Editing Missiles
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Hello, I have several questions about editing various aspects of missiles for a custom theater. (ITO)
#1: AGM-78 Standard missile is flying way too fast
In-game it reached Mach 5.5Launch alt? Launch speed?
It’s so fast it could barely pull enough G’s to maneuver to it’s target when fired from close/medium range
Congrat, you discovered the most problematic issue of AAMs since…
from the beginning…In close range missile can accelerate more than you need. This is the problem with non controlled thrist and speed-lift realation.
Most sources say it should fly around Mach 2
They bad. AGM-78 is from sea based platfroms has larger kinemtic range as RIM-66 SAM, which is needed much larger max burnout speed at very long range. At high alt launch you can be sure it can reach M3.5-4 without any problem.
I believe I can change the speed by editing the AGM87.dat
ideas on what to edit or what values to use?Speed is not scripted it is defined by aerodynamics, cross section, cd, cl values and wieight and forces are counted from this. If you do not know this area is better not touch it.
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They bad. AGM-78 is from sea based platfroms has larger kinemtic range as RIM-66 SAM, which is needed much larger max burnout speed at very long range. At high alt launch you can be sure it can reach M3.5-4 without any problem.
This is scripted? Or based on which source?
Actually, I guess at high enough altitudes, based on the problem we are discussing, you are 100% correct. Launched at for example, 230 thousand feet, its pretty safe to assume mach 4 is very achievable. Maybe it would be useful to limit the problem set here? What altitudes do we care about?
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Spitballed some numbers from ‘On Estimating the Drag Coefficient of Missiles’. Barring egregious errors on my part (certainly more than possible), drag at mach two on the deck seems much lower than I would have anticipated, using a Cd of 0.25. Now, that either means I dont have a great idea of how much drag there ‘should’ be, or it means my drag coefficient is way off. Not sure which is more likely. I got 880 lbsf drag for that condition, which seems quite low - certainly that much drag would not do much to retard the impulse there.
Depends on Mach number, 0.25 is low though. My order of magnitude figures are around 0.5 at subsonic and high supersonic, peaking at around 0.8 in the transsonic region - all this for a “pointy” missile. Exact figures depends on radome shape and fins. The radome itself is usually a compromise shape between RF and aerodynamic properties. It is not easy to come up with 100% accurate figures for every single missile but there is some litterature available.
A quick note on Gs for missiles : a misconception is that you need to match the turn radius of the target to be able to get it, and that is not true. If the missile is established on a collision course, and then the target manoeuvers at N Gs, a missile with a perfect sensor picking up on the manoeuver instantly will need to manoeuver at N Gs as well, no more, no matter what the missile speed is. Since sensors and guidance algos are not perfect, you usually need a bit more, but if you go 4 times as fast as the target you dont need 4² = 16 times more g.
Now, getting on a collision course usually require some turn rate that is harder to get with a fixed G limit and very high speed. This is why it is wiser to turn a bit in anticipation of the target for an AAM, or to avoid launching an AGM with an angle offset. This is also why missile defense advice is to put the missile on the 3-9 line and turn nto it : doing so maximizes LOS rate, so if the missile sensor/autopilot lags a bit the angular error will be maximum and the missile will struggle to get back on a collision course.
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Launch alt? Launch speed?
Congrat, you discovered the most problematic issue of AAMs since…
from the beginning…In close range missile can accelerate more than you need. This is the problem with non controlled thrist and speed-lift realation.
They bad. AGM-78 is from sea based platfroms has larger kinemtic range as RIM-66 SAM, which is needed much larger max burnout speed at very long range. At high alt launch you can be sure it can reach M3.5-4 without any problem.
Speed is not scripted it is defined by aerodynamics, cross section, cd, cl values and wieight and forces are counted from this. If you do not know this area is better not touch it.
Launch Speed: Mach 1
Altitude: 20k
Sorry I don’t have more data, I’ll do more testsBut it seems a little strange for the Standard to be flying faster than the Harm by 2xMach……
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This is scripted? Or based on which source?
Actually, I guess at high enough altitudes, based on the problem we are discussing, you are 100% correct. Launched at for example, 230 thousand feet, its pretty safe to assume mach 4 is very achievable. Maybe it would be useful to limit the problem set here? What altitudes do we care about?
This is simply physics. Even with most optimal ballistic trajectory you need much higer burnout speed than M3.5 to reach the range of RIM-66 as SAM. Literally even the worst and crappiest AAMs could break M2.0 at 5 km alt from airplane at M0.8, stone age AIM-9B, R-3S or smallest R-60. Such a large missile as AGM-78 at high alt can break without any problem M3.0
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further to gasmans point regarding AGM-88 vs AGM-78: the whole point of the High-speed Anti Radiation Missile (HARM) was to go faster than the STANDARM, right?
Gasmans screenshot above shows either the HARM speed needs increasing or the STANDARM’s needs to slow down. I for one don’t believe a HARM can exceed M 5.0, (then it would be borderline hypersonic) - so logic dictates the STANDARM model is too fast if it can hit M5 and the HARM cannot under similar launch conditions.
We can disagree over ranges between Mach1.8-2.5 (perhaps higher), and we may have mixed data based on rocket motors - im not smart enough to understand or argue the maths around rocket science - but i doubt the -78 should be faster than the missile that was designed to replaced it. We may not be able to get exact speeds due to classifed nature, or due to modelling limitations in BMS, but how about we start with the premise that the STANDARM as modelled is too fast for starters? -
Actually, the HARM was made to be faster than a Shrike. The AGM-78 problem was that it was exceedingly heavy.
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intereresting. so you think the speeds are accurate? it just seems very counter intuitive to have your newer missile operate slower than you replacement It also goes against the sources quoted, do we have anything that demonstrates a -78 going M5+?
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Some sources are not open.
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17 nautical miles isnt exactly long range, though. RIM-66 doesnt narrow it down much molni - three different rocket motors for ‘RIM-66’.
Also worth noting is that the High Speed ARM is flying almost half the speed of the missile it replaced… why not call it to SLow ARM instead?
Interestingly, ausairpower cites the SR113 as containing only 303 lbs, or 137.4 kg, of propellant. Seems low for a high speed missile.
Ah! But its a lightweight missile, only 360kg at launch. That makes more sense.
Well, I figure the HARM should have a delta-v around 1.1 kps… not drastically dissimilar to the STARM. Its unclear to me whether BMS internally uses the rocketry equation for missiles?
The thrust for both missiles is quite different, and I dont have any sources to compare either to. The BMS AGM-78 has 1 second of no thrust, 4 seconds of 17 klbsf thrust, then 10 seconds of 5.1 klbsf thrust. The BMS AGM-88 has 4 seconds of 10.6 klbsf thrust, then 36 seconds of 888 lbsf thrust. That is probably the major source of the HARM in BMS flying much slower.
If you take as granted that the STARM flies too fast, you could rectify that by decreasing the sustain thrust and increasing the duration of that sustain. While I suspect that is likely the case, without more data on it its a bit beyond my capabilities to assess - I dont think I can make an estimate for burn rate and thrust profile without more information. Nothing stopping you from using the LAR method - Looks About Right - to get the missile to fly slower.
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Burn duration can be guesstimated from old live fire videos of SM1s.
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17 nautical miles isnt exactly long range, though.
This value concering on what?
Do not forget the target of RIM-66 is moving and even is non maouvering it means at edge of range missile speed is t least ~400 m/s. Target of AGM-78 does not move so even the missile is subsonic can hit the target.
The big question how different was the engine thrust char. of AGM-78 comparing to RIM-66.Interestingly, ausairpower cites the SR113 as containing only 303 lbs, or 137.4 kg, of propellant. Seems low for a high speed missile.
Almost all AAM and similar SAM missiles have very similar propellant weigth/launch weigh, about 0.32-0.37.
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This value concering on what?
Do not forget the target of RIM-66 is moving and even is non maouvering it means at edge of range missile speed is t least ~400 m/s. Target of AGM-78 does not move so even the missile is subsonic can hit the target.
The big question how different was the engine thrust char. of AGM-78 comparing to RIM-66.Almost all AAM and similar SAM missiles have very similar propellant weigth/launch weigh, about 0.32-0.37.
The RIM-66 block III and block IV, the only two RIM-66 versions that share a motor class with the AGM-78, had a notional range of 17 nautical miles to complete an intercept. The block V and onwards, using the Mk56 rocket motor, had a range of around 25 nautical miles.
l3, that is a very good point. The only question there though, is that of the difference between the Mk27 mod 0 used in the RIM-66A block III and block IV, and the Mk27 mod 4 used in the AGM-78. I dont know what the difference is, but someone thought it a significant enough difference to label it such. Thrust profile of the grain would be a very good reason to label the motor a mod X…
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The RIM-66 block III and block IV, the only two RIM-66 versions that share a motor class with the AGM-78, had a notional range of 17 nautical miles to complete an intercept. The block V and onwards, using the Mk56 rocket motor, had a range of around 25 nautical miles.
If you drop a bomb at 24000k feet with 300 m/s its impact distance projected on ground is close to 10 km (6 nm). It is very hard to belive that an AGM-78 max. impact distance on ground from high alt is only 17 nm. Just doing a loft with the missile without thrust can result such impact range… Such one value range data is useless for comparsion…
I have somwhere the range enevelope oh Kh-58 missile. At low level is less than 40 km, az very high alt the range in above 120 km… So 17 nm at SL is maybe true but launching at high alt…
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If you drop a bomb at 24000k feet with 300 m/s its impact distance projected on ground is close to 10 km (6 nm). It is very hard to belive that an AGM-78 max. impact distance on ground from high alt is only 17 nm. Just doing a loft with the missile without thrust can result such impact range… Such one value range data is useless for comparsion…
I have somwhere the range enevelope oh Kh-58 missile. At low level is less than 40 km, az very high alt the range in above 120 km… So 17 nm at SL is maybe true but launching at high alt…
You havent read the post then… I did not say that the AGM-78 had a range of 17 nm. Go back and reread the post, then try again.
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You havent read the post then… I did not say that the AGM-78 had a range of 17 nm. Go back and reread the post, then try again.
Do you have a bad day or someting?
So the intercept point of RIM-66 can be 17 nm? It seems to me real. The same missile in case is launched from a very high platfrom could have 2,5 times larger range (not launch range) even against air target and much larger against ground target…Concerning on first post. As long as is no exact data about AGM-78 you have to find or use Kh-58 as reference pont. At very high alt M5.5 is likely too big but M4.5 would not be.
(AIM-54 is much more inaccurate currently in BMS4, has similar or worse kinematics than AIM-7F/M…)
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Yes in fact, but thats no excuse for a poor attitude on my part. My apologies.
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(AIM-54 is much more inaccurate currently in BMS4, has similar or worse kinematics than AIM-7F/M…)
Yeah, I need to tackle that one of these days.
I also confirm that the flight profile makes a huuuuge difference for a long range SAM. So much so that a VLS is pretty much the optimal way to fire a SM2 or a SA-10 anyway This is why they were able to gain a ton of range by going from a pure SARH with the SM-1 to a datalink/TVM type with the SM2, all else being equal - they could tailor the flight profile for max performance.
Regarding the missile flight model in BMS, it is actually pretty detailled and you can get very close to RL performance if you have enough data. Engine thrust, aerodynamic lift & drag, atmospheric model etc. are all modelled decently and give realistic results. The guidance is a bit monolithic (you only have proportional navigation, with a loft G bias at first) but you can tweak the gains depending on the part of the flight, allowing you to adust the flight profile quite a lot.
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Concerning on first post. As long as is no exact data about AGM-78 you have to find or use Kh-58 as reference pont. At very high alt M5.5 is likely too big but M4.5 would not be.
Potentially, if it was launched at or above mach 0.8 and at high altitudes. Again, launched at say 90,000 ft you could see mach 4 to mach 5. Well, launching aircraft speed plus mach 3.7, basically.
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Gents,
Is the consensus that the modeling in BMS is probably about 25% too fast?
Can I please have the physicists give a direct response to this question? (Blu3wolf, l3crusder, moli)?
We are trying to decide to go forward with modifying it for our 80s ITO mod or not - we need to know if we should modify it based on our hunch that it’s too fast or if we should just leave it and move on. Thank you.