What-If 0021: "Machine Gun Jetpack"

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Re: What-If 0021: Machine Gun Jetpack

Postby Himself » Wed Dec 26, 2012 6:44 am UTC

Reminds me of the time I propelled myself down the streets of San Andreas using a tank mortar.
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Re: What-If 0021: "Machine Gun Jetpack"

Postby OrbitalFacePalm9001 » Thu Dec 19, 2013 5:52 am UTC

What if we tried more power?

GUN THAT FIRES DIAMONDS AT 9001 TIMES THE SPEED OF LIGHT PER NANOSECOND EVERY NANOSECOND
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Re: What-If 0021: "Machine Gun Jetpack"

Postby Pfhorrest » Thu Dec 19, 2013 6:58 am UTC

OrbitalFacePalm9001 wrote:What if we tried more power?

GUN THAT FIRES DIAMONDS AT 9001 TIMES THE SPEED OF LIGHT PER NANOSECOND EVERY NANOSECOND


"9001 times the speed of light per nanosecond every nanosecond" has units of distance*time-3, i.e. jerk, or rate of change of acceleration, which in turn is of course rate of change of speed.

Accelerating at 9001 times the speed of light per nanosecond should be perfectly possible so long as you only keep it up for less than 1/9001 of a nanosecond. Jerking up to that acceleration should be possible for even longer. So what you suggest is something that could certainly be done. You just set the diamonds into motion very quickly; that is, you bring them to their final state of motion almost instantaneously, instead of gradually building up acceleration to gradually bring them up to speed.

Problem is, you haven't told us what the final state of motion of those diamonds is to be, much less their mass, much less the rate of fire. So for all we know, this diamond gun may supply no more power than any ordinary machine gun.
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Re: What-If 0021: "Machine Gun Jetpack"

Postby OrbitalFacePalm9001 » Thu Dec 19, 2013 10:34 am UTC

Pfhorrest wrote:
OrbitalFacePalm9001 wrote:What if we tried more power?

GUN THAT FIRES DIAMONDS AT 9001 TIMES THE SPEED OF LIGHT PER NANOSECOND EVERY NANOSECOND


"9001 times the speed of light per nanosecond every nanosecond" has units of distance*time-3, i.e. jerk, or rate of change of acceleration, which in turn is of course rate of change of speed.

Accelerating at 9001 times the speed of light per nanosecond should be perfectly possible so long as you only keep it up for less than 1/9001 of a nanosecond. Jerking up to that acceleration should be possible for even longer. So what you suggest is something that could certainly be done. You just set the diamonds into motion very quickly; that is, you bring them to their final state of motion almost instantaneously, instead of gradually building up acceleration to gradually bring them up to speed.

Problem is, you haven't told us what the final state of motion of those diamonds is to be, much less their mass, much less the rate of fire. So for all we know, this diamond gun may supply no more power than any ordinary machine gun.

It fires 192828379278478378378377e89289389283373883 diamonds every nanosecond and the diamonds are 500 grams.
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Re: What-If 0021: "Machine Gun Jetpack"

Postby Flumble » Thu Dec 19, 2013 10:49 am UTC

All else aside, there are less than 1e100 atoms in the reachable universe, so asking for about 2e89289389283373906 diamonds is a bit much.

...unless you want to fire them for less than 1e-89289389283373836 seconds. But measuring that amount of time is a bit tedious, since we already have a theoretical problem at Planck time, roughly 1e-44 seconds.

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Re: What-If 0021: "Machine Gun Jetpack"

Postby gmalivuk » Thu Dec 19, 2013 1:15 pm UTC

And we still don't know how long you'd be firing for or how fast they end up being at the end of that bit of acceleration.

Nor, for that matter, does it matter in the least that they are diamonds. Diamonds are hard, sure, but what gets us power here is mass, and diamond isn't particularly denser than any other rock.
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Re: What-If 0021: "Machine Gun Jetpack"

Postby davidstarlingm » Thu Dec 19, 2013 2:49 pm UTC

Pfhorrest wrote:Accelerating at 9001 times the speed of light per nanosecond should be perfectly possible so long as you only keep it up for less than 1/9001 of a nanosecond. Jerking up to that acceleration should be possible for even longer.

I'll take your word for it. :P

But seriously, I love how things like this are dissected here.

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Re: What-If 0021: "Machine Gun Jetpack"

Postby Shidoshi » Thu Dec 19, 2013 2:53 pm UTC

About the laser-blocking full-body suit, that doesn't really seems plausible since, unless you are reflecting the laser, your suit would be absorbing the energy from the laser and would soon burn out.

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Re: What-If 0021: "Machine Gun Jetpack"

Postby kowalski96 » Fri Dec 27, 2013 10:51 am UTC

In Dr Karl (A popular science personality in Australia)'s most recent book, Game of Knowns, he wrote a chapter based upon and expanding upon this What-If. It's a good read (as is the whole book) for anyone interested in this What-If.

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Re: What-If 0021: "Machine Gun Jetpack"

Postby Rincewind_wizzard » Mon Mar 24, 2014 9:59 am UTC

How would this propulsion work if you didn't carry the gun and ammo with you, but were accelerated by bullets fired from below? Into some shield that would protect you from getting killed, obviously.

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Re: What-If 0021: "Machine Gun Jetpack"

Postby gmalivuk » Mon Mar 24, 2014 7:14 pm UTC

Rincewind_wizzard wrote:How would this propulsion work if you didn't carry the gun and ammo with you, but were accelerated by bullets fired from below? Into some shield that would protect you from getting killed, obviously.
Then you wouldn't need to worry about the rocket equation (i.e. "fuel makes you heavier") or the thrust-to-weight ratio of the guns themselves. On the other hand, even if we assume a weightless vacuum (so the bullets never slow or fall), each bullet imparts less and less momentum as the shielded payload goes faster and faster, and there's no way to get the payload to go faster than the bullets. Also, we don't get the additional 30% boost from ejected gases.

For an AK-47 shooting 10 8-gram rounds per second at 715m/s, it starts out imparting the 57.2N of thrust mentioned in the What-If. However, the actual thrust per weapon is 0.08(715 - v) when the payload is traveling at v m/s.

Suppose we have 500 of them attached to a much more massive launch platform (say, an asteroid), and we want to launch a 100kg payload. With 40(715 - v) Newtons of thrust, we get 0.4(715 - v)m/s^2 of acceleration. So v' = 286 - 0.4v and v = 715(1 - e-0.4t)
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Re: What-If 0021: "Machine Gun Jetpack"

Postby Bill63 » Mon Mar 02, 2015 12:07 am UTC

I am a complete newbie. Not only have I never posted to this Forum before I don’t think I have ever posted to *any* Forum. I was quite impressed by Randall Monroe’s *What If?* book and have some thoughts about the *Machine-Gun Jetpack* episode.

I could not find an email address for Randall so I will just write this post in the form of an email to him:

I am an Emeritus Professor here at the University of Illinois at Urbana-Champaign. I have solid academic credential (e.g., Fellow of the AAAS, more than 10,000 scholarly citations to my work, etc.) However, my thoughts on the *Machine-Gun Jetpack* are based only on an avocational interest in the history of science and so might best be taken with a grain of salt.

My basic point is to note the delightful concordance between your piece on the *Machine-Gun Jetpack* and the fact that Robert Goddard, the physicist who constructed the first liquid fuel rocket, initially attempted to solve the problem of space flight with a “multiple-charge machine-gun rocket system” [p. 92 Lehman, M. (1988) *Robert H. Goddard: Pioneer of Space Research* NY: Da Capo Press--originally published in 1963 as *This High Man: The Life of Robert H. Goddard* NY: Farrar, Straus].

This similarity, allows you to situate the *Machine-Gun Jetpack* scenario in the larger issues of the historical development of rockets and space travel. In addition, while thinking about this topic, I have come to see a intriguing parallelism in the conceptual structure of many of your *What If?* episodes and the research strategy Goddard used throughout his professional career.

On Oct 19, 1889, while up in a cherry tree, the 17 year old Goddard had an epiphany--he decided that it ought to be possible for a human to travel to Mars. (We know the exact place and date because every year for the rest of his life Goddard celebrated this as his “Anniversary Day”.) In his Autobiographical fragment Goddard says when he came down from the cherry tree, “Existence at last seemed very purposive” (Lehman, 1988, p. 26). So in the terms of my extended analogy we can think of Goddard as asking, “What if we tried to fly to Mars?” Then, in the same way that you do in most of your answers to a *What if?* question, Goodard attempted to imagine different possible physical solutions.

His first proposal, before he had mastered much physics, was to use the centrifugal force produced by a weight whirling around a horizontal shaft (Lehman, 1988, p. 26).

As he matured as a physicist (B.S. in Physics from Worcester Polytechnic Institute, 1908; Ph. D. in Physics from Clark University, 1911) he worked out the math and concluded that it was, in principle, possible to reach escape velocity. Much of this work appears in: Goddard, R. H. A method of reaching extreme altitudes. *Smithsonian Miscellaneous Collections*, 71 (2), 1919.

Goddard first explored standard black powder fireworks rockets and decided they could not do the job. He began to examine the “thrust-to-weight ratios” you discussed in your *Machine-Gun Jetpack* piece. He focused on the fuel “efficiency” of various types of rocket motors and developed a De Laval nozzle that increased the efficiency of solid fuel rockets by better than an order of magnitude. I do not really understand the details of his cartridge feeding mechanism. It is apparently described (with pictures) in U.S. Patent No. 1,103,503 granted in 1914. Even though he got some of the machine-gun rockets to fly, it turned out to be very difficult to make this technique work reliably. Examples from his experimental notes: “chamber cracked”, “cartridge jammed” (Lehman, 1988, p. 124); “ugly bulges from the pressure”, “breech-block jammed” (Lehman, 1988, p. 88); “Hickman tried two cartridges, one exploded in the magazine, jammed or detonated from shock” (Lehman, 1988, p. 93).

After about 4 years of work Goddard gave up on the machine-gun rockets and decided to try liquid fuel rockets (he had inserted a short section on this alternate approach in the 1914 patent). He wanted the most powerful chemical reactions possible and focused on liquid hydrogen and on liquid oxygen. Liquid hydrogen was not available in large quantities so he substituted gasoline. However, he could obtain liquid oxygen so he decided to use it. He then spent the next decade or so trying to learn to how to master liquid oxygen with many spectacular explosions along the way [in the xkcd tradition?]; but eventually he succeeded in carrying out a number of rocket flights reaching over a mile in altitude.

Goddard also realized that once one got out into space an ion drive would be very efficient and actually built a small drive (Lehman, 1988, p. 118-118). I wonder if the folks involved in the current *Dawn Mission* to Ceres know about this. Goddard really was a person who could see into the future.

This ends my extended analogy and I hope shows you why I think that you and Robert Goddard are kindred spirits. One additional xkcd-like side issue. Goddard took much flack for his visionary ideas. After his proposals about flying to other planets became public, the New York Times wrote an editorial that asserted: “Professor Goddard with his ‘chair’ in Clark College...does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react--to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools” (NYT, Jan 13, 1920). Goddard’s response wins the Isaac Newton Memorial Award--he attached a pistol loaded with blanks to a pivoting rod, extracted the air in his apparatus, and showed the vigorous recoil of the pistol in a vacuum.

Finally, I will end this message with a travel recommendation. I gather you live in Cambridge so I recommend a day trip over to Worcester. You can find the location of the first flight (March 16, 1926) of a liquid fuel rocket on what is now the 9th Fairway of the Pakachoag Golf Course in Auburn, MA (just outside of Worcester). The iconic cherry tree was in the back of his birthplace at 1 Tallawanda Drive, Worcester (but the great hurricane of 1938 took it down). The Goddard Library at Clark University has a display of Goddard memorabilia around the walls of the main reading room--among them is at least one of the machine-gun rockets. They also have his papers, patents, lab books, etc. Finally, to end on a stranger-than-fiction note, some of Goddard’s cartridge rocket experiments took place at the Magnetic Building on the campus of Worcester Polytechnic Institute. This building is still there, but is now the headquarters for a secret society and is called the *Skull Tomb*.

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Re: What-If 0021: "Machine Gun Jetpack"

Postby gmalivuk » Mon Mar 02, 2015 2:23 am UTC

Bill63 wrote:I am a complete newbie. Not only have I never posted to this Forum before I don’t think I have ever posted to *any* Forum. I was quite impressed by Randall Monroe’s *What If?* book and have some thoughts about the *Machine-Gun Jetpack* episode.

I could not find an email address for Randall so I will just write this post in the form of an email to him:

I am an Emeritus Professor here at the University of Illinois at Urbana-Champaign. I have solid academic credential (e.g., Fellow of the AAAS, more than 10,000 scholarly citations to my work, etc.) However, my thoughts on the *Machine-Gun Jetpack* are based only on an avocational interest in the history of science and so might best be taken with a grain of salt.

My basic point is to note the delightful concordance between your piece on the *Machine-Gun Jetpack* and the fact that Robert Goddard, the physicist who constructed the first liquid fuel rocket, initially attempted to solve the problem of space flight with a “multiple-charge machine-gun rocket system” [p. 92 Lehman, M. (1988) *Robert H. Goddard: Pioneer of Space Research* NY: Da Capo Press--originally published in 1963 as *This High Man: The Life of Robert H. Goddard* NY: Farrar, Straus].

This similarity, allows you to situate the *Machine-Gun Jetpack* scenario in the larger issues of the historical development of rockets and space travel. In addition, while thinking about this topic, I have come to see a intriguing parallelism in the conceptual structure of many of your *What If?* episodes and the research strategy Goddard used throughout his professional career.

On Oct 19, 1889, while up in a cherry tree, the 17 year old Goddard had an epiphany--he decided that it ought to be possible for a human to travel to Mars. (We know the exact place and date because every year for the rest of his life Goddard celebrated this as his “Anniversary Day”.) In his Autobiographical fragment Goddard says when he came down from the cherry tree, “Existence at last seemed very purposive” (Lehman, 1988, p. 26). So in the terms of my extended analogy we can think of Goddard as asking, “What if we tried to fly to Mars?” Then, in the same way that you do in most of your answers to a *What if?* question, Goodard attempted to imagine different possible physical solutions.

His first proposal, before he had mastered much physics, was to use the centrifugal force produced by a weight whirling around a horizontal shaft (Lehman, 1988, p. 26).

As he matured as a physicist (B.S. in Physics from Worcester Polytechnic Institute, 1908; Ph. D. in Physics from Clark University, 1911) he worked out the math and concluded that it was, in principle, possible to reach escape velocity. Much of this work appears in: Goddard, R. H. A method of reaching extreme altitudes. *Smithsonian Miscellaneous Collections*, 71 (2), 1919.

Goddard first explored standard black powder fireworks rockets and decided they could not do the job. He began to examine the “thrust-to-weight ratios” you discussed in your *Machine-Gun Jetpack* piece. He focused on the fuel “efficiency” of various types of rocket motors and developed a De Laval nozzle that increased the efficiency of solid fuel rockets by better than an order of magnitude. I do not really understand the details of his cartridge feeding mechanism. It is apparently described (with pictures) in U.S. Patent No. 1,103,503 granted in 1914. Even though he got some of the machine-gun rockets to fly, it turned out to be very difficult to make this technique work reliably. Examples from his experimental notes: “chamber cracked”, “cartridge jammed” (Lehman, 1988, p. 124); “ugly bulges from the pressure”, “breech-block jammed” (Lehman, 1988, p. 88); “Hickman tried two cartridges, one exploded in the magazine, jammed or detonated from shock” (Lehman, 1988, p. 93).

After about 4 years of work Goddard gave up on the machine-gun rockets and decided to try liquid fuel rockets (he had inserted a short section on this alternate approach in the 1914 patent). He wanted the most powerful chemical reactions possible and focused on liquid hydrogen and on liquid oxygen. Liquid hydrogen was not available in large quantities so he substituted gasoline. However, he could obtain liquid oxygen so he decided to use it. He then spent the next decade or so trying to learn to how to master liquid oxygen with many spectacular explosions along the way [in the xkcd tradition?]; but eventually he succeeded in carrying out a number of rocket flights reaching over a mile in altitude.

Goddard also realized that once one got out into space an ion drive would be very efficient and actually built a small drive (Lehman, 1988, p. 118-118). I wonder if the folks involved in the current *Dawn Mission* to Ceres know about this. Goddard really was a person who could see into the future.

This ends my extended analogy and I hope shows you why I think that you and Robert Goddard are kindred spirits. One additional xkcd-like side issue. Goddard took much flack for his visionary ideas. After his proposals about flying to other planets became public, the New York Times wrote an editorial that asserted: “Professor Goddard with his ‘chair’ in Clark College...does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react--to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools” (NYT, Jan 13, 1920). Goddard’s response wins the Isaac Newton Memorial Award--he attached a pistol loaded with blanks to a pivoting rod, extracted the air in his apparatus, and showed the vigorous recoil of the pistol in a vacuum.

Finally, I will end this message with a travel recommendation. I gather you live in Cambridge so I recommend a day trip over to Worcester. You can find the location of the first flight (March 16, 1926) of a liquid fuel rocket on what is now the 9th Fairway of the Pakachoag Golf Course in Auburn, MA (just outside of Worcester). The iconic cherry tree was in the back of his birthplace at 1 Tallawanda Drive, Worcester (but the great hurricane of 1938 took it down). The Goddard Library at Clark University has a display of Goddard memorabilia around the walls of the main reading room--among them is at least one of the machine-gun rockets. They also have his papers, patents, lab books, etc. Finally, to end on a stranger-than-fiction note, some of Goddard’s cartridge rocket experiments took place at the Magnetic Building on the campus of Worcester Polytechnic Institute. This building is still there, but is now the headquarters for a secret society and is called the *Skull Tomb*.

That's a very cool and informative post. Thanks for joining just to share it with us.

(I don't think Randall reads the forums, you might also want to email him at whatif@xkcd.com. It's normally just for What-If questions, but since this is obviously related to a what-if, I imagine it counts.)
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Re: What-If 0021: "Machine Gun Jetpack"

Postby Neil_Boekend » Mon Mar 02, 2015 7:26 am UTC

Welcome Bill63. I am delighted to make your acquaintance.
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