Re: OT: Repeatably lobbing "projectiles"

That is unfortunately a rather accurate characterization, I concede.

Jeroen Belleman

A marble that rolled down a ramp and hit a hook-shaped launch thing would be very precise.

Surely those are issues within range of your abilities? You're free to scale and reconfigure the thing. If you want an exact solution, at least specify your exact requirements. Otherwise we'll think you're here just to argue. You know how to state exact specs, don't you?

Jeroen Belleman

You give up too easily. Most people can solve an engineering problem if they are given accurate specifications and objectives. In my limited experience, I've never received all the necessary information. In the few times when I did receive accurate specifications and objectives, the client was made endless changes and "enhancements" until little clue as to what he wanted and somewhat less on what I was expected to accomplish. Over the years, I've deduced that this typical and that Don Y is a good model for my typical client.

Still, something can be done with the muddle. The first step is to determine what problem Don Y is attempting to solve. Don Y does a tolerable job of providing a partial solution, but without a clue what problem his partial solution is intended to solve, his partial solution is at best a moving target. My guess(tm) is that it does nothing useful, so this initial step can be ignored.

The next step is to determine what we have to work with. Since there are no specifications except for 20 ft range, the launcher and projectile can be almost anything. A 40 ft diameter ball would do the job and never miss the target. However, that's probably not what Don Y is expecting. A smaller ball, with a 20 ft steel wire attached, would also work. The 20 ft steel wire limits the flight distance to a

At this point, I usually would present my worst and most ludicrous ideas to the client, who would immediately being crying, screaming or yelling. Since that clients endless changes and meddling have dragged the estimated delivery date far too close to the deadline, the client has to select something. It's at this time that he gets serious about defining specifications (with my assistance). The rest of the project is comparatively easy and consists mostly of lost weekends and lack of sleep.

I could go on, but it's raining, the lights are flickering, and I suspect that I'll loose power shortly.

It seems that you are looking for a low friction (and thus repeatable) sear mechnism. This is almost certainly a solved problem.

Possibly something involving an over-center mechanism and low friction bearings will be suitable.

If that's the problem and not the temperature of the spring, or some other uncontrolled property of the aperatus.

I haven't seen any suggestions yet that don't have the same release friction problem, except perhaps the rotating arm in sleeve,

Yes, it was just an alternative that might have had some of the characteristics you need.

Another possibility is a variant on a pea-shooter that has a steady jet of compressed air entering a long tubular barrel through a port in the side. A continuous line of 'peas' is slowly pushed up the barrel and as each one passes the port, it is accelerated by the air stream. You only need to control* the air steam continuously, there is no need to micro-mange each pulse.

We have been told the range is about 20ft (but no accuracy limits) and there have been hints about what the projectile must not do (bounce or injure bystanders) but the main constraints on the projectile were not known at the outset and this was mainly what was making the problem more difficult than it needed to be.

The sorts of properties that make a huge difference to the design are:

Rigid , flexible, floppy or completely shapeless like a bag of water? If fairly rigid: spherical, cylindrical or some other shape? Approximate range of mass? Homogenous or made from bits of widely differing density?

There is a vast difference between trying project a quarter-inch-diameter lead ball 20ft and trying to do the same with a bucketfull of cows guts.

If we are allowed to specify the shape of a projectile (within limits) a ball-shaped projectile may look like the obvious choice, but it is not particularly aerodynamic and may have errors induced by erratic turbulence I would suggest that the projectiles could be tear-drop shaped to give them the best aerodynamics, with slightly spiralled tail fins that engage with rifling in the barrel to spin stabilise it in flight.

If you don't want it to bounce on arrival and would rather not use hard solid material which might cause damage, you could use a shaped thinwalled polythene bag full of sloppy semi-liquid that would be firm enough to take up the spin but would cause the bag to burst on impact.

In flight, semi-liquid droplets tend to take up the shape of a teardrop naturally, so the bag need not be particularly stiff, but it would need some sort of hoop around its point of largest diameter to ensure that it sealed well in the barrel. The fins could extend from the hoop backwards along the body to rigidise it and impart the spin.

The barrel needs to have multiple ports around the circumference where the air is injected, supplied by a 'manifold' in the form of a collar surrounding the barrel. At least two projectiles would have to be waiting in the barrel, they could be pushed forward by a positive displacement plunger so as to drive the hoop of the first one past the air ports, the second one acting to block the barel and prevent the escape of air backwards.

The plunger would then be moved backwards and another projectile introduced into the space through a slot in the side of the barrel. There is no need for a sealed 'breech' as the second projectile acts as a breech block. That sort of loading mechanism lends itself to automation or even gravity feed from a vertical magazine.

Your original post (message id <vhmm2k$hpg1$ snipped-for-privacy@dont-email.me) omitted (at least) any statement of the requirement for:

Which was only revealed after two sequential posts from Liz and one intervening reply from you.

I didn't consider designing a fantasy projectile lobber. Replacing my engineer hat and with my computer game player hat, I now have a better idea of what you're trying to accomplish. The processes you describe are very similar to writing a computer game using "the hero's journey" as a template:

What you've done is setup a fantasy playing field that is 20 ft long. Various SED (sci.electronics.design) members are invited to design a poorly defined machine which will lob projectiles repeatedly and accurately. The extent, size, composition etc of the playing field are not defined. Similarly, the composition of the projectile is not defined.

Due to the lack of design criteria, the objective of the game is not to design something that will function in a defined manner. Rather, it's for the participants to determine what the game-master (Don Y) has in mind. To do this, each player is allowed to ask questions and offer suggestions to the game-master. He may choose to help, explain, provide detail, offer clues, or he may choose to hinder, confuse, wander off into the weeds, or provide useless clues. He may also criticize and pass judgment on suggestions. His choice.

So far, the game only follows a few of the 17 stages suggest by Joseph Campbell. For example, a knowledgeable mentor might appear in SED to offer experience and suggestions.

From what I've seen so far, the game continues until everyone involved is terminally frustrated and gives up. However, the game does have possibilities. Normally, such a game has a built in reward system, where the players are granted "points" for successfully achieving intermediate goals. To prevent player frustration, some kind of prize or reward system much be implemented. Without keeping score, the game is largely brainstorming, which usually ends with an argument between those claiming credit for the best solutions. Therefore, some kind of scoring system should be implemented. There's also the problem of dealing with unacceptable and ludicrous solutions. The scoring system much include a way to exclude these solutions. Anyway, some changes in the game rules will need to be made to prevent player frustration. However, I'm frustrated, so I'll give up now.

Incidentally, the power stayed on during last nights rain. In my haste, I forgot to proof read and spell check my previous message. Sorry(tm).

With a long thin projectile, random tumbling could become a problem. If it were spin-stabilised at least it would only be moving sideways through the air in a predicatable and repeatable manner at the end of the trajectory.

Perhaps a spherical projectile would be better for short distances.

Use ice for the projectile, there would be no litter at all. If it were pointed and had the right aerodynamic properties it would embed point-first in the ground and not bounce - but it could be dangerous if it hit anything other than soft ground.

Yes, that would work. The critical point is that the air flow and maximum obstructed pressure are already established before the projectile enters the accelerating region, so there is no need for accurate valve timing.

How about a spherical bag (toy balloon?) full of water sitting in a polystyrene cup which maintained its shape as it was accelerated up the barrel. The cup and balloon would separate in the air, with the cup falling near the launch point where it could be retrieved and re-used.

Yes. I was following on from the 'row of peas' already stored in the barrel, but if you have a magazine on the side you could load one at a time. That could get messy if the projectiles were water-filled bags.

Snowballs!

The above is simply further evidence of the fact that you are trolling the posters here in the group.

This is a classic troll response when the troll is caught up in the web of their own lies.

No - because the cup has to be in contact with the projectile to maintain its shape while it is accelerated up the barrel. That means the air pressure must be behind the cup.

The cup could be stopped rapidly at the mouth of the gun by some sort of obstruction, but that would risk damaging it and/or fouling the projectile.

That problem was solved long ago with a discarding sabot:

The plunger has to stay behind the air inlet so as to block the back end of the barrel.

Or for the scrap value of a microwave oven transformer.

There will be magnetic friction as it lets go, but this should be repeatable if the armature placement is repeatable,

Some sort of "springboard" perhaps.

I suspect that there is no limit in pounds per watt that an electromagnet might hold up. Or it's at least enormous.