Better FM for the SCUD-B

The obviously disabled SCUD-B we have as of the date of post had three of lines commented out that disabled it. But there was a reason, it's flying like a Tomahawk. It's not good to see a ballistic missile behave this way and I feel a bit embarrassed it got shown like this.

Like with the modded 2018 SCUD, I modified the data and got it flying more ballistically, but I never shared that work and this was foolish because, if you are going to improve something, don't keep it to yourself.

This time, the cat is out of the bag, so I'll offer a different FM that is more like a SCUD, based on publically available data where possible (attached at the bottom). But tonight I didn't really feel like playing with the FM, there's not many notes on the settings and I believe it to be a flawed model because tweaking mass on accelerationand burn time (or putting in correct figures) gives you incorrect behaviour. So, I think I will leave it to you guys to work out the last 10% of it and in the course, I'll add what I learned playing on the version I did in August 2018 based on the mod that arrived on the forums.

First up it's a stock SCUD-B, not an upgraded Iraqi GW1 one or any N Korean Hybrid. That's a published range of 350km. So I reduced that from the 460km range in ED's. Second I took the measurements I found out there. I used a variety of sources, but this one was very enlightening, a research paper on the N Korean ER version. It's a good read to see how analysts work out the capabilities of missiles, their acceleration, checking sizes and postulating weights. Enjoy: https://www.38north.org/wp-content/uploads/2016/11/Scud-ER-110816_Schiller_Schmucker.pdf. I also used https://www.aps.org/units/fps/newsletters/1999/july/canavan-paper.html which described the ballistic arc.

In my initial tests of tweaking I found that the max G of the missile is very much the primary affector of the flight profile arc. ED put 17G for some reason. This thing could never pull 17G and my best range was between 4-6G which is also mentioned in the research papers. Whilst I understand what G is, there is no reason the ED model should be forced into a parabolic shot because the max G is lower. But it absolutely does. If you change the G, you will see the different arcs described.

Next I looked at max speed and altitude. A 350km shot was looking close to 250,000 feet high at the zenith. in 2018 i seemed to reach the right altitudes, today I couldn't, the shot is shallower for some reason. The missile also flies unimaginally straight at the tail end of the shot when it is properly ballistic. Something like an 8 degree dive unpowered, deccelerating through 1500kts. No idea why it should have that much lift, it maintains that right to impact.

For speed I believe we should have been getting around Mach 5. It doesn't happen either , there seems to be some cap on the speed that I cannot work out in ED's missile behaviour. I'm getting two thirds of that and this messes the entire thing. If you can work out why, please tell me! Due to the lower speed you have to have much longer burn times. This plays with two settings, the median speed and lowest speed. Not sure how I came to the figure or what they exactly mean, but I put them in and they kindda worked.

The mass, and time to acceleration were super short. I found that too low acceleration was tied to G, which also meant a slow accelerating rocket ended up hovering sometimes. I'm going out on a limb here but I think air density isnt modelled for these rockets. All three settings affected the way it launched, but not in a way I found logical. I'm merely a hack. See if you can work it out. Basically put in the right figures in one place and you need to change another valid one vastly to get it to work with that.

ED's 20m initial sigma values are wrong/WIP for a SCUD with INS. I put these at the stated and widely public values of 450m accuracy. 20M was laser accurate from all the stories we heard and to be honest, half the time SCUD's ended up in the wrong city or middle of the wrong desert.

OK, things that I don't like/not working.

The "Life_Time" is something like the battery when not propulsed in seconds. On 350km shots this SCUD self explodes 80-90% of the way there way before Lifetime is exceeded. I can't explain it. Long shots at max range aren't working but I got OK figures on 25%, 50% and 75% and all these landed within the sigma values on the target with a reasonable ballistic start and a somewhat flat ending.

I can't hit M5 or 250,000feet at zenith and I can't work out what the mass is, it certainly doesnt perform correctly with the correct masses, so have a look at that.

4.5G shorter shots can't turn down fast enough, so I went with 6G which is high for it's size.

The last part of the trajectory seems to glide ridiculously. I'd love to know how to eliminate that. It might just be the ED model.

The burn time is wrong it should be roughly 70-80s. I uprated it to account for the max G being too small which needed to be that to create the arc.

Everything in the Lua i've done by myself based on finding Russian comments in code on some other missiles and using Google Translate, some comon sense and a lot of trial and error. It's just number tweaking. I am most likely massively wrong on some things. But warts and all, this is still better than what is in the game now and if you want to work on the model more or publish your experimental findings and give back to the community (instead of what I originally did) then everyone else can learn a bit more.

GLHF and happy Scudding.

Head_Type = This is an enumerator. The notes say something like conical or round iirc

sigma = in metres, the accuracy/dispersion

M = I think mass but not sure the units as you need to really play with this to get it to work. It affects the acceleration and can stop launch, cause funny stuff

H_max = The maximum height it can go

H_min = -1, --why -1? does it stop the missile striking? (lowest height)

Diam = in millimetres, the width of the rocket. Not sure if it accounts for widest point or fins or smallest or what. ED's values was slightly thinner than SCUDB

Cx_pil = 1, --notes don't make sense no idea

D_max = max range but see below there's two, why?

D_min = min range. What affects does this actually have?

Head_Form = Dunno, left it as 1

Life_Time = shoudl be the battery or self destruct timer

Nr_max = max G - seems to affect acceleration stage as well as turning. Also affects altitude (lower, higher traj)

v_min = -- velocity minimum? what is that?

v_mid = --velocity middle? What is that?

Mach_max = --doesn't seem to be reached, go over 2.5

t_b = --dunno

t_acc = 8.0, --time it takes to reach top speed in seconds

t_marsh = the length of time the rocket motor lasts (burn time) (in French/Russian?! Marcher=to go?)

Range_max = why is this repeated? What is the difference between D_max and Range_max?

ScudLauncher.lua

If head form is one it might refer to MIRV warheads, which is promising for what may be in the future beyond Scud

Good initiative, thanks. I have a few parameters available to help you.

Maximum flight altitude(top of the ballistic curve): 86 km

Ballistic curve ascending arc angle: 45 °

Rocket speed: 1430 m / s

Rocket engine operating time: 62-64 seconds

Distance deviation: 160-610 meters

Lateral deviation: 100-350 meters.

Another note on the missile flight itself. At altitudes where the vacuum is not working stabilizer rocket and rocket rotates around its center of gravity. The stabilizers begin to function at a altitide of about 20,000 meters when descending.

Thank you Pikey :thumbup:

It's flying much better now.

Making a bit of a wild guess here. But as an old artillery man thinking back to the parameters we used to get a grenade on target and taking into account the difference between a powderburn grenade and a self propelled rocket I miss 2 parameters that affect both. First is rotation of Earth in relation to flight time. But I don't think the world in the ED universe rotates. So that's no issue. And air density (affects speed and range). So perhaps D_Max and D_min is air density (or rather drag). Just an idea.

D_Max = max range of missile

D_Min = minimum range of Missile

I think the range it fires at the target

Range_max = overall range I think