Editing Missiles
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Well, M5.5 seems on the high side of thing, I would love an ACMI for that.
With a 120,000 lbs total impulse, assuming a ISP of around 240s, give or take, the vacuum top speed is around 3700 ft/s ADDED to the velocity of the launcher, aka around M3.7 more than the launcher (exact mach number depends on the altitude). Formula for that at the bottom of the post. Atmospheric top speed should be a bit lower but in the same ballpark - drag is a factor but not a huge one for the top speed, with a short burn time drag just do not have enough time to slow the missile down a lot. In game, launched at M0.9 from 25 000 ft, you get a top speed of around M4.0.
So I have no idea where they get their M2.5 top speed figure, but it just is not possible with the motor they had on. Especially when you consider that it comes directly from the RIM-66 “SM-1” missile, which is a Surface to Air Missile quoted as having a top speed of around M3.5 launched from a fixed platform. From a moving platform with launching speed of around M0.8 - 0.9, there is just no way the top speed is M2.5.
Formula for vacuum top speed, BTW :
Vaccum top speed (ft/s) = ISP (s) * g (ft/s²) * log_e(Total Missile Mass at launch/Missile Mass with motor fuel entirely consumed)About the trail : no need for a particlesys change. The trails are defined in trails.txt (in the same folder as particlesys.ini), so you can just swap the trail for an AIM-7 one, for ex. To do so, change the mistrail value in the missile .dat file to the new trail id number.
https://www.dropbox.com/s/319k1pvzvzd1sr1/ITO80s%20Test1.zip.acmi?dl=0
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Where did you find the propellant mass of 229 kgs? And where did you use the total impulse given?
The mass value is plausible for the volume, although hard to nail down a small range of values without knowing more about the grain profile - and the exact composition and therefore density of the propellant.
The SM-2s are cited as having a speed of Mach 3.5 by the wiki. Hard to gauge its accuracy, though. The RIM-24B used the same motor, the Aerojet MK27, and its cited as having a top speed of mach 1.8. The SM-2 does not use the same motor, though, so that top speed is quite plausible. Only the RIM-66As, the SM-1 MR Block III and block IV, used the MK27.
Id not be so quick to write the drag out of the picture… For the altitudes we are considering, I wouldnt be too surprised at all to see it shave off a few hundred m/s or more. Its a short burn duration, and that means a very high rate of acceleration - but the faster we are going, the more wave drag we are getting… Only need to get to a speed where there is an appreciable fraction of 16 thousand lbs of drag, to see the extra thrust not help much at all for acceleration.
You can estimate more or less the impulse/propellant weight based on RIM-66 range figures and tests with in-game flight model. No need for exact grain profile, density, etc. , what matters most is ISP and propellant weight.
I’m not saying that drag does not play a factor in top speed, of course it does. But the top speed is obtained at the end of the burn, which is very short. So drag simply do not has enough time to slow the missile top speed by a ton when launched from high altitudes. RIM-24s had an early version of the MK27, and they flew a very flat profile that gets you the most drag at the beginning of the flight that does play quite a bit, especially launched from near the ground. The AGM-78A does not have this much of a problem, launched from higher up.
BTW : The distinction SM-1/SM-2 is merely a distinction on guidance type, you have SM1s that flew much faster than SM2’s. (The whole Standard family is somewhat of a mess). But considering that SM-2s fly an optimized profile that let them get out from low-altitude drag faster, the top speed of early SM-2s is actually much closer to what the vacuum top speed should be, so they are a better indication of whether the engine data is correct or not.
All this is to say : the M2.5 speed for the AGM-78 is plainly not accurate. Like all range, speed figures for missiles, all of them have to be taken with a grain of salt. When you actually run simulations of what plausible drag and engine combinations gets you in speed/range/etc. in various configurations (like the distinction fired from the ground/fired from higher up), inconsistenies like this become obvious
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Definitely the case that top speed figures dont answer anything without accompanying profile information.
That said, the SM-1s that shared a motor with the SM-2s used the Mk56 engine - an entirely different rocket motor. Comparing apples and oranges here.
Edit: anyway… the reason I ask where you used the total impulse value and where you determined the propellant mass, is because I wanted to know which values you plugged into Tsiolkovsky’s venerated equation. It seems like you are saying that given the total impulse, and the specific impulse, you find a delta v of 3700 fps… then when I ask where you found the propellant mass, you tell me it was determined experimentally through falcon? I must be confused, because that sounds rather like you are begging the question.
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AP composite propellants apparently have a specific gravity between 1.7 and 1.9. This is a little confusing to discover, because it suggests quite a high mass of propellant given the volume of the rocket motor. I have to begin to question the accuracy of the figures I have for the dimensions of the rocket motor (13.5 in by 105 in, http://enu.kz/repository/2011/AIAA-2011-6941.pdf). Measuring it from the schematics online however, the rocket grain seems to be closer to 67 inches in length - so I guess the remainder would be the nozzle.
Assuming that is correct, this would give an upper bound of 312kg for the propellant mass. Less the core, and less the difference if the s.g. is lower than 1.9.
The core would take up at least 20% of that mass, but not more than 50%. That leaves a wide range of feasible values, though. A multifin grain profile would allow for a dual thrust profile and would use in the vicinity of 25% of the grain in the core. I have a feasible value of 216kg for the propellant mass, which is not significantly different from your value of 229kgs. That would give us a delta v of about a kps, again basically as per your results. So we agree on the basics there - it seems our disagreement comes from the drag more or less solely.
As regards the missile data file, it might be interesting to start by upping the missile weight to a more realistic value. I note the stock BMS value is a tad low at 840.90 lbs - unless this is a quirk of the .dat file format? The lowest value Ive seen online for the AGM-78 has been 1370lbs, and some spec sheets have claimed more weight again, not less.
Upping the missile weight in the .dat file is going to have quite the effect on the burnout speed.
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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.
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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.