Taming the 'VMax kick': A launch pad modification that just might help

[cvanc]

Hi troops-

We've all seen a flight go ripping off at a wicked angle from rod whip. It's often associated with one of the super fast propellants out there (VMax, Warp 9, Lokis,... you know the ones).

A discussion was had at Mini Midwest Power about this. I feel this is a pad problem. The pads many clubs use were designed and purchased long before these ultrafast fuels were marketed, and frankly, they just don't seem to be up to the task. They're not rigid enough.

But what to do about it? Buying or building a bunch of new pads doesn't sound like fun. Perhaps what we own now could be modified to work better?

The conversation turned to the idea of ISOLATING the blast deflector from the rest of the pad. What if the blast deflector was removed and simply laid on the ground under the nozzle? That way the 'push' of motor thrust against the deflector is not coupled into the legs & rail assembly.

This was tried on a couple of flights and it seemed to help, but more testing is called for. I just wanted to put it out there for others to discuss and comment on. Have you done something to try and solve this problem, and if so, what? Did it seem to help? Did it cause any new problems?

Thanks.

 

[markkoelsch]

I think that might help. I do not think the designers of any of the commercial pads I have seen ever contemplated an N10000, and I think most pads are under designed and under built for this and other big, nasty motors.

Ultimately I think the logical choice is largely semipermanent pads. Concrete base with antenna type tower and possibly wires to the ground to the top of the tower. Even then, who knows what the limit of such a pad may be?

Frankly, I think we are hitting the practical limit of high power. Motors are the easy part in a sense. Ground support for these really big powerful rockets is in question. Ask the guys from Thunda Down Under about ground support for the v2.

See Steve Eves Saturn 5. It was a big rocket with a really big pad. The pad was moved 4 or 5 feet as memory serves.

 

[blackjack2564]

Frankly, I think we are hitting the practical limit of high power. Motors are the easy part in a sense. Ground support for these really big powerful rockets is in question. Ask the guys from Thunda Down Under about ground support for the v2.

.

Nah Mark..we'll just keep building bigger ground support! LOL

Close ups of the V-2 [full scale ] pad.

The 10x10 1/2in steel base plate weighed 1/2 ton.

Outriggers weighed couple hundred pounds apprx. 20ft long.

Base V-2 sat on very close to real support.

4 in.steel well drill pipe, 28ft long welded to base plate.

Receiver pipe inside rocket went up center only 25ft. was PVC and fit perfectly [approx 1/4 in. of play] over steel rod.

Rocket was assembled on pad in 3 sections....fincan....body...NC.

Basically a Giant Estes rocket launching off a 4 in rod... on a 1 ton pad. LOL

My dimensions are by memory, most certainly not accurate, but close enough for you to get the idea!

To answer OP's question....most larger clubs have trailer mounted pads that can handle VERY large heavy rods/rails/uni-strut, that will support several hundred pound rocket 20-30 ft tall.

Really big project almost always require the fliers to build one, rocket specific.

Properly designed pads aren't that much of an issue, the cost of building them, is.

Most clubs capable of handling large launches & rockets have their own designs built & on hand.

QCRS has plenty of large pads capable of flying N-10,000's N-5800's P-motors. I helped build some of them, maybe you haven't seen them yet. They just put out a few smaller ones for Mini MW power. Tim's got trailer pads in the yard. Heck N-10,000's have flown at MWP without any issues.

 

[markkoelsch]

Ok CJ. I still think my statement is close to reality for a variety of reasons.

Massive pads are heavy, large, and expensive. Transporting them is not easy as detailed by your description of the V2 pad. Some of the trailer mounted pads can handle larger stuff.

If you have flown a class 3 rocket and been a part of the FAA process you know how onerous that can be. I know as I did Tim's first application. Most folks have no idea how much work that is to do.

I am not saying that folks should not do these things. I still think that we are hitting a practical limit due to a number of reasons including cost, engineering, materials, regulatory, and guidance. Some people will always go beyond...

 

[blackjack2564]

Looking at it under those terms...............I must agree with you sir!

I tend to forget, most folks won't go the distance, nor do they have someone like you in their corner.....by the way, thanks for all you do & have done.

 

[markkoelsch]

Carl, sorry to hijack your tread a bit. I am now on the mend as I had surgery today- Carl, you know about what as I mentioned last time I saw you.

Back to topic. I think the next logical step is new/upgraded pads. The pads need to be a bit heavier for sure. I think this includes going to a heavier rail like a 1020 or 1530. This would be much stiffer assuming the rest of the pad is up to the task.

 

[cbrarick]

I like a good vmax, you just gotta step up your game on the pads. I fly gizmos on K2045 no problems but I'm flying off 1515 rails. I've done the I800 but again flew on the 1515 stuff. I flew the N10000 in a stretch big gizmo but was on the MDRA hyraulic tower. You just got to stay away from light pads, rods and the 1010 stuff.

Kinda funny, low power guys talk about rod whip on C6's and we don't for Vmax/warp 9....lesson unlearned??

JMHO, doped up from surgery Tues....

 

[llickteig1]

I have observed and commented to other club members about the affect of high thrust motors on proper flight angles. You have called it the "VMax kick"; I have seen evidence of it with large Blue Thunder and other propellant/motor types with a high amount of initial thrust. This effect doesn't seem to happen very often, but when it does occur it is a head scratcher because it isn't the initial rod whip or weather-cocking we're used to seeing more often. It just feels different.

We launch all HPR rockets from very stout pads that can accommodate any size of rod and our 1010 and 1515 rails. The pads are low to the ground, and the blast deflector is a permanent part of the pad. (We do have a couple XXL pads for the really beefy birds).

The comment I will make is that I believe the anomaly observed is not necessarily rod whip per se from the rail flexing but rather a condition where the high thrust causes the pad to shift or jump which alters the angle of the rail before the rocket clears it. The size of the rocket is less of a factor than the kick of the motor, which leads me to the conclusion that the rigidity of the rail isn't necessarily the problem. Whether the pad shift is caused by the thump of the initial propellant blast or the thrust pushing from beside or underneath the blast deflector is unknown.

I can see how isolating the blast from the pad support structure would help in many pad designs I have used at other clubs. For our pads, isolating the blast deflector might not help because I think the thrust could be pushed under the center of the pad. Our pad's legs are crossed in a large X and made from 1" square steel tubing. I think a possible solution is to stake down the ends of the X to firmly hold the pad in place. I'd like to try that to see if that helps.

What would be really cool is some hi-def, slo-mo pad-cam video that demonstrates the problem and results of tests to see what improves the situation.

YMMV, --Lance.

 

[RocketHunter]

I have observed and commented to other club members about the affect of high thrust motors on proper flight angles. You have called it the "VMax kick"; I have seen evidence of it with large Blue Thunder and other propellant/motor types with a high amount of initial thrust. This effect doesn't seem to happen very often, but when it does occur it is a head scratcher because it isn't the initial rod whip or weather-cocking we're used to seeing more often. It just feels different.

We launch all HPR rockets from very stout pads that can accommodate any size of rod and our 1010 and 1515 rails. The pads are low to the ground, and the blast deflector is a permanent part of the pad. (We do have a couple XXL pads for the really beefy birds).

The comment I will make is that I believe the anomaly observed is not necessarily rod whip per se from the rail flexing but rather a condition where the high thrust causes the pad to shift or jump which alters the angle of the rail before the rocket clears it. The size of the rocket is less of a factor than the kick of the motor, which leads me to the conclusion that the rigidity of the rail isn't necessarily the problem. Whether the pad shift is caused by the thump of the initial propellant blast or the thrust pushing from beside or underneath the blast deflector is unknown.

I can see how isolating the blast from the pad support structure would help in many pad designs I have used at other clubs. For our pads, isolating the blast deflector might not help because I think the thrust could be pushed under the center of the pad. Our pad's legs are crossed in a large X and made from 1" square steel tubing. I think a possible solution is to stake down the ends of the X to firmly hold the pad in place. I'd like to try that to see if that helps.

What would be really cool is some hi-def, slo-mo pad-cam video that demonstrates the problem and results of tests to see what improves the situation.

YMMV, --Lance.

Here's an example of the effect you described with a K1100T at 0:48:
[video=youtube;Bscl1EAQMM8]https://www.youtube.com/watch?v=Bscl1EAQMM8[/video]

We fixed it by using a slightly heavier, lower to the ground pad with a flat blast plate:
[video=youtube;ULawqUKbBok]https://www.youtube.com/watch?v=ULawqUKbBok[/video]

 

[rharshberger]

Why not have a spine made for the back of the rod from half inch steel or aluminum cut on a CNC plasma with holes (circular or triangular) that tapers toward the rod tip, similar to the old Nike Ajax rails.

 

[markkoelsch]

Why not have a spine made for the back of the rod from half inch steel or aluminum cut on a CNC plasma with holes (circular or triangular) that tapers toward the rod tip, similar to the old Nike Ajax rails.

Take a look at the 1020 or 1530 extrusions. Pretty much accomplishes the spine.

 

[rharshberger]

Take a look at the 1020 or 1530 extrusions. Pretty much accomplishes the spine.

Definitely the 1020 and the 1530 are heavier due to the fact they are doubled however, there is still the side to side. A spine or even just mounting the rail to a piece of 4" pipe with short standoffs and two diagonal braces about halfway up the length would stiffen it in all directions. Yes it would be more expensive than the 1020 or 1530. Kinda like this: (at least the backing rail)


IMO the 1530 and the 1020 still won't be up to the task with L and above motors. We don't get many Vmax motors yet at our launches so its not as bad for us as some of the larger groups, and our club is limited to K or below without pre-clearance, and with pre-clearance to L motors. Then again I could be totally wrong about the stiffness of the two extrusions you are suggesting might work.

 

[markkoelsch]

Carl, in post surgery recovery mode I pulled out some video Justin shot at Bong. I think I might have figured this out, at least what happened with our little adventure last summer at LDRS (big rocket, big vmax motor, rocket heading in weird direction, and landing in a swamp).

The video was not of your flight, but of a K1100 Blue Thunder. The pad you flew off of and this one were the same TWA pads. The adjustment for elevation is via a large screw, but the position is held by friction. This K1100 and clearly your vmax applied enough force to overcome this friction. To detail the video. If I were to explain it from an angle/protractor type of explanation the rocket started with a angle of 100-105 degrees. There was a very good slow motion of this. Motor hits hard on the integrated blast deflector, the thrust overcomes the friction, and in a few frames it changes from the aforementioned angle to say 80 degrees with rail whip in no time. I mean this happened before the rocket left the rail, and clearly the pad stayed in this new angle some 20-30 degrees in a different direction.

The pad is clearly not sturdy enough for this type of motor. I strongly suspect the design is insufficient for this type of motor. I think the ticket is an elevation jack screw like on my Impulse Aerospace Quad Pod. I will snap pictures later and attach to this thread.

 

[jcato]

I have observed and commented to other club members about the affect of high thrust motors on proper flight angles. You have called it the "VMax kick"; I have seen evidence of it with large Blue Thunder and other propellant/motor types with a high amount of initial thrust. This effect doesn't seem to happen very often, but when it does occur it is a head scratcher because it isn't the initial rod whip or weather-cocking we're used to seeing more often. It just feels different.

<snip>

The comment I will make is that I believe the anomaly observed is not necessarily rod whip per se from the rail flexing but rather a condition where the high thrust causes the pad to shift or jump which alters the angle of the rail before the rocket clears it. The size of the rocket is less of a factor than the kick of the motor, which leads me to the conclusion that the rigidity of the rail isn't necessarily the problem. Whether the pad shift is caused by the thump of the initial propellant blast or the thrust pushing from beside or underneath the blast deflector is unknown.

{Lance, I'm just using your post to respond to, as it most clearly identifies the issues}

Folks, this whole problem is just Newton's Third Law in action -- that every 'action' results in an equal and opposite 'REaction'. There is a 'downward force' applied to the blast deflector that *exactly* equals the upward force imparted to the rocket --- just look at the thrust curve and you can see the magnitude (and timings) of the force imparted to the pad (blast deflector) (until, that is, the rocket leaves the pre-launch location - and then the force falls back to zero at some point where the thrust of the rocket motor is too far away to have any effect).

BUT... that initial force (or 'VMax kick') - right at ignition - is simply EQUAL (and downward) to the initial thrust of the motor. Newton's Third Law - plain and simple.

With the widespread use of a single angled deflector, it's no wonder there's all these pads 'dancing' all over the place. With all due respect, this single-angled deflector is really not that good of a design. If the mass of the pad is some several multiples of the rocket (and sufficiently capable of resisting the force imparted by the motor), it can (could) handle it (at least in the past) -- but, now with all these big, high thrust motors, that's no longer the case.

If you will look at how NASA handles these things (thinking here mostly the Saturn 1B, Saturn V and Shuttle), you will notice the shape of the blast deflector is an inverted 'V' - and that is for a reason: If you split the exhaust (equally) in two opposite directions, the horizontal force generated will cancel out - the result being NO horizontal force imparted to the pad. (Something similar occurred with the Mercury-Redstone launch table - but, in this case, that was a 4-way deflector.)

Note that that force will follow the 'angle of incidence equals the angle of deflection' (from the physics of light) - so, a 45degree (single) deflector will turn the entire force of the exhaust precisely horizontal (in ONE direction) - probably the worst angle one could have. (Don't worry, that's not the (major) problem - the 'major' problem is simply turning that force in ONE direction). However, 45 degrees is *exactly* the angle you want on a inverted 'V', because that is necessary to make sure the two thrust vectors are coincident (co-linear) and, thus, opposite - and, thus, cancelling. (A steeper angle would resolve into some slight upward force on the pad/deflector - a shallower angle would resolve in a slight downward 'resultant' on the pad/deflector. Thus, shallower would be a better 'error' if you couldn't get it 'dead on' 45.) This also requires the vehicle be precisely (as one can) centered over the the apex of the 'V', but we never found that to be too difficult to accomplish. Any mis-alignment would result in a little more 'push' on one side over the other -- but, again, negating 90% or 95% of the lateral force is far superior to negating none of it.

RocketHunter's video (first video) of the K1100 illustrates better than any other example I've seen - man, that pad was 'Dancing with the Stars' <g>.

Now, that post I put up here a few weeks ago of my Nike-Tomahawk, here:



... is a J800 -- but this same pad also hosted this self-same rocket on our subject K1100 in Argonia (on a rocket that was heavier (I'm guessing, may not be) than the one in RocketHunter's video - and on a 1/2" rod, no less) and the pad did not move at all (and was only weighted down with a 20# piece of Inconel) -- and I would posit that would mostly be because that inverted 'V' deflector cancelled out this lateral force.

(Maybe even more relevant is that this 15 (some-odd) lb two-stage rocket was launched with a 1/2" diameter rod - I wouldn't try this today because the plans were to adapt this deflector scheme to the red tower (see at the bottom of this post) - but, the fact remains that this rocket flew 4 times from this pad (3 - J800s and 1 - K1100) - and the photo above shows the boost was rather nearly vertical. Stop RocketHunter's K1100 video at 0:55s and compare that to this Nike-Tomahawk - both rockets 4 feet or so above the rod (i.e. already a 'free body in space'). The other Tomahawk liftoff shots were very similar.)

Some other shots of the pad here:



(the split in the launch table back near the rod anchorage simply allows for the blast deflector to be removed separately from the rod - to either be replaced with a different (and more appropriate) deflector for any needs of special designs (which has never happened) -- OR, to allow the blast deflector to be re-mounted on another launcher (like my red tower that I've uploaded previously -- which was (back then) 'in planning' for this upgrade - this also hasn't happened as of this hour)):

I can see how isolating the blast from the pad support structure would help in many pad designs I have used at other clubs. For our pads, isolating the blast deflector might not help because I think the thrust could be pushed under the center of the pad. Our pad's legs are crossed in a large X and made from 1" square steel tubing. I think a possible solution is to stake down the ends of the X to firmly hold the pad in place. I'd like to try that to see if that helps.

Isolating the deflector and/or staking the pad could surely add some additional measure of rigidity to the pad structure, to be sure. Negating that force in the first place only makes your job all the easier. IMO. Why struggle dealing with some anomalous forces when a little thought can simply remove them altogether?

-- john.

 

[markkoelsch]

{Lance, I'm just using your post to respond to, as it most clearly identifies the issues}

Folks, this whole problem is just Newton's Third Law in action -- that every 'action' results in an equal and opposite 'REaction'. There is a 'downward force' applied to the blast deflector that *exactly* equals the upward force imparted to the rocket --- just look at the thrust curve and you can see the magnitude (and timings) of the force imparted to the pad (blast deflector) (until, that is, the rocket leaves the pre-launch location - and then the force falls back to zero at some point where the thrust of the rocket motor is too far away to have any effect).

BUT... that initial force (or 'VMax kick') - right at ignition - is simply EQUAL (and downward) to the initial thrust of the motor. Newton's Third Law - plain and simple.

With the widespread use of a single angled deflector, it's no wonder there's all these pads 'dancing' all over the place. With all due respect, this single-angled deflector is really not that good of a design. If the mass of the pad is some several multiples of the rocket (and sufficiently capable of resisting the force imparted by the motor), it can (could) handle it (at least in the past) -- but, now with all these big, high thrust motors, that's no longer the case.

If you will look at how NASA handles these things (thinking here mostly the Saturn 1B, Saturn V and Shuttle), you will notice the shape of the blast deflector is an inverted 'V' - and that is for a reason: If you split the exhaust (equally) in two opposite directions, the horizontal force generated will cancel out - the result being NO horizontal force imparted to the pad. (Something similar occurred with the Mercury-Redstone launch table - but, in this case, that was a 4-way deflector.)

Note that that force will follow the 'angle of incidence equals the angle of deflection' (from the physics of light) - so, a 45degree (single) deflector will turn the entire force of the exhaust precisely horizontal (in ONE direction) - probably the worst angle one could have. (Don't worry, that's not the (major) problem - the 'major' problem is simply turning that force in ONE direction). However, 45 degrees is *exactly* the angle you want on a inverted 'V', because that is necessary to make sure the two thrust vectors are coincident (co-linear) and, thus, opposite - and, thus, cancelling. (A steeper angle would resolve into some slight upward force on the pad/deflector - a shallower angle would resolve in a slight downward 'resultant' on the pad/deflector. Thus, shallower would be a better 'error' if you couldn't get it 'dead on' 45.) This also requires the vehicle be precisely (as one can) centered over the the apex of the 'V', but we never found that to be too difficult to accomplish. Any mis-alignment would result in a little more 'push' on one side over the other -- but, again, negating 90% or 95% of the lateral force is far superior to negating none of it.

RocketHunter's video (first video) of the K1100 illustrates better than any other example I've seen - man, that pad was 'Dancing with the Stars' <g>.

Now, that post I put up here a few weeks ago of my Nike-Tomahawk, here:

View attachment 264956View attachment 264955

... is a J800 -- but this same pad also hosted this self-same rocket on our subject K1100 in Argonia (on a rocket that was heavier (I'm guessing, may not be) than the one in RocketHunter's video - and on a 1/2" rod, no less) and the pad did not move at all (and was only weighted down with a 20# piece of Inconel) -- and I would posit that would mostly be because that inverted 'V' deflector cancelled out this lateral force.

(Maybe even more relevant is that this 15 (some-odd) lb two-stage rocket was launched with a 1/2" diameter rod - I wouldn't try this today because the plans were to adapt this deflector scheme to the red tower (see at the bottom of this post) - but, the fact remains that this rocket flew 4 times from this pad (3 - J800s and 1 - K1100) - and the photo above shows the boost was rather nearly vertical. Stop RocketHunter's K1100 video at 0:55s and compare that to this Nike-Tomahawk - both rockets 4 feet or so above the rod (i.e. already a 'free body in space'). The other Tomahawk liftoff shots were very similar.)

Some other shots of the pad here:

View attachment 264958View attachment 264959View attachment 264957

(the split in the launch table back near the rod anchorage simply allows for the blast deflector to be removed separately from the rod - to either be replaced with a different (and more appropriate) deflector for any needs of special designs (which has never happened) -- OR, to allow the blast deflector to be re-mounted on another launcher (like my red tower that I've uploaded previously -- which was (back then) 'in planning' for this upgrade - this also hasn't happened as of this hour)):

View attachment 264960




Isolating the deflector and/or staking the pad could surely add some additional measure of rigidity to the pad structure, to be sure. Negating that force in the first place only makes your job all the easier. IMO. Why struggle dealing with some anomalous forces when a little thought can simply remove them altogether?

-- john.

John, you are correct. The issue is that we are dealing with existent pads, which while sufficient for the rockets of the era in which they were built are not up to the kick that many rockets motors have now.

I personally flew with the OP when one of these events occurred at ldrs 33. The pad was not sufficient to have a 50 pound rocket flying an M4770 on it. Simple really. The issue is that now that we have identified the issue how to move forward- retrofit existing pass or replace some of them with a better design.

 

[jcato]

John, you are correct. The issue is that we are dealing with existent pads, which while sufficient for the rockets of the era in which they were built are not up to the kick that many rockets motors have now.

I personally flew with the OP when one of these events occurred at ldrs 33. The pad was not sufficient to have a 50 pound rocket flying an M4770 on it. Simple really. The issue is that now that we have identified the issue how to move forward- retrofit existing pass or replace some of them with a better design.

Probably *both*, Mark.

Actually, the first step is getting folks to recognize that there *is* a problem that needs to be fixed -- but discussions such as these indicate that the awareness of the issue is coming to light and spreading. Biggest hindrance to that is simple resistance to change - but it is encouraging that there are those who recognize 'change' is the only way out.

Some of these other discussions (here) about isolating the deflector from the pad proper is a workable solution -- as is have a flat (but it *needs* to be flat - if connected in any way to the pad) 'blast plate' on the ground, a rather simple workaround. If connected to the pad, can't let it 'wobble' around, as that is then allowing it to come right back to this 'single angled' plate.

It would also seem that a retrofit inverted 'V' deflector design could be developed in such a way as an easy bolt on to existing pads is possible (seems like there are a few 'standard' pad designs out there that some adaptation of the 'V' could be accommodated) -- but I just don't know the extent of 'unique' designs and how much of an issue it would be to work a 'V' deflector to fit all (or most) of them. That would only come through a survey of sorts to ferret out the different designs.

Moving forward from that, it would seem the hobby could get a few folks together and develop a 'reference design' that addresses these issues and publicize it widely -- with a recommendation from the hobby orgs to follow that path. Gradually, the new design would take over. The bottom line, however, is to get the word out that these 'single deflector' designs are inadequate for the larger vehicles and should be abandoned in any future design thinking. Once the 'thinking' changes, the pads will as well.

-- john.

 

[bobkrech]

The issue has little to do with the motor and everything to do with the rigidity and stiffness of the launch pad. If you look at the videos in post #9 above, you will see that the unweighted leg lifts clear off the ground while the weighted legs bend under the transferred launch load. If the 3 support legs were staked into the ground, the pad would be much more rigid and the leg could not lift and transfer the excess load to the other 2 legs. (NOTE: Triangles are inherently stiff. The structural material or the joint has to fail before they deform. Staking three tripod legs to the ground makes a 4 sided pyramid which is the most rigid structure you can make. For the pad base to move, a stake to the ground as to pull out. or one of the joints/welds has to break before the base will more.)

The other common mistake is to attach a stiff 1010 or very stiff 1515 rail to the launch pad tripod with a side-bolted 1/2" aluminum rod. That "convenience" negates the stiffness of the launch rail. At a minimum, when you convert a stiff rod pad to a rail pad, you should thread the hole on the end of the 1010 rail with a 1/4-20 stud at least 1.5" long or a 1515 rail with a 5/16-18 stud at least 2" long. You then need to drill and tap a 6" x 0.5" OD or larger diameter steel or stainless steel rod for the 1/4-20 stud or to drill and tap a 6" x 0.75" OD or larger diameter steel or stainless steel rod for the 5/16-18 stud and connect the rod to the rail and tighten with a pair of vice grips. The will allow you to mount the rail on-axis into the circular rod holder without additional off-set torque. If the ends of the adapter rods are square as well as the rails, then the rod becomes an extension of the rail and if the top of the pad is square with relationship to the bore, the load from the rod will transfer directly to the rod holder and can not bend which makes a very stiff joint.

These two simple modifications will result in a very rigid and stiff rail pad that won't exhibit a Vmax or Warp9 "kick".

Bob

 

[jcato]

The issue has little to do with the motor and everything to do with the rigidity and stiffness of the launch pad.

Unless the motor is a big one and then it has a whole lot to do with the motor. I would take your comments further to say the issue has everything to do with the structure accepting, resolving and transferring the forces imposed upon it to the ground... which applies equally to buildings, bridges and, yes, rocket launch pads.

Many of these pad designs were adequate when the motors were smaller in that their rigidity was enough to accept the lateral forces (in our present case) and not be adversely affected by it (i.e. deflecting (or allowing the rail to deflect) to the point that the trajectory of the vehicle was affected). This is why I said, "Unless the motor is a big one..." above. As the motors (and their initial thrust spike) got bigger, then the ratio of the forces they imposed over the ability of the pad to 'take it' passed unity with the results we see today - as represented by the 'post #9 video'. And that kind of behavior is clear indication that these 'structures' are not performing in a manner appropriate to their (intended) function. That indicates the design is flawed.

If you look at the videos in post #9 above, you will see that the unweighted leg lifts clear off the ground while the weighted legs bend under the transferred launch load.

Well, all three legs were 'weighted' (and probably equally) until the lateral forces 'rearranged things' - but I noticed that as well - even with slo-mo, it appears that leg was off the ground for several seconds (the rocket was clear and gone before it returned to ground). The thing that is kind of curious is why they didn't rotate the rail/deflector 180 degrees (to be opposite one of the legs) and use that outstanding leg to help maintain rigidity. It might not have completely prevented what happened (I would doubt it), but it would have lessened the effect. As it is, the arrangement of the legs with the blast deflector was about the worst it could have been. (Now, this arrangement might have been the best if one were relying on staking to restrain the pad -- giving maximum moment arm to the 'staking force'.)

One other interesting aspect of this launch was how the rail 'twanged' at the point where (it appears) the top rail guide left the rail - because until that point, the rocket was pretty much parallel with the rail (albeit not necessarily vertical), but then rapidly diverged from the rail - which may have exacerbated the deflection of the flight path. I'm assuming the rocket/rail 'package' had some inertia and stored up a little elastic energy that was then released when the top guide was free of the rail. Interesting phenomena, nonetheless. All the more reason to investigate secondary struts 'trussing' the rail a little (I've always felt that just sticking that rail vertical on the pad structure without some secondary struts was pushing things a little too far).

If the 3 support legs were staked into the ground, the pad would be much more rigid and the leg could not lift and transfer the excess load to the other 2 legs. (NOTE: Triangles are inherently stiff. The structural material or the joint has to fail before they deform. Staking three tripod legs to the ground makes a 4 sided pyramid which is the most rigid structure you can make. For the pad base to move, a stake to the ground (h)as to pull out. or one of the joints/welds has to break before the base will mo(v)e.)

I agree that staking would go a long way to preventing what we saw here. Unless, of course, there were some heavy rains two days before the launch and the field is soft and soggy -- then the stakes would need to be 2-3 times longer to provide the restraint needed (assuming the organizers had planned for this and had longer stakes available).

My point (with my first post above) is to understand how (and where) the forces are being generated with these launch operations and designing a system that negates them as much as possible (and, thereby, negates the overriding need to stake in the first place). That's just 'good design'.

Consider two scenarios: in both cases, the rocket is the same, the motor is the same (K1100 in this case), the pad is the same (and let's go ahead and stake it to the ground) - with the singular exception being the blast deflector.

The ONLY difference is that one pad (Case A) has a single angle deflector (like in the video) and the other (Case B) has an inverted 'V' deflector.

Case A will have to resolve a 300# (initial thrust spike on a K1100) lateral force imposed at launch (even if staked to the ground) - while Case B will have a 10-15# lateral load (assuming some slight mis-alignment of the deflector - otherwise, it would be zero lbs) and while staking might be nice here, it's almost optional...

.. and this is for the same rocket on the *same* motor on the *same* pad.

Staked to the ground or not, there isn't an architect nor engineer anywhere in the world that wouldn't jump at the chance to reduce the lateral loads on his structure by over 95% (to as much as 100%) with such a minor effort as (in this case) simply changing the blast deflector (Yeah, I know, bridges and buildings don't (generally) need 'blast deflectors', but you see my point).

Don't get me wrong, staking is a good thing and I agree with you on that point. We're just looking at two different aspects of the problem - and finding viable solutions in each place.

-- john.

 

[Ravenex]

I agree that a v blast deflector would reduce lateral loading. However, no one has mentioned how high the rocket and blast deflector are above the ground. Given the width of the base and the height of the blast plate it's doesn't surprise me that the pads do this. If the rail was mounted so the pivot was 2ft up the rail the bottom of the rod and the blast plate would be much lower to the ground. Also I'm wondering with single stick legs if there is some bounce involved. Without braces those legs are effectively big springs, when the initial downward forces are applied it would load the leg which would unload as the rocket exhaust left the blast plate.

 

[crossfire]

Carl
I agree with you 100%. A while back I talked to 2 of the club leaders about this same problem. A lot of flyers are flying smaller rockets with big motors and are still flying off pads that were made 15-20 years ago. Years ago the big motors were used in big air frame rockets and most of the time flew off much larger pads. The pads need to keep with the times. Clubs don't need to replace all their pads just get 2-3 stronger pads.