Desalination, 29 (1979) 17-23 ~vierScientificPS~Comp~ay.Amsterdam-_PrintedinTheNetherlands

TEBBADYNAMICS AS APPLIED TO ICE

Alan Pope, Consulting Engineer 816 Val Verde SE, Albuquerque, Nev Mexico 87108 (USA)

In late 1959 and early 1960 -- up to May 1st when Gary Powers was shot down over Moscow. U-2 aircraft operated out of a small airfield near Ankara, Turkey. They flew up over Swastopol. Odessa, Kiev, Uoscow. Leningrad, and into a then-secret airbase near Bode. Norway. The pictures they brought back shoved that the Russians were doing the same thing we were -- namely digging huge holes in the ground (which would later be called silos) for their intercontinental ballistic missiles. Being at that time in the weapon business. I suggested that we attempt to streamline bombs so that they would go deep into the ground so that a minimum yield would do the job. This prin- ciple is, of course, as old as the hills. Any farm boy who has ever blown

up a stump knovs you don't put the explosive on top of the stump; you dig a hole and put it underneath. But few people realize the actual ratio of ex- plosive Is SO to 1. 1000 tons on the surface makes a 135-foot crater; so does 20 tons down 60 feet.

So we got the job and, after the usual literature search, concluded that it couldn't be done. There was a 25,000 lb. bomb in stockpile used to attack submarine pens but, being marginally stable in the ground, sometimes it went down a hundred feet and sometimes it didn't. Since we were committed. we decided to proceed vlth a thorough research program which would at least give us an understanding of how a projectile penetrates.

Normally penetration is studied with a rifle and a box of sand. By great good fortune, we decided against this; we learned later that the old way would have led us completely astray. Combining the facts that we needed an onboard instrument package and that the then-smallest atom bomb was 7" in diameter, we opted for a 9” tube vith 1" walls. The telemetry people weren't enthusiastic about providing a package that would survive impact from 25,000 feet, but they gave us the best they had and it turned out to be ample. I don't remember why we made our vehicle 12 feet long -- maybe to get the weight. That also turned out to be a good decision.

Our first drop in July, 1961, was a smashing success in that nothing got

smashed. The earth penetrator came doun from 25,000 feet and impacted at about 1200 fpm. It penetrated very hard ground about 30 feet. There were

four surprises. First, the curve of deceleration vs time was not expotential, but flat. Second, instead of a maximum G in the thousands, it was about 400.

Third, there was no entry crater. Fourth. the hole for a 9" vehicle was 7" across. Thus there never was any hope of pulling a penetrator out of its

hole. You have to dig all the way to its nose in soil or rock. In ice. as

we learned later, once the base is exposed, steam will melt the beast free.

I am reminded at this point of another company's design after terradynamics was announced. It had a barb on its nose to keep it from jumping back out of

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the ground. That was about as useless as wheels on a cow.

As we dropped hundreds of penetrators and studied the recovered vehicles and

the holes, a picture of penetration finally emerged. The nose throws the soil aside so violently that a cavity is formed for several diameters along the body, which must be long enough to extend into the reclosed region ii stability is to be maintained. Obviously. the longer and more gradual the nose, the less the lateral acceleration of the soil, up to where the nose is

too weak and curls over. This lets the body turn and break up- Above say 2000 fps, depending on the soil, the lateral momentum of the soil is so high that it crushes the soil radially and the hole does not completely close. In some cases this results in a loss of stability and a break-up of the ve- hicle. Work is now proceeding in the very high speed regime with good SUC- Ci5SS.

As the vehicle slows, the cavity diminishes in diameter and length until the

soil scrapes along the vehicle sides, giving it the highest deceleration of

the h7hole trip. There is a little rebound at the end. Ice is the same, only more so, with deceleration rising with time.

Empirical formulae were developed which predict total penetration with about 20% accuracy, since soil is such a variable. The important parameters are: velocity to the first power, weight/frontal area to the half power. nose shape and a soil constant. Thus doubling the velocity, while quadrupling the energy, only about doubles the depth. W/A must be far higher than typi-

cal bomb shapes -- a minimum of 10 psi is suggested. This is what makes it impossible to study penetration with a rifle. It takes a steel projectile

30 inches long to have a W/A of 10, even though it is solid.

Up to impact speeds of 1400 fps we have gotten by with vehicles made of 1020

steel, but above that, again depending on the soil, D6A or maraged typed should be employed. It is particularly hard to make a joint that survives. Usually the nose is solid and the afterbody hollow in order to get the aero- dynamic c-g. far enough forward. No study has been made to determine where the underground c-g. must be. The stabilizing forces in soil are very large, and 45% seems okay.

After penetration, the vehicle is located by probing down the hole, with a

pipe, using compressed air to clear the probe's path. Early attempts at hav-

ing the body emit a noise for triangulation proved useless.

After a successful drop program using desert hardpan as a target with depths up to 220 feet, and Florida where we limited our vehicles to permit easy re- covery and only went to 70 feet, we moved north to Fairbanks, Alaska to pene- trnte permafrost and glacial ice. There were no big surprises. Permafrost is not very thick around Fairbanks -- say 20 feet -- and the cold muck be- neath it prevented recovery. While the soil hole is straight and only

loosely filled, in cold muck it closes up in a few minutes. (I have gone down the recovery holes; I don't recommend it for people with claustrophobia)

We then went south a hundred miles and made two drops into Gulkana glacier from a hovering helicopter. The units were the same, 900 lbs, 9" diameter, (thus a W/A - 14 psi), with a 6 caliber nose. They impacted at 480 fps, and both went 18 feet. This corresponded to a hard soil. The same vehicle with a sharper nose and higher speed could penetrate 100 feet or more. The angle between the ice and the velocity should be 45* to assure penetration.

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Penetration in ice follows classical laws. Ice is always somewhat porous and thus may be compressed. Next to the body the pressure causes melting of the ice and the water layer and protects the vehicle's surface so that there is not the slightest damage to it, even to the paint. We did not do a para-

metric study on icebergs, but a conservative approach would be to start with an L/D of 10 and a W/A of 10 psi and see what happens.

I appreciate that my audience hos thought about icebergs a lot more than I,

but if you don't mind an amateur conjecturing a little, here are some com- ments. I see six applications of high-speed penetration worth looking into.

1. To implant beacons for tracking.

2. To measure the thickness of sea ice to see if ice breakers will be able to get through.

3. To measure iceberg tllickness. (Electronics is easier.)

4. To implant explosives to crack off pieces, or a glacier.

5. To implant anchor posts.

6. To survey the thickness of a wide glacier.

Let me discuss each from a penetrator standpoint.

1. Beacons. A great deal of work has been done at Sandia Laboratories on shallow implants where positive implant is desired with little penetration.

Thus the technology completely exists for "sticking in" but ending up with

cn antenna on or above the surface.

2. Thickness of sea ice. A great deal of work has also been done as regar&

measuring the thickness of sea ice up to 10 feet thick without limitationsof

location, weather, or darkness. This type of penetrator uses a transmitter at the end of a trailing line longer than the estimated ice thickness or

penetration depth. The signal from the on-board accelerometer goes directly to an integrator on board the drop aircraft and ice thickness is obtained instantaneously. During the research program a flame-orange dummy was drop- ped at the same time as the penetrator- It stays on the surface and locates

the hole for a ground party to use for measurements -- thickness, tempera- ture, salinity, and tensile strength. No spalling occurs at the sea-ice interface and thickness accuracy within 10% is normal. We tried a small

corner reflector to be used instead of the full-scale dummy, but without success.

A second type of ice thickness measuring device employs a two-part penetra- tor. The rear part, containing the transmitter, breaks off and remains on the ice surface while the heavy penetrator, which contains an accelerometer

with signal conditioning equipment, penetrates the ice and reels out an um- bilical line which connects it to the transmitter. After emerging from the bottom of the ice, the penetrator eventually breaks the wire and is expanded. This same system is used to drop a sonobuoy through the ice and form a sort of permanent listening post.

Difficulties have been encountered with keeping the antenna from breakingoff and the reel-out system intact, but both have been solved for impacts up to 400 fps. Higher speed might well require further research. The advantage

of the two-part system for ice thickness measurement is that the transmitter can be recovered for re-use.

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In some cases, when the water beneath the ice was shallow, its depth was also measured.

The final design for measuring sea ice thickness ended up at 50 lbs and a diameter of 2.75 inches. It had a conic nose and at 500 fps would perforate and measure 10 feet of ice. Unit costs without the transmitter vere well below $1000; a possible target is $200.

Error sources and sizes were as follovs:

A. Time of impact. 10G was considered the start of the ice.

B. Impact velocity was from ballistic drops. Error 1%.

C. Calibration of the accelerometer, -r 5%.

D. Exit time vas considered when acceleration dropped to 80G.

E. Integration error, negligible.

Though the tensile strength of fresh water ice is greater than that of sea ice, penetration in the fresh water ice is greater. Possibly it is related to toughness rather than tensile strength. (In the soil program, the soil constant has not been correlated with any single parameter.)

3. Iceberg thickness. While no programs have continued the above vork to

include icebergs, there are no reasons why the same sort of measurements can- not be accomplished. Since the detachable afterbody has velocity limita- tions, it vould appear advisable to use the trailing antenna system. The drag of an antenna is not inconsiderable, but extra W/A should be provided to assure the desired impact velocity.

It would be reasonable to ask if one could not simply drop an inert penetra- tor and measure the ice thickness through the hole. Probably so, but since the ground crew would have to remain at some distance for safety, and since they would have to use some equipment to break through any refreezing. the procedure does not seem economical. The sea water returns rapidly back in the hole to its own level and the hole is filled with broken ice.

4. Implanting explosives. There is no reason why typical military aircraft intervalometers cannot be used to implant a string of penetrating bombs. These could be fired simultaneously, either by a ground crew interconnecting trailing wires longer than hole depth or by using radio code signals the

same way. Alternate bombs could be designed to give deep and shallow depths.

5. Anchor Posts. Early tests in soil indicated that there was no possibil- ity that penetrators could be pulled from the ground, and those implanted in ice could well behave in the same manner. In soil this is not only due to the penetrator ending up very firmly grasped by the earth when it comes to rest, and a smaller hole to get it out of than its diameter, but because the loose soil bridges the hole and prevents passage of the vehicle without tear- ing up a lot of real estate.

We made a test of this very early in the program. We drilled an open hole 12" in diameter and 100 feet deep. We then lowered a cylinder 3" in diameter to the bottom on a steel cable. Next we threw in some 25 feet of loose soil. What it would take to pull the cylinder out ve never learned, as we broke a 20,000 lb. cable trying.

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Xt woald be conjecture as to whether ice exhibits the same characteristics as soil in this respect. It would seem possible to trail a cable to a pene- trator and employ a radius device at the surface to permit horizontal tov.

6. Wide glacier thickness measurements. A signal advantage exists for mea- suring the thickness of soil on ice over a long distance when the surface conditions are poor or inaccessible as regards conventional drill rigs. A study made of a soil survey for a second "Panama Canal" using earthen dams across the Blo Atrata and the Rio San Juan io Columbia showed that while surface dr%lling to determine the location of bedrock would take two years, it could be done in a day using penetrators.

The depth of a wide glacier could thus be carried out in a few hours, de- pending on the spacing betveen data points.

Returning to rhe general f-ield of earth penetrators, it is quite true that a trade-off exists between the ease of using surface drill rigs and sacrific- ing accuracy of site location. Since wind and weather may be waited for, bombing accuracy may be optimized. Even so, a few hundred feet CEP is prob- ably a practical number to try for. In view of the costs of a program, a resonable approach is to sacrifice sophistication for low cost, and simply drop a string of penetrators which start early and end lare so that the area to be surveyed is both hit and covered.

Here are a fev "don'ta".

A. Don't count on pinpoint accuracy.

B. Don't count on guidance as the high weight and momentum and low lift makes a path chance very slow.

C. Don't count on a simple recoilless rifle for implant, Weights are too high.

D. Don't kaste time on spinning the penetrator so that it would screw it- self into the ground.

As a matter of completeness, an attempt was made many years ago to build a rocket capable of implanting a 1000 lb. penetrator supported a few feet Li- bove the ground. To this end a rocket was designed 9" in diameter to pro-

duce a thrust of 250,000 lbs. for 250 milliseconds. This in turn required a bum pressure of 20,000 psi -- about ten times the normal range. While preliminary tests looked good, the full-scale motor blew out about as much fuel as it burned and the totaL impulse was 10~. A special, very large re- coilless rifle has been built that can implant the smaller size of peaetra- tors, but the rifle (veighing some 70,000 lbs.) is barely transportable.

In closing, I certainly don't want to claim to be an arctic expert after only three weeks on the ice out from Fairbanks and at Thule, Greenland.. Bowever. during that time my learning curve was straight up, and maybe a fev personal observations vi11 save those who get to go ahead with the cold weather work some time. physical pain, or embarrassment. So here goes:

If possible, +ie up with some military base. This not odly gets a lot of equipment at your disposal, but it gets you a "gopher" (which in American slang means somebody vho will "go for" this or **go for*' that. For example, when ve first vent to Fort Wainwright, in March of 1963, it was only to plan a drop program. To our dismay, the base commander said we"d have to do the

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drop immediately, as the warm Chinook winds could come at any time and turn the land into a morass. We explained that we could fly up some penetrators in 24 hours, but in no way could we do all the other things needed in such a short time.

"What other things?" said the colonel.

"Well, we'll need three field telephones with a mile of wire."

The colonel turned to the gopher. "Captain. get the three field telephones with a mile of wire."

"Yes, sir."

"We'll need a 48-inch well digger."

"Captain, get them a 48-inch well digger."

"Yes, sir."

"We'll need a road a mile long to get into the drop area."

"Captain, build them a road a mile long tomorrow morning."

"Yes, sir."

., --. and a chopper, and field radio, and clothing and transits -- no, make

that theodolites. Transits can't look up-u

Now the gopher was clearly sweating. But he was resourceful. Looking for a way out, the said, "Colonel, we can't drop there. It's on an airlane." He sat back and relaxed. But not for long.

The colonel wasn't a colonel by chance. He solved that problem in a milli- second. "No sweat. These will be accidental drops."

A day later, we got our birds and ran an accidental drop program.

tiext, clothing. The army also knows about arctic clothing. Borrow it if you can. If not, the local ski shop will help. Heavy underwear and a lined

jumpsuit is fine down to about O" as long as you are working. If you have to stand around and wait, you'll need those baggy pants and a heavy parka. The hood on the parka has a soft iron wire in the fur so you can bend it in- to a snoot if the wind blows or if it gets really cold. Most of the time I kept the hood down and used a Elountie cap; it doesn't restrict your vision or hearing as much. (By the way, you can't shop in arctic clothes. In about three minutes you run screaming outside to cool off.)

It may seem silly to put on those bulky bunny boots every time you go out,

but do it. Your feet may not get cold, walking a few hundred yards in

regular shces, but those hot soles will melt the ice -- and down you go! The bunny boots stay cold.

No matter how badly you need them, eyeglasses won't work on snowmobiles. The wet, warm air from your face will cloud them over. Let somebody else

do the driving.

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The arctic gloves are about the only item that doesn't work well. You will

find yourself balling your fists or putting your hands in your pockets to keep them warm. This can make your hands sweat, and if you then grab cold metal you may get stuck. If you do, pull free right away; you leave the minimum hide this way.

You will quickly learn to leave your car engine idling while you have dinner unless there's an electrical outlet available to plug in the calrod. (Don't be embarrassed if you forget the drop cord when you drive off. Everybody does this.) Don't leave beer outside; it will freeze solid.

Find out about the safety rules and don't break them. If a Phase III storm confines you, stay confined and visible. People have died looking for others who had sacked out in an odd place. Learn about the safety shelters. The tools are outside so you can break in.

If you go far by aircraft, the pilot will know that the magnetic compass points straight down and is useless. He has to rely on his direction finder. Barely, but only once to any pilot, the target station will go off the air and the flight be too far out to pick up the return beacon. Ask permission to play with the direction finder and keep alternate stations ready.

The most fundamental rule of all is to have somebody who knows your route and schedule alerted to start a search at the right time and place, if you don't show.

Finally, there really is something terrifying in the northland. It is called

a "destroillet". This is a toilet used where sewer lines can't be put under-

ground. Its principle is simple. The usual flush handle, instead of flush- ing, activates a propane burner which blasts on and evaporates and burns all excrement and blows it up a stack. There is, of c*ufse, a microswitch on the toilet seat so that inadvertent activation of the burner -- which could

vary all the way from quite exciting to downright deplorable -- is most un- likely. Even so, I found that all of our crew were so emotionally disturbed by thoughts of possible mishaps that more than the one day we spent at Point Barrow would have done us in.

Good luck with your iceberg work!