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Alright people. I've got a challenge to put out there for anyone wanting to have a crack. I'm looking to pay someone to optimise HGDT figures, with a minimum of 650m/s and the following prerequisites.
Insulation - none
Chamber volume maximum - 150 cc
Chamber fan diameter - 0
Chamber fan flow - 0
Profile - blunt
Disc diameter must equal barrel diameter
Propellant - Propane
Mix - 150
Barrel bore must equal projectile diameter
Barrel length maximum - 110 cm
Projectile mass to diameter ratio no less than 33:1 (metric)
Barrel and chamber pressure cannot exceed 16,000 psi.
$50 goes to the person with the highest velocity above 650m/s. This can be done through PayPal. Challenge closes 12am Sunday 10th of July, EST.
Post your velocities on this thread, then when competition closes you can upload your screenshots of HGDT.
First off, I would love a crack at $50, but as your fellow forum member I have to advise you against this.
Just playing around and a little research can get you what you want without having to drop $50 for whoever manages to get the highest velocity with your specs in a configuration that may or may not be feasible for you to build.
Short version, don't waste your money.
Cheers Mark, I appreciate the heads up although I'm quite happy to pay the $50 for someone who can produce those numbers. I have spent plenty of time playing with the figures to get just shy of it, so for me it is worth it to see someone beat it.
I'm sure someone here would do it if you asked them nicely in a PM or whatever medium, or even just offered $5 or so, or a site donation. I think a few people here would feel uneasy about charging $50 to run a GGDT simulation.
Unless it's blatantly obvious and I'm thick as faeces in the neck of a milk bottle, but could you define mass to diameter ratio?
Is it simply a 1cm diameter projectile must weigh 33 grams presuming a 1:33 ratio? I've not encountered the term, and a search didn't help either.
Yeah that's fair enough mate. I did make a site donation the other day, after all the site has been invaluable to me over the last few years.
To be honest though I've been playing with HGDT all afternoon and I'm not sure if I can even get to 600 now. But hey I've said it, so I'll stick to my guns (lol) and if anyone gets to 650 it's theirs.
Yep that's spot on. It's not a known term, just my way of explaining it.
I'm getting that feeling too now.
The projectile specs are based on a 50 cal BMG round, with a mass of 40 grams, and diameter of 12.95mm. I wanted to maintain the same scaled round, hence the mass to diameter ratio.
The mix is simply what I would like the gun to operate at to achieve the performance numbers I was after. That's not set in stone though and may change.
I wanted to keep the chamber volume as low as possible while keeping the muzzle energy and velocity to a level I was happy with as well. At 150cc, less than 100 shots will drain a scuba tank shooting at 150x mixes so the smaller the chamber the better.
Disc failure pressure is the maximum efficient operating pressure, anymore than 4000psi and the chamber and barrel pressures rise a lot without much change in performance. Same with the ignition source numbers.
I managed 623ms by the way.
A barrel 150cm long gets your 650ms figure using 2cm disc diameter, barrel and projectile (66g). An extra 40cm isn't a lot to add for an extra 40ms or so. Chamber was 4cm diameter by 8cm long. Burst disk pressure 14900PSI.
100 shots is quite a lot, I've built three hybrids and probably have fired them a combined total of 60 or 70 times. How much does it cost to fill a SCUBA tank anyway?
623 is quite good man. I'd be interested in seeing what you used there. I'm trying to keep everything within the limits I set but I may be able to compromise on the barrel if it's the only thing I can do.
Filling a scuba here is around $7, but I live in a remote area around 1000kms away from the nearest fill station so I'm just looking at getting a compressor. They're around $2500 for a second hand one but is my only option. My design is a bit different from the norm, being a mix between a coaxial and normal inline piston valved semi auto hybrid. So when it's eventually done, hopefully within this decade, I'll be firing it quite a lot.
Chamber: 4cm diameter by 12cm long
Valve: Disk bursts at 14900PSI, 2cm diameter
Ignition points: 10
Barrel: 2cm diameter 110cm long
Projectile: 2cm diameter 66g
Oh yeah I got ya. Problem with putting the burst disk pressure up so high is that it also raises the chamber pressure and barrel pressure beyond 16,000psi that's all. I'm trying to keep it below that so the valves I plan to use are within their working pressure.
Also, have you ever come across any run time errors with HGDT? I'm finding whenever I deal with high numbers, it spits out an error code then closes. Spoke to David Hall, the guy who built it and he said he didn't know of any errors like that though. Could be just me
Oh yeah wow, I didn't notice how much the chamber pressure increased with the extra 9 ignition points. Jumps up to around 20,000PSI, for an increase of about 10ms.
Is there a problem with sticking a longer barrel on it? Hybrids aren't very portable to begin with, especially one like this.
well if I were you I'd stick to 10-30 X mixes and use a much lighter round
Children are the future
unless we stop them now
Best I can manage while meeting the specs given is 611m/s with a 2.1cm bore, 4 ignition points and the disk failing at ~9500 psi (HGDT's results for a given input necessarily aren't constant from run to run). The chamber in that sim is 5.76cm internal diameter and 5.76cm long.
I think your $50 is pretty safe
With a 150cm long barrel, speeds above 660m/s can be achieved without too much difficulty. Also, POLAND_SPUD appears to be trolling SB15. Pay him no attention
Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
All right, I got 799 m/s. But for a real design IT WILL NEVER WORK. suffice to say that the aspect ratio of the chamber is... well... *em*
HGDT reports the maximum chamber pressure as 221ksi, but the graph only shows a peak at around 16ksi. It is also extremely inconsistent in that the velocity varies 200m/s between runs with identical parameters.
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