8
$\begingroup$

While I was looking into alternatives to gunpowder I stumbled onto a comment talking about an ignited chemical propellant that could be placed in front of a bullet and effectively suck it forwards out of the barrel, similarly to a vacuum cannon. However I haven't been able to find the comment again.

So in other words does anyone have any ideas as to the possibility of an "inverted" propellant? Which would suck a projectile forwards out of the barrel?

$\endgroup$
1
  • $\begingroup$ The "best" use for an inverted propellant would be in combination with an normal propellant, as to create a larger pressure differential. But that 0.5 extra bar of effective pressure you are maybe getting isn't worth the effort. If you wanted more effective bullets, improving barrel-bullet friction would be far more useful . $\endgroup$
    – vinzzz001
    Commented May 21 at 13:45

6 Answers 6

25
$\begingroup$

It sounds like a poorly conceived idea.

Reason?

With increasing pressure you can go up as much as you want, the limit being the strength of your barrel. If 20 bar of up pressure are not enough, you engineer the gun to go to 40, 60, 80 or more.

With down pressure instead you can suck as much as you want, you won't go beyond 1 bar, which is your environmental pressure. So your only way to increase the bullet energy would be to make longer and longer barrel. And Dirty Harry would not be as cool while wielding a 20 m long revolver.

$\endgroup$
2
  • $\begingroup$ this is only true for compressible fluids (e.g. gas). An incompressible fluid can achieve negative pressures $\endgroup$
    – cms
    Commented May 17 at 13:35
  • $\begingroup$ @cms Which then obliges you to have a gun where the projectile is suspended in an incompressible fluid, which is much more difficult (especially when it comes to making sure your fluid doesn't leave the gun when the bullet does). $\endgroup$
    – Cadence
    Commented May 18 at 5:21
7
$\begingroup$

This - or something adjacent to this - has been done before.

Ultimately, there's no reason not to use regular propellant. Normal things explode just fine, and if you have the tech to make something explode backwards, you probably also have the tech to make something explode forwards just as powerfully. How your guns work on a mechanical level and to whom they are distributed and their lethality are often valid worldbuilding points, because they actually mean something for your world, whereas things like "is the propellant a positive or negative pressure generation system" leans very heavily into the region where readers/players/viewers are going to hear that and think "oo, cool technobabble, let's get on with the story though."

As far as actually making it possible, you can produce just as much force on an object by vacuuming out the region in front of it vs. increasing the pressure behind it. The linked video is of a supersonic baseball cannon (no, I'm not screwing with you) a part of which involves producing a vacuum in the front of the barrel to increase the pressure differential across the baseball and thereby increase the acceleration as it proceeds down the barrel, out the muzzle, and through a large number of catcher's mitts.

However, there is a limit to how much pressure you can actually produce with this mechanism (as noted in the video). You can only create a pressure differential of one atmosphere, because that's the surrounding environment, whereas with a regular explosive propellant you can create pressure differentials of upwards of 3400 atmospheres. Barring somehow producing proper negative pressure, regular propellant is always going to be much more powerful than negative-pressure propellant.

$\endgroup$
4
  • 1
    $\begingroup$ I see, the reason I was looking into the idea was as an initial propellant for a smart bullet propelled by liquid rocket engines, so the propellant was placed in front instead of behind where it could damage any components. Apparently the compound the person had mentioned was something that already exists $\endgroup$
    – Wey Newman
    Commented May 16 at 19:01
  • 2
    $\begingroup$ @WeyNewman The business end of a rocket is usually pretty robust, what with having to withstand the combustion and exhaust. But if you're looking for a unique first stage mechanism, you could try a coilgun. $\endgroup$
    – Cadence
    Commented May 17 at 1:01
  • 1
    $\begingroup$ ummm, you can't "produce just as much force on an object by vacuuming out the region in front of it vs. increasing the pressure behind it". As explained in a different answer and in your own answer further down, vacuum is limited to 1 local atmosphere. $\endgroup$
    – matt
    Commented May 17 at 8:44
  • 4
    $\begingroup$ Also, the vacuum in front of the ball in that cannon is to reduce air resistance - the pressure differential is >20 bar, so a vacuum only increases that by about 1. Not having air in front of the baseball as it accelerates is what allows the baseball to reach the much higher speeds in that video. Without the high pressure behind it providing the acceleration, the baseball would not go very fast at all. $\endgroup$ Commented May 17 at 16:39
5
$\begingroup$

As stated by the other answers, vacuums are a bad idea. However, consider this answer a loose interpretation of your final sentence.

A coilgun "sucks" metal projectiles forward out of the barrel.

These use electromagnetism, not a vacuum to accelerate the projectile. They could be shorter than a canon, and you can even make handheld versions.

$\endgroup$
1
  • $\begingroup$ You could also make it yourself. $\endgroup$ Commented May 19 at 14:09
4
$\begingroup$

Using suction to "pull" the projectile has one major drawback:

as you suck at the projectile, you are also sucking at the air outside the barrel with the same but opposite force. The air rushes into the barrel against the projectile coming out, wasting 50% of the energy you provided. The only way around this is to seal the end of the barrel until the moment the projectile is about to exit, but this is extremely finicky to pull off.

Alternative option is to use extremely rapid piston action to create under-pressure in front of the projectile, but this begets the question: why not use the piston itself as a projectile and simplify things?

$\endgroup$
2
$\begingroup$

I'm going to throw to the wind

The only chemical I know of that absorbs oxygen (not air... oxygen) is pyrogallic acid. Compared to what you need to make a slug an effective bullet, it's darn tootin' slow. And I don't know of any chemical that absorbs the entire chemical makeup of the atmosphere.

Besides, it's just not fun. Science says this won't work. Things don't move simply because a vacuum exists. They move due to the rush of air as it's pulled toward whatever is trying to create a vacuum (and the rush of air is only because the gas is trying to expand to fill the space, but that doesn't happen at anywhere near the speed of an explosion). Further, the mass of the air in the barrel (no matter how long... at least for a practical-length barrel) is nowhere near the mass of the slug, so there's just not enough "umph" to move the slug. Worse than that, at the point of implosion you still have a something that's in the way of the bullet comprising of the mass of the gathered air and the mass of whatever caused the implosion. If I'm wrong and the slug actually got moving, it would hit that spot and, IMO, stop dead in its tracks — a bit like two magnetic poles coming into equilibrium. Boring!

Let's move to and assume the existence of Chemical X that can cause the air to move toward the implosion at a respectable percentage of the speed of light. Yeah, baby!

Now, let's put the implosion at the back of the barrel behind the slug such that it's drawing air from the outside. Once the implosion is done, the passage to the outside is closed. (This isn't as impossible as it sounds. The mechanism would work like a check-valve for a water system.)

Once the implosion is done... the air is released! BANG! If you're thinking, "Aw, man... all you did was invent an over-engineered air gun," yup, you're right.

But, you have to hand wave Chemical X because I kinda suspect no chemical exists that can do as I just described.

Result? Something more than a popgun but less than a railgun. So say we all.

BTW: This Stack has seen a lot of, "my world doesn't have gunpowder" questions. It's not just a little unrealistic. It's really unrealistic, which makes the tag a bit odd. The real problem is that gunpowder isn't in any way unique. There are many chemical compounds that can make a substantial BANG other than gunpowder and the ecological consequences of a world without one or more of sulfur, carbon (via charcoal), and/or nitrogen and oxygen (via potassium nitrate) are staggering.

$\endgroup$
2
  • 1
    $\begingroup$ Or other ways to launch a projectiles. There's some historically lethal airguns, steam cannnons or even kinetic projectile hurlers $\endgroup$ Commented May 17 at 13:09
  • $\begingroup$ Iron also absorbs oxygen very effectively, though not very quickly. $\endgroup$
    – cobbal
    Commented May 17 at 19:44
1
$\begingroup$

The Achieved Vacuum Can't Exceed the Size of the Implosive Charge

It is as simple as that. Put an implosive charge of one cubic centimeter in front of the bullet, and the absolute limit (which can't be reached) of the vacuum in front will be one cubic centimeter. As you can imagine, this will not propel the bullet very far - especially as the vacuum will also pull in air from the front of the gun, counteracting the pull on the bullet.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .