute was totally protected when protective fabric AND wadding is used correctly.

 

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