0351 Tools

Time fuse is used for non-electric initiation of a charge. It transmits a delay spit of flame from an fuse igniter to an non-electric blasting cap, causing the detonation of that blasting cap and any charge it is primed into.

The key property of time fuse is its ability to be used as a countdown timer. Time fuse provides an alternative method of detonation from electrical initiation, which, generally speaking, is command-based. Each initiation system has advantages and disadvantages.¹

The primary advantage of electrical initiation is that a Marine can control both if and when to detonate the charge. There are many instances where these advantages are critical to mission accomplishment. On the other and, electrical initiation generally requires that a Marine be at the end of a spool of wire to trigger the firing circuit. If the Marine in control of the firing device is incapacitated, comes under fire, or loses control of the firing device, the charge will not go off. Worse, if an enemy captures control of the firing device, they can deploy the charge to their advantage. Thus, in some situations, command-based detonation can become a liability, rather than an advantage.

Another limitation of electrical initiation is the length of the wire spool(s). For some charges, it can be impractical or unsafe to detonate a charge within the distance imposed by the available wire.

Time fuse does not suffer from these limitations. Direct command is traded for the high-probability that, if everything is arranged properly, a charge will detonate within a certain amount of time. Once a time fuse is lit, the only practical way to stop detonation is to cut or extinguish the time fuse. Furthermore, the only limit on distance becomes how far away the Marines can move in the time provided by the time fuse.

In order to employ time fuse effectively, Assault Marines need to be able to determine:

1. The accuracy and burn rate of the time fuse, and
2. the amount of time fuse needed to achieve their target burn time.

Time fuse should burn at a rate of 40 seconds per foot, plus or minus 5 seconds. Time fuse outside this range may fail to work properly or be an inaccurate time keeper, and should not be used. In order to determine whether a time fuse is reliable, we test its burn rate. The burn rate, in turn, allows Marines to determine how much time fuse is needed to achieve their target burn time.

You test a six foot length of time fuse, and it burns for 4 minutes and 24 seconds.

1. First, convert the minutes into seconds. 4 minutes × 60 seconds/minute = 240 seconds. Then, add the remaining seconds to the result. 240 + 24 = 264 seconds.
2. Next, divide your burn time in seconds by the length of time fuse. 264 seconds ÷ 6 feet = 44 seconds per foot.
3. 44 seconds per foot is the burn rate. This is within the acceptable range for time fuse. You can use this fuse.

• If your test burn time is less than 3 minutes and 30 seconds (210 seconds), or more than 4 minutes and 30 seconds (270 seconds), your time fuse is not good. There's no need to calculate the burn rate unless you have to use that time fuse.
• Conversely, if your test burn is between 3 minutes 30 seconds and 4 minutes 30 seconds, that time fuse is good. You should proceed to calculate its burn rate.

Enter the burn time for six feet of time fuse:

Once you have determined the burn rate for your time fuse, you'll need to calculate the length of time fuse needed to achieve your desired burn time.

Remember, the burn rate tells you how long it takes for one foot of your time fuse to burn. Thus, to determine the amount of time fuse you will need, you need to:

1. Take your target time and convert it into seconds.
2. Divide the target time by your burn rate.

The result is how many feet of time fuse you need to achieve your target time.

It's very likely that your result will include a decimal. To cut your time fuse accurately, you'll need to convert any decimal feet into inches and sixteenths of an inch.

To make that conversion:

1. Make note of the number of feet from your initial calculation of the fuse length.
2. Take the decimal of that result and multiply it by 12. The number to the left of the decimal is the amount of inches you need to achieve your target time.
3. Take the decimal of that result, and multiply it by sixteen. The number to the left of the decimal is how many sixteenths of an inch you'll need. If the number is a decimal, round it up or down, then mark it down.

You now have the number of feet, inches, and sixteenths of an inch of time fuse needed to achieve your target burn time. Use those figures to cut the appropriate amount of time fuse.

Your target burn time is 10 minutes, and your burn rate is 37 s/ft.

1. Convert 10 minutes into seconds. 10 min × 60 sec/min = 600 seconds.
2. Divide the target time by the burn rate. 600 s / 37 s/f = 16.216 feet. Make note of the number of feet, 16.
3. To determine the number of inches, multiply the decimal by 12. With a calculator, you can do this easily by subtracting the number of feet from your result. In this case, 16.216 - 16 = 0.216. You can then multiply that number by 12 to get 2.594. If you're without a calculator, you can round to 0.22, and then multiply by 12. The result in that case is 2.64. In either case, make note of the number of inches, 2.
4. Next, determine sixteenths of an inch. With a calculator, you would run 2.594 - 2 = 0.594. Multiplying that result by 16 will yield 9.513. Round this up to 10, to get a result of 10/16 of an inch, and make note of this number. If you're without a calculator, multiply 0.64 × 16, which yields a result of 10.24. This is rounded down to 10.
5. The final result is 16 feet, 2 inches, and 10/16 of an inch. For a time fuse that burns at a rate of 37 seconds per foot, this length of time fuse will burn in approximately 10 minutes.

Enter decimal feet to convert to fractional feet:

Breaching provides an assault force with rapid, dynamic access to a hostile stronghold. Access is the key capability provided by breaching; in most instances, an assault is not possible without a breach of some kind. A failed breach will result in a failed assault. Successful breaches also enhance assaults, by augmenting their speed, surprise, and violence of action.

Situation dictates the scale of a breach. There may be instances where a mechanical breach (less violent, safer for occupants) of the front door (minimal surprise) will be sufficient and appropriate. On the other end of the spectrum is the concrete charge, which creates an entry where one is least expected, with lethal consequences for the occupants. An Assault Marine should be proficient with the full breadth of available approaches.

Explosive breaching provides incomparably rapid, adaptable, and dynamic access. This does not mean it is suited for every situation. Its disadvantages include the additional requirements for equipment and preparation, complexity of setup, and increased danger for occupants and breachers. The violence of the breach, which is likely to incapacitate or at least disorient the occupants, is offset by the need to protect the assault team from the explosives. The breacher must evaluate the trade-offs for each breach.

This last point underscores the importance of breaching math. A breacher should use sufficient explosive to ensure access and create conditions favorable to the assault. At the same time, the breacher must also protect the assault team (and any potential bystanders) from the explosives. The assault team should be kept as close as possible to the breach, so as to rapidly exploit it, but far enough away to not be harmed by it. We use the calculation of Net Explosive Weight and Standoff Distances to make this determination.

For a successful assault, the assault team should be placed as close as is safely possible to the breach. If a team is placed too far away, they will be unable to fully exploit the surprise and damage created by the breach. In the case of weaker charges, it's possible that the occupants will have time to prepare and respond to the breach if it is not quickly exploited. On the other hand, placing a team too close to the charge has its own dangers.

The risk is more than death or great physical harm. A team placed too close to a charge can survive the blast but suffer from ruptured eardrums or heavy disorientation, both of which will endanger the assault. Remember, explosives work primarily because of a rapid conversion into heat and expanding gas. The primary safety limit imposed for breaching is the sudden change in air pressure.

A pressure of 3.4 PSI (pounds per square inch) is enough to damage unprotected eardrums. Because Marines are expected to wear hearing protection, the safety limit is expanded to the point where the pressure from a charge reaches 4 PSI. Marines that are shielded from the blast by some form of cover (such as a blast blanket) can be placed even closer to the charge, up to 8 PSI.

Calculation of the standoff distance reveals the exact distance where the assault team can be safely placed. The formula to calculate standoff is DISTANCE = K-FACTOR × ∛NEW.⁵ The K-factor is a conversion factor used to determine the point of safe pressure for a given NEW (a radius from the charge location). A K-factor of 12 is used if the assault team is shielded; 18 is used if the team is unshielded.⁶ These factors account for blast overpressure. To determine the safe distance from hazardous fragmentation, a K-factor of 300 is used.⁷

Multiplying the K-factor by the cube root of a charge's NEW will yield a distance in decimal feet. This is the closest the team can be placed for a given condition (shielded or unshielded). In practice, it is common to round up the decimal and use that figure, as an extra measure of safety; e.g., for a standoff of 26.23 feet, the assault team is placed at 27 feet. Check your unit's SOP.

Some explosives, like blasting caps and det cord, are measured in grains, not pounds. Because the NEW is measured in pounds TNT equivalent, you need to first convert grains to pounds.

There are 7000 grains in a pound. To convert from grains to pounds, divide the number of grains by 7000. The formula is POUNDS = GRAINS ÷ 7000. Remember that for det cord, you need to first multiply the grains by the number of feet. For example, three feet of det cord is 150 grains (3 feet × 50 grains per foot = 150 grains).

The NEW calculator on this page converts the grains in M-6 and M-7 blasting caps and M456 det cord automatically. However, if you plan on using the NEW calculator with other types of blasting caps or det cord, or any other explosive measured in grains, you'll need to first convert grains into pounds. The Grains to Pounds Converter will do that calculation for you.

Note: the results in these examples will vary slightly depending on whether one is rounding or using the most precise result from a calculator.

A. You need to get the next explosive weight for a charge that has 2 non-electric blasting caps, 50 feet of det cord, 5 x 1.25 pound blocks of C-4, and 25 x 1 pound blocks of TNT.

1. Determine how many grains of explosive are in the blasting cap. Each non-electric blasting cap has 13.5 grains. 13.5 × 2 = 27 grains.
2. Convert the grains into pounds. 27 grains ÷ 7000 grains/pound = ~0.004 pounds, rounded to three decimal digits.
3. Multiply your result by the REF for non-electric blasting caps, which is 1.60. ~0.004 × 1.60 = ~0.006 pounds TNT equivalent. You can round this up to 0.01 pounds TNT equivalent. Set this number aside.
4. Determine how many grains of explosive are in the det cord. 50 feet of det cord, which contains 50 grains of explosive per foot, means there are 50 × 50 = 2500 grains of explosive.
5. Convert the grains into pounds. 2500 grains ÷ 7000 grains/pound = ~0.357 pounds.
6. Multiply the result by the REF for det cord, which is 1.66. ~0.357 × 1.66 = ~0.59 lbs TNT equivalent. Set this number aside.
7. Determine how many pounds of C-4 you are using. 5 blocks × 1.25 pounds per block = 6.25 pounds of C-4.
8. Multiply the result by its REF to get the TNT equivalent weight. 6.25 pounds × 1.34 REF = ~8.38 lbs TNT eqv. Set this number aside.
9. The last explosive doesn't need conversion. TNT is already equivalent to itself. 25 blocks × 1 pound per block × REF of 1.00 = 25. Set this number aside

10. You know have the TNT equivalent weight for each explosive:

    • Blasting Caps: 0.01
    • Det Cord: 0.59
    • C-4: 8.38
    • TNT: 25

    We need to add these together to get our NEW. 0.01 + 0.59 + 8.38 + 25 = 33.97 pounds TNT equivalent. That is the NEW for this charge.

B. You need to get the net explosive weight for 1 electrical blasting cap, 25 feet of 250 grain per foot det cord, and a single 100 pound chunk of octanitrocubane (which has an REF of 2.38).

1. Determine how many grains of explosive are in the electrical blasting cap. Each electrical blasting cap has 13.5 grains. 13.5 × 1 = 13.5 grains
2. Convert the grains into pounds. 13.5 grains ÷ 7000 grains/pound = ~0.002 pounds, rounded to three digits.
3. Multiply the result by the REF for non-electric blasting caps, which is 1.60. 0.003857 × 1.60 = approximately 0.003 pounds TNT equivalent. You can round this up to 0.00 pounds TNT equivalent. Set this number aside.
4. Determine how many grains of explosive are in the det cord. 25 feet of det cord, which contains 250 grains of explosive per foot, means there are 25 × 250 = 6250 grains of explosive.
5. Convert the grains into pounds. 6250 grains ÷ 7000 grains/pound = ~0.89 pounds.
6. Multiply the result by the REF for det cord, which is 1.66. 0.89 × 1.66 = ~1.48 lbs TNT equivalent. Set this number aside.
7. Next, convert the octanitrocubane to pounds TNT equivalent. 100 pounds × 2.38 REF = 238 pounds TNT equivalent.

8. You know have the TNT equivalent weight for each explosive:

   • Blasting Cap: 0.00
   • Det Cord: 1.48
   • Octanitrocubane: 238

   We need to add these together to get our NEW. 0.00 + 1.48 + 238 = 239.48 pounds TNT equivalent. That is the NEW for this charge.

Instructions:

1. Select an explosive with the drop-down menu.
2. Enter the quantity used, e.g., 2 blasting caps, 3 blocks of C-4.
3. Select the type of unit used, or, for a custom explosive, its weight in pounds. For example, 1 1/4 or 2 1/2 blocks of C-4, or 3 lbs for a custom explosive.
4. For a custom explosive, enter the explosive's REF.
5. To add or remove a charge, select the plus or minus button in the rightmost row.
6. To calculate the NEW, press the Submit (NEW) button. To get the NEW and Standoffs, press the Submit (NEW + Standoff) button.
7. To reset the calculator, press the Reset button.

The formula for calculating standoff distance is: DISTANCE = K-FACTOR × ∛NEW. Note that the distance result is in decimal feet.

As a reminder, this table shows which K-factor to use for a given usecase:

Use a K-factor of 12 if the assault team will be behind a blast blanket or some form of cover. Use a K-Factor of 18 if the assault team will be in the open. A K-factor of 300 is used to determine how far hazardous fragmentation from the blast will go.

Once you know your usecase, the calculation is performed as follows:

1. Calculate the cube root of the NEW. A calculator is recommend.⁸ Calculating the cube root by hand is outside the scope of this page, but you can find many tutorials online.⁹
2. Multiply the result by the K-factor you need. This will be either 12 or 18, AND 300 – you should always know what impact fragmentation will have around the breaching site.¹⁰
3. The result will be a distance in decimal feet. Typically, this result is rounded up if it is a decimal; e.g., for a standoff of 26.23 feet, the assault team is placed at 27 feet. Where extra precision is needed, you can convert decimal feet to fractional feet, discussed above

A. You have NEW of 8 lbs TNT equivalent. Your assault team will be breaching with a blast blanket. Friendly forces are located 50 yards away from the assault team, with the breaching site in view.

1. Get the cube root of the NEW. ∛8 = 2.
2. Next, multiply the NEW by the K-factor. Use 12, since the team is shielded. 12 × 2 = 24.00 feet. This is the closest the assault team can be placed to this breaching site.
3. To determine the distance of hazardous fragmentation, use a K-factor of 300. 2 × 300 = 600.00 feet. Because friendly forces are located only 50 yards away (150 feet), they are within the fragmentation radius. They should be informed to take some form of cover.

B. You have a NEW of 0.10. The assault team is in the open. Friendly forces are, again, 50 yards away.

1. Get the cube root of the NEW. ∛0.10 = ~0.46.
2. Next, multiply the NEW by the K-factor. Use 18, since the team is unshielded. 18 × ~0.46 = ~8.35 feet. This is the closest the assault team can be placed to this breaching site. Typically, the team is placed 9 feet away.
3. To determine the distance of hazardous fragmentation, use a K-factor of 300. ~0.46 × 300 = ~139.25 feet. In this case, friendly forces are outside the fragmentation radius. It might be a good idea for them to take cover regardless, but their exposure to hazardous fragmentation is likely negligible.

This table provides the NEW and Standoff Distances for common breaching charges. You must confirm that you use the exact same amount of materials as described in the table. Otherwise, both the NEW and Standoff Distances will be off.¹¹

David Wen Riccardi-Zhu / dwrz@dwrz.net

Note to Marines: first-name basis is OK.

1. Increase confidence in demolitions and breaching math. Succesful use of explosives, in training or at war, turns on accurate calculation. A Marine that incorrectly calculates the necessary length of time fuse, or the required standoff distance, not only risks mission failure, but also their life, the lives of their fellow Marines, and the lives of any innocent bystanders.

These tools are meant to reduce the probability of such outcomes. Any competent Assault Marine (0351) will double check their math and have other Assault Marines independently verify their calculations. Nevertheless, confirming with a calculator – which is not affected by fatigue, stress, or dimming memory – can add a layer of confidence. The objective is to bolster, rather than replace, the proficiency of Infantry Assault Marines.

2. Assist Assault Marines' retention of their demolitions and breaching skills. Often, the time available to practice and develop these skills is limited. Ranges are infrequent, and for Reserve Marines, the retention of other skills competes for limited training time. To address these conditions, this page provides quick, accessible reviews of demolition and breaching math.

3. Enhance leaders' trust in the proficiency of their 0351s. For many Marine leaders, the Risk Management Chart for demolitions and breaching is unappealing. After all, death and injury are the likely consequences of a mistake or mishap with explosives. With the severity of the risk clear, the determinative question becomes the probability of the risk. How likely is it that something will go wrong?

These tools aim to lower the probability of risk as well as the leader's perception of it – to make the possible appear improbable or rare. While leaders should trust the effectiveness of the Corps' training institutions, a little external verification can help assure leaders that their Marines remain proficient.