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Mars Pathfinder
Frequently Asked Questions

Entry, Descent and Landing

Updated 10 April 1996

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How can you use such a small parachute on Mars? What has been done to ensure successful operation? What is stopping the lander from landing on a large rock?
What stops the parachute from falling on the lander? Why is Pathfinder not landing near the so-called "face"? Why does Pathfinder use petal/airbags vs. traditional landing gear?
Why is Pathfinder landing at night (Mars local time)? This airbag thing sounds a little shaky. How many times do you expect the lander to bounce before it comes to a rest? And what's to keep it from flipping over while its bouncing? What happens if there are strong winds in the landing region when the craft is surrounded by the airbags?
What is "planetary quarantine"? Did you consider the use of a parachute similar to the types connected to small personal airplanes that can be deployed at high speeds and have a mechanism to continually allow the parachute to expand as it slows? Could the solid rocket motors, by firing, affect either the lander or the kevlar bridle?
If the lander is not falling exactly vertically, could the solid rocket motors carry it way off course, or even break the assembly free of the parachute? Could the heat shield, the backshell or the parachute come into contact with the lander inadvertently? Could one of the airbags fail to inflate?
If something does go wrong, how will you be able to tell what happened later? Does telemetry continue? Have you calculated a probability of a successful landing? Is there a possibility of the airbags hitting a sharp rock and popping?

If the Mars atmosphere is less than 1% than that of Earth, how can a parachute of the size you are using be sufficient? It would seem to me that you would need a parachute close to 1000' wide to achieve the same effect.

Would you please explain the dynamics of placing a lander on Mars, and why a small parachute would work on Mars as it does on Earth?

You ask a very insightful question. The bottom line is you're right, parachutes this small aren't sufficient on Mars! On Mars Pathfinder, as on Viking, we use a "small" 40.5 ft (12.5 m) chute. It was scaled so that, with our lighter lander, it does about as much for the our descent speed as does Viking's. Our terminal velocity seconds before getting to the ground (where the atmosphere is "thickest") is still about 65 m/s (146 mph)!!

You are correct, it would indeed take a a larger chute to get slower "normal" Earth-like terminal velocities. Our chute on Mars is about the equivalent of a chute 38 times smaller in area on Earth (6.5 ft across!), and this includes the effect of Mars' lower gravity! A chute that could lower our lander to the Martian ground at a gentle 10 m/s (22 mph) would have to have an area about 42 times larger than our "little" chute (or a diameter of 263 ft)! That's 42 times the mass (and volume) of our 10 kg chute, or 420 kg, more than the mass of our entire lander! It wouldn't fit! We would need to have a "gossamer" (ultra-light weight material) parachute and then figure out how to get it open at high speeds!

This is why we turned to solid rockets to stop our lander just before we hit the ground. Viking, too, used liquid rockets to slow the terminal decent. Also Pathfinder's airbags protect the lander from the local terrain variations (bumps, craters, rocks, hills, etc.) after the rockets do their thing.

So why do we do we use a chute at all? Well, parachutes might not be all that good a laying a lander gently down on the Martian surface, but they do a spectacular job of braking something moving very fast. Remember, the drag FORCE a chute generates (therefore its deceleration), is proportional to the square of the velocity and only linearly proportional to the atmospheric density; so even a thin atmosphere and a "small" chute will do much to slow our entry vehicle down once the heatshield's aerobraking has been mostly achieved.

This is also true of heatshield, our entry vehicle (like Viking's) enters the upper atmosphere at 7 km/s (or more than 15,000 mph!). Most of this is reduced by the friction with the heatshield. But even 2 minutes later, our vehicle is still screaming in at nearly 400 m/s (900 mph) when the parachute opens before slowing down to 65 m/s near the ground. I'd say that reducing our velocity by a factor of 6 (a factor of 36 in kinetic energy), isn't all that bad for only 10 kg of extra payload mass, wouldn't you?

So, the short answer is, you're right, parachutes don't work on Mars like they do on Earth (neither do airbags, but that is another story), but they do a great job when you need to slow down something that is whipping through the Martian atmosphere FAST!

--Rob Manning

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As with other missions, the ability to deploy the panels for solar use, high gain communications, or any other use for that matter, has not been as good as could be. What has been done in ensuring successful operation now or in the future? It seems without them opening up all the way it can cause major problems. Like Galileo's high gain antenna and Mars Surveyor's 20 degree shortfall in deployment of its solar panels.

The problems associated with moving parts are difficult ones to solve. Since the cost to send one kilogram of material into space is so high, spacecraft designers must be very stingy in allocating mass to the engineers who make the mechanisms. You might be surprised that the typical spacecraft mechanism can be destroyed with your bare hands!

The other part of this equation is that MOST of the time, mechanisms must only need do their jobs under rather benign weightless conditions in space, BUT they must also be able to handle the much rougher conditions that precede getting there: ground handling and launch. It is these phases of the mechanism's life that are the most traumatic. They are the most difficult to quantify as well. I don't think the designers of the Galileo high gain antenna mechanism would have expected that the antenna would be closed for so long before finally opened in flight and that it would have had to survive three cross-country road trips in a van! (Both of these events were a direct result of the Challenger disaster.)

There is no magic formula for making mechanisms work in all situations, but we have been learning just how subtle these problems can be. The trick is to learn from your (and other people's) mistakes. Mars Pathfinder has more than its share of moving parts. We knew that going in, so we went out of our way to be a bit paranoid about it. We hired the very best spacecraft mechanical engineers we could find. Going to Mars made the job a bit more difficult in some cases because of our need to have the mechanisms work under very harsh environmental conditions (harsher even than in deep space). For example, the Rover, the IMP camera and the high gain antenna actuators must all work under very cold conditions (as low as -90 deg C). Most lubricants do not lubricate at those temperatures. We had to make sure that the actuators were either warmed before they were used or had adequate torque margins for the motor to overcome the sticky lubricant before it warmed up with use. In some cases we "overkilled" the problem (e.g. the lander petal actuators) and provided much more torque than we thought we really needed - just in case. (I could go on and on.) It is safe to say that the mechanisms on Mars Pathfinder were a LOT of work. But we tested and tested them (even beating them up!) under many rough conditions until we were finally satisfied that they will work fine when we need them to.

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What is stopping the lander from "landing" on a large rock, and making it impossible to open up the craft to do its thing on Mars?

We all wondered about this at the beginning. So we tried it! First of all, it turns out that we had a hard time getting the inflated, 17 ft beachball of a lander close to a big rock! As long as those airbags stayed inflated, it wanted to roll away from anything big and pointy. Secondly, even when we did manage to coax it right next to a wicked boulder, the petals opened right up even if it meant having the whole lander do a backflip! It took some work, but we actually made that happen once in our Mars Yard at JPL and without damage! It really helped that the petal actuators (a motor and a gear train mounted on each of the three petal hinge lines) had the torque margin to actually LIFT the lander off of the ground (they can even indefinitely support the lander in a sort of "iron cross" once open). And these tests were done under Earth's gravity, which gave the rocks a distinct advantage. With many many tests behind us, in not one case would it have got stuck.

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What stops the parachute from falling onto the craft and gumming things up that way?

Lots of people asked us that question at the beginning. If you don't give this some serious thought, there is a real risk that the lander could get covered by the chute (a bit more than embarrassing). Fortunately we designed the timing and sizing of the solid rocket firing (remember there are three mounted inside the backshell) such that when the lander inside its inflated airbags comes to a stop some 12 m above the Martian surface, the software activates a cutter that cuts away the bridle thereby freeing the backshell from the lander. The rockets, with still a quarter second of impulse left over, launch the backshell up and away taking the parachute with it (at an angle, tumbling as it goes). Meanwhile the lander and airbag go bouncing away in the other direction! (Don't you wish you could be there to watch it all happen?). You can click here to see an artist's rendition of this procedure...

--Rob Manning

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To satisfy the curious, Pathfinder should have landed near the so-called Face/Pyramid area. Or is that a hoax?

That there is a rock formation on Mars that looks somewhat like a face is certainly true, but it is also true there are many similar naturally occurring structures on the Earth, Moon and Mars that resemble faces, animals and even man-made designs. The best way to see what large geologic structures exist on Mars is to use the high resolution cameras on board the recently-launched Mars Global Surveyor. Mars Pathfinder can not be accurately aimed to any site smaller than a typical US county. We are specifically targeting the ancient Ares Vallis outflow channel. Fortunately the channel is big so we will not miss it! This site is ideally suited to Mars Pathfinder's geologic mission. We believe a huge flood carved that channel and deposited a large number and variety of rocks from the highland water source into the flood basin where we intend to land. Our miniature robotic geologist, the Sojourner Rover, will be able to analyze these various rock types and give us an idea of how they were formed and about Mars' early history.

In addition, all of the power for the spacecraft is collected from solar panels. In order to get maximum power, one of the landing site requirements was to have the sun be high in the sky. This restricted the landing site to +/- 20 degrees from the equator of Mars. Cydonia (where the so-called "face" is located) is at too high of a latitude for the lander to receive adequate power.

--Rob Manning

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What are the main advantages of the combination of petals/airbags (Pathfinder) vs. the traditional landing gear (Viking)? Is it all about weight?

When originally conceived as the MESUR project (Mars Environmental Survey), Mars Pathfinder was going to be the first of a large number of identical units that would be dropped all over Mars. Although JPL initially proposed Viking-like landers, it was deemed to be too expensive. NASA very much wanted to send landers to Mars at very low cost compared with previous missions like Viking. In addition, we wanted to minimize the amount of a priori knowledge about the particular landing sites. To do that we needed to design a highly robust landing system that would allow some of these landers to land in places on Mars that are very rugged. Small landers using traditional landing gear can be quite susceptible to the local terrain. The airbag system proposed by NASA Ames Research Center (as well as by Russian researchers) gave us a means to land in some of the most inhospitable locales on Mars. The funding for the multiple lander mission (MESUR) was dropped after the full funding of what became Mars Pathfinder (called at that time MESUR Pathfinder) was approved. So it was cost considerations that drove us to this design. Future landers going to challenging terrains on Mars will likely use a combination of controlled descent like Viking and Mars '98, and impact attenuation and automatic righting like Mars Pathfinder.

--Rob Manning

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Is the fact that trajectory constraints restrict Mars Pathfinder to a local Martian nighttime landing at Ares Vallis unavoidable or is it due to the publicity surrounding a July 4th landing? From an engineering standpoint, would a daytime landing increase chances of mission success by immediately providing power to the lander's solar petals?

Trajectory constraints limit us to either an early morning landing (3 am) or a late evening landing (~6 pm). These times are determined by the launch and arrival dates (the specific dates result in a given angle between the approach trajectory and the Sun - the two landing times correspond to an upwind landing (retrograde) and a downwind landing (prograde)). The local time at JPL (PDT) of landing will be about 10 am, so we won't really avoid any publicity (in fact, we like it). Actually, we planned to landing early in the morning because of several engineering reasons. The first is that gives us a long day ahead in which to attempt to deploy the rover. Because the lander takes several hours to cool down after landing, it really doesn't mind landing in the early morning. The advantage is that we have many hours before the first real night we will see. The other advantage is that an early morning landing is probably the most gentle from a wind perspective. That helps us during EDL, when wind shear and strong gusts are a concern.

--Richard Cook

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This airbag thing sounds a little shaky. How many times do you expect the lander to bounce before it comes to a rest? And what's to keep it from flipping over while its bouncing?

I would use the word "bouncy" rather than "shaky". The lander is completely surrounded by airbags like a giant (5 m or 17 ft diameter) beachball; it can bounce, roll and come to rest in any orientation. So there is no preferred orientation. Remember that the lander will automatically right itself once the bags have been deflated and retracted.

We fully expect many bounces before the lander comes to rest suspended inside its airbag cocoon. Although we have designed and extensively tested the airbags to withstand multiple very hard bounces (the equivalent of dropping it from a 10 story building on Mars), we expect that the lander will see about 10 or less significant bounces with what we consider a "nominal" (average) impact after being dropped from about 12 m (40 feet) above the Martian ground. 12 m is the "target" bridle cut altitude. Because it is probable that the terminal descent solid rocket (RAD) motors in the backshell will not be perfectly upright when they fire, the lander will likely get a sideways push from the rockets that will also cause the lander-airbag combination to roll along the ground for tens of meters before coming to rest. The bags were specifically designed to BOTH be impact absorbers AND automobile tires!

--Rob Manning

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What happens if there are strong winds in the landing region when the craft is surrounded by the airbags? Could the craft just start rolling and continue to do so much longer than the estimated time? If this happens could it roll until damage is done to the protective bags? Or do you just sit back and relax until the wind goes down before final deployment? Maybe the density of the atmosphere is low enough so this will not be a problem.

Good question! As I mentioned in another FAQ, due to the very low density of the Martian atmosphere, the force imparted by a wind on Mars is much less than wind at the same speed on Earth. For example, a very high (and rare) wind speed of 100 mph on Mars would feel the same as a moderate Earth breeze of only 10 mph. Despite the large surface area that the airbags present to the wind, that force would not be enough to keep the airbags from rolling for very long. Even if it did continue to roll and was helped by a slight terrain slope, after 11 minutes on the surface, the flight software will slowly begin to deflate the airbags by activating the four airbag retraction actuators (one for each of the four bags). These motors reel in multiple retraction cords mounted inside the bags. One of these cords (one on each bag) will pull open vents that allow the airbag gases to leak out. As soon as the bags begin deflating, the kinetic energy of any residual rolling motion will quickly be dissipated much like the drag of a partially flat tire. In the "airbag roll down the hill tests" which we performed in the JPL parking lot (one of our more entertaining tests), we confirmed that it would be very hard to sustain a rolling motion for very long. In those tests we measured and calculated the rate of kinetic energy loss (i.e. slow-down) on a flat surface for a given initial velocity. Rocks, sand and gravel only tend to slow down the lander further.

--Rob Manning

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What is "planetary quarantine"?

There are internationally accepted guidelines that NASA has adopted as rules that govern the number and distribution of Earth "spores" (bacteria or other biological contaminate) that we can sprinkle (intentionlly or otherwise) on the surface of another planet (or moon) believed to have had the potential of harboring life in the past or present.

The two Viking landers were sterilized in a large oven and then encapsulated just before they were rocketed to Mars. This means that there were minimal concerns about the spacecraft inadvertantly crashing onto the Martian surface and spraying Earth spores everywhere. Baking the Viking spacecraft was considered very controversial in its day however. The builders were very concerned that the high temperatures would damage or degrade the materials used in the construction. On Pathfinder we were very reluctant to bake the whole spacecraft. Instead, we opted for baking bits and pieces (low gain antenna, parachute, etc.), and cleaning the rest (wiping or immersing in cleaning solvents).

The end result is that the outside of Pathfinder is clean to within the allotment set by the international guidelines (we know this because we were continously taking bio-assays to count the bugs). But what if we inadvertantly put it onto a collision course with Mars and then somehow lost control of the spacecraft? Wouldn't the Earth-bacteria sealed on the inside escape onto the surface when it crash landed?

We don't know, but we could avoid the issue altogether by putting the spacecraft on a collision course in the first place. By keeping the trajectory near the edge (limb) of Mars we could make sure that, if the spacecraft is lost control of, either it misses Mars altogether, or crash lands with a velocity slow enough so it doesn't spew the bugs all over the surface. Of course, this won't happen!

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Did you consider the use of a parachute similar to the types connected to small personal airplanes that can be deployed at high speeds and have a mechanism to continually allow the parachute to expand as it slows?

You are stumping the "expert". I am really not at all close to being a parachute expert (despite my pontificating). I am sure you are quite correct that new more complex parachute designs could very well be developed along the lines you propose.

I do know that there are big challenges associated with the qualification (design verification) of parachutes destined to be used on Mars, especially if they are required to be opened at supersonic speeds as ours is (Mach 1.7). It is quite difficult to reproduce the opening conditions on Earth. Wind tunnels have their drawbacks (size & wall effects) and high-altitude high-mach number tests are very expensive (out of our league for Mars Pathfinder). That is why we did not consider more complex designs. In fact, a going-in position at the beginning was that we would save money by borrowing the design and test results from the billion-dollar Viking mission from 20 years ago.

For chutes that open up at low speeds (say after a drag chute has slowed the lander down), then maybe the problem could be solved more cost effectively. It is a fun and potentially quite useful solution that should be explored. (Imaging a chute you could "steer" right to your favorite landing site!) There are many more design solutions yet to be discovered!!

Could the solid rocket motors, by firing, affect either the lander or the kevlar bridle?

Of course the rocket firing does affect the lander and the bridle. We spent a good deal of time modeling and measuring the forces that the rocket firing will have on the backshell structure, bridle and its attachments on both ends, as well as the acceleration the rockets impart to the lander. We actually fired flight-like rockets inside the spare backshell with an identical bridle. Also the lander and the lander electronics were tested in a centrifuge which created the forces we expect due to entry, RAD (rocket) firing and final impact.

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If the lander is not falling exactly vertically, could the solid rocket motors carry it way off course, or even break the assembly free of the parachute?

Very good question. For the same reason a pendulum is rarely vertical, there is a very low probability that the lander and the backshell will be exactly vertical at the moment that the rockets fire. We spent a lot of effort to figure just how bad that angle could get. It turns out that the aerodynamics of the parachute and the backshell limit the angle to somewhat less than 20 degrees. This is the main reason why the parachute is shaped like it is. The large (9 ft) "skirt" (the "band" in the Disk-Gap-Band parachute configuration terminology) doesn't help slow the lander down so much as it stablizes and helps minimize swing. Even with angles near 20 deg, we fully expect that the rockets will do a pretty good job of imparting a large horizontal velocity to the lander before it ever hits the ground (up to 50 mph). For this reason we had to design and test the airbags to handle impacts at shallow fast grazing angles. So not only do the airbags have to be good impact absorbers, but they also have to act like a giant aircraft landing gear tire! Since we are only trying to aim the lander to an area 100 km by 200 km long, we don't mind if the lander rolls for a kilometer or more.

As far as the paracute is concerned, during the rocket firing, the parachute's role pretty much comes to an end. The tension on the parachute suspension lines drops to near zero because of fast rate of deceleration. We do not expect (and our many simulations confirm) that large angles of the backshell during the 2 sec of rocket firing will change that significantly.

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Could the heat shield, the backshell or the parachute come into contact with the lander inadvertently?

Another good question. There are two parts to your question.

For the problem of "recontact", we spent a lot of effort to make sure that recontact does not occur after separation. We did this in two ways. First we spent a good deal of effort to create models (mathematical/computer ones) of most of the components (including the parachute). We characterized the forces of things like the separation springs, aerodynamics and gravity, measured clearances and mass properties, determined the quantitative uncertainty in these parameters (tolerances), and finally we simulated the separation events over and over again using the full range of possible uncertainties - guaranteeing that recontact does not occur in any of the possible cases. So that we could double check that our models were created correctly, we then do a small number of separation tests using real hardware and confirm that the model correctly predicts the outcomes for the test. This latter step is what we call "model correlation". If this all sounds like a lot of work, rest assured that it is! This is why making spacecraft isn't cheap.

The other part of the problem is in what happens to the backshell, parachute and heatshield after separation. In the case of the heatshield, we know that it will hit the ground before the lander and a long way away (it separates when the lander is miles above the surface). In the case of the backshell, we ensure that the rockets are still firing when the bridle is cut at the lander. The residual rocket impulse pops the backshell up and away at up to 100 m/s (225 mph). It will go up in a long arc because the center of mass of the backshell is slightly offset. Since the parachute stays with the backshell as it pops up, it too will sail away from the lander. From the lander's cameras, we hope to be able to see the backshell and the parchute in the distance after we land.

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Could one of the airbags fail to inflate?

Yes, anything could happen! We all know Murphy's laws. But we strongly doubt it. Why? Because we tried to design the bags so that it is very unlikely to happen (that's our only defense against those laws of his). We asked ourselves, "What might prevent one or more of the bags from inflating?".

Of course, it is possible that the EDL (entry descent and landing) software could somehow forget to schedule the airbag inflation (it turn out the software could forget to hit the "button" and it still works due to a timer backup system we designed). Testing and verifying software takes a lot work (we are still at it!).

Each of the four bags had to be "strapped down" to their respective petal. To untie them the software must command a (dual) relay closure which applies voltage to a pair of (dual redundant) explosive squibs (also called pyrotechnic initiators) which a activate cable cutter. There is a cutter on each of the four bags that all fire at the same time. If a cutter doesn't fire then that bag won't inflate. Fortunately, we know from experience that these cutters will cut provided one of the two initiators fires; so we made sure that they fire by using dual redundant circuitry and power to the initiators.

Once the bags have been unstrapped (this occurs 1/4 sec before the gas generators ignite), the three gas generators (GGs) are ignited in the same manner as the cutters, each with dual redundant circuitry and initiators. Although the bags work best when all three GGs work, if only two of them work, then all four bags will still inflate because the bags are interconnected. They are interconnect so that they can share gas and equalize the pressure among them. In this case the bags will still do an adequate job of protecting the lander from impact provided that the impact conditions are also not the worst case.

To make this complex system reliable, we had to balance risk throughout the design process, juggling the cost of additional complexity with relative reliability.

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If something does go wrong, how will you be able to tell what happened later? Does telemetry continue?

During the EDL process itself, we get very little information about what is going on, and even that information is indirectly obtained. We don't get digital data (telemetry) until about 3 hours after landing. The data we get back then however includes a rather detailed record of what happened during EDL.

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Have you calculated a probability of a successful landing?

No. Although we considered it and we did apply a unique method of measuring our belief in the probability of success, there is really no way to determine the probability from the bottom up. What we would like of course, is some unbiased method were we could calculate the number of cases where the mission would not work if you were to launch and land 100 Mars Pathfinders. Unfortunately you can't really do that. We can and did pieces of that calculation. For example, we simulated EDL over and over again under many different conditions. We made sure that in no more than 1 percent of the cases, the final impact conditions exceeded the airbag test case conditions (that fraction of a percent did not mean that the landing was a failure however).

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Is there a possibility of the airbags hitting a sharp rock and popping?

There is a possibility that the airbags will hit a sharp rock. Although our Mars geologist claimed that that is a low probability, we nevertheless want to make sure that our bags could handle a large sharp nasty rock even under the worst (fastest/shallowest) impact condition. We knew that very large rocks (even sharp ones) were not a problem because they tended to act like small hills. There is sort of a "magic" size of sharp rock which could do the maximum damage. We also knew that "needle" rocks (like the tufa towers near Mono lake in California) are highly unlikely even on foreign planets like Mars - there are only just so many ways that rocks can be formed. From experiments and geometry (the lobes of the bags are 1 meter in diameter) we decided that the worst sharp rock was a 0.5 to 0.7 meter sharp rock point that is firmly attached to the Mars ground. So how do you make sure it can handle such a rock? Test it! We built a large sloped platform inside the Plum Brook 120 ft vaccum chamber in Sandusky Ohio and we studded it with sharp 0.6 m rocks (the nasty kind that can rip skin). These rocks were bolted down to the ramp and a full scale heavy lander inside the bags was pulled down with bungy cords so that it impacted these rocks at better than 25 m/s (60 mph).

After some trial and error over several months in late 1995 and early 1996 (redesigning bags as we went) we found that the bags would take this abuse with minimal damage and only minor puncture. It turns out that vectran bags do not "pop" when they are punctured. The incredible tensile strength of this fabric (and the special "rip-stop" weave) prevents propagation of tears.

--Rob Manning

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