Arun: I thought that detailed explanation would have triggered some actual number-crunching, but I realize that you are a busy man. Browsed the links you provided, with deep interest. <P>Yes, those "nuts-and-bolts arguments" are indeed bothersome, just as the 2nd Law of Thermodynamics is a real pain. Destroys so many bright ideas.<P>Specific Impulse is indeed a great metric, but its not everything. For example, if you want highest Isp, you shine a flashlight out the back end of the vehicle and get the thrust from the light itself: fantastic Isp since the photons move at the speed of light and weigh next to nothing. Unfortunately this won't give you the best acceleration because you can't produce enough thrust (unless the Aliens tell us how). Solid rockets are low-tech, dirty, inefficient, but also very cheap and easy to produce. <P>I suggest that you do in fact "run the numbers" yourself. Also try to include production cost of a missile with machined inlets, air ducts, and all the complexities of transitioning from solid rocket to airbreathing ramjet. <P>Rocket SAMs capable of reaching 15km are no more massive than a sounding rocket which can reach 100 km, and those are still fairly mobile. <P>I am still going by gut feeling and sense about the pressure ratios, peak temperatures etc., but I would be very surprised if properly calculated numbers come out any differently, whatever those web pages say. Now lets look at your 3 web page links: <P>1. The SHAAFT link (interesting name) talks about a HYPERSONIC airbreathing vehicle. Slightly different contraption from the Akash, which I think will only reach about Mach 2.5 or maybe Mach 3. When you run the numbers, see how much better the ramjet becomes when you use Mach 4 or 10, and cruise ranges of 10,000km as the requirements, than for the operating parameters of a SAM. <P>At Mach 2.5, at 10000m altitude, the peak combustor pressure of a ramjet may be about 4 atmospheres after you take out the losses due to shocks and air friction. A rocket combustor will easily reach 20 atmospheres. <P>The formula for peak stagnation pressure is: <P>Pstag = Patm*{(1 + 0.2M^2)}^3.5<P>At 10000 m, Patm ~ 0.25 atmos. (note: 1 atmos ~ 1 sea-level standard value of atmospheric pressure = 1.03E+05 N/m^2, or 14.7psia). At Mach 2.5, this gives Pstag ~ 0.25* (2.25^3.5) atmos; which is too difficult for me to work out without getting out of my recliner and finding my calculator. (4.27 atmos, actually). Shock and friction losses will take out about 10 to 20% of that inside the engine, not counting the drag due to the inlet in the external flow. If your inlet is poorly machined, you are going to lose 35% of the incoming stagnation pressure due to normal shocks forming in the inlet, versus the weaker and more desirable oblique shocks.<P>The ramjet will operate for a longer time, and be very fuel-efficient (high Isp). It will therefore reach a higher velocity, eventually. Great for the supersonic cruise missile as I argue. Also for the "hyperplane", which is what your SHAAFT page was about<P>But your argument about higher acceleration than a rocket is simply not sensible. And as you said, SAMS just boost up to altitude and speed ASAP, then coast with aerodynamic controls to hit their target. In this case, the best launch IS the implusive launch, (like from a gun), where you reach max speed and altitude ASAP by using up all your propellant ASAP. If you can stand the g-forces and produce enough thrust. The rocket comes closest to this, because you do dump out the propellant at a tremendous rate, starting at zero velocity. <P>The ramjet may be super-efficient, but it distributes the fuel burn over a longer time. <BR>Lets throw some more stones here. The "choked" mass flow rate through a given nozzle is directly proportional to the stagnation pressure (Pstag) and inversely proportional to square root of stagnation temperature. The temperature is probably limited by material considerations, so not very much higher for rocket vs. ramjet (though I may be wrong, and it may be 1.5 times than of the ramjet caase). But the pressure can be 5 times that of the ramjet, so the mass flow rate is anywhere from 4 to 5 times that of the ramjet!! Doesn't matter much if it is nitrogen or hydrogen chloride, mass is mass. The final exit velocity reached, is also higher for the rocket, since the stagnation temperature is higher. So, the thrust is a lot more, UNLESS you now claim that the nozzle geometry is altered radically between the rocket and the ramjet. This in turn would require another major design cost, and a weight cost for changing the geometry. <BR>My whole question is: "Why did a deadline-strapped, resource-strapped DRDO go to such complications instead of slapping 3 solid rocket stages from ISRO sounding rockets together, putting a payload and guidance system on, demonstrating it and sending it to mass-production?"<P>"Copied the Russian system" is the standard answer. But in this case, I can tell you that the Akash project was a whole lot more than reading Russian instruction manuals, or reverse-engineering Russian systems. <P>See the current designs of the NMD vehicles (not that they work ) All rockets. Those even have to travel much further out into the atmosphere than Akash does, still they don't try to use ramjets for any stage. Reason can't be cost: its the fact that you can't accelerate nearly as fast. If the SHAAFT was so great for acceleration, hey, don't you think it would be the ideal intermediate stage for the NMD kill vehicle? Does Lockheed or the US DOD have less resources and experience in high-speed flight than DRDO?<P>Its true that rockets have to carry their own oxidizer. But in a solid rocket, the oxidizer and the "fuel" are mixed together nicely, so that when the stuff melts and vaporizes, its all ready to react. Very fast reaction, efficient heat transfer etc. Much simpler than trying to get solid propellant to burn in a fast airstream. <P>I did not understand your statement about nitrogen compression aiding the jet, etc. Please explain.<P>2. The "Trimode" web page, with all due respect, is rather speculative, to put it VERY mildly. A pneumatic launch (air-gun) to send a projectile at full liftoff weight to Mach 2 or at least supersonic speed, so that the ramjet fires up. Very interesting, but I doubt whether the thermodynamics and gas dynamics hold up. The first minor problem is that you come out the end of the tube at Mach 1+, so that the whole vehicle, including its first-stage fuel tank, has to be designed to take the acceleration of that phase: 0 to 300m/s within, what, 10 feet? 30 feet? How long is the launch tube going to be, considering that it has to withstand all the pressure and point up at 45 degrees or so? OK, I'd like to see the fuel tank weight on that, storing enough propulsion for all but the Space portion of its propulsion! Its true that the modified Bofors artillery shell does have base-burning propulsion, which is in some ways similar to a supersonic ramjet, but I think that is a small part of it, meant mainly to reduce the drag at the base and add a little bit of range. Its not primary ramjet propulsion.<P>Further, the "Trimode" author describes this system as a viable and efficient idea for "launching space hotels" etc. Wow! I'd like to welcome him to my Alien Invasion Theory (AIT) Group. The unfortunate difference is that whatever I might write on BRF, I don't post web-pages claiming to be serious, and my name on it, with that kind of stuff. <P>On a previous post, I suggested that TSPAF will now develop a ramjet SAM which will start from rest in ramjet mode, to go "India+1. Sorry, I was kidding. To produce enough thrust from a ramjet to overcome gravity, you need to be going at least Mach 0.6, probably faster. <P>When you run your "rocket simulator", try using material from textbooks on gas dynamics and propulsion, please; not hype web pages. In summary, the SHAAFT web page conclusions are not relevant to the Akash problem; the Trimode web page is sheer nonsense dressed up with the usual "I am an engineer. If you use my Valuable info to design weapons I will report you (!!!) NASA uses these concepts."<P>The question still remains: Why did DRDO go to all the trouble of an integrated rocket-ramjet just to be able to field a medium-range SAM? Answer: nuclear-capable supersonic cruise missile.<P>Johann: Interesting rules of thumb. The more I read these rules from the nuke weapon effect prediction community (like that article on Mumbai after a 15kT nuke explosion) the less terrible nuclear weapons sound. Tank columns moving through with no worry after a tactical nuke burst at 6000 feet above, hey? So all I need to do is buy a Humvee look-alike and I can cruise around Atlanta with no more worry about nuclear incineration. <P>Does it make sense that a tactical nuke is no worse than a daisy-cutter bomb; in fact much less destructive if the soldiers are sitting in trenches?<P>Somehow these numbers do not make sense. But of course I am too lazy, unlike you guys, to actually get out a piece of paper and my 15-year-old TI-30 Solar calculator. <P>On the other hand, my Akash-as-ground attack weapon scenario just used 6000 feet as a starting point: I had no idea how to estimate optimal detonation altitude for such a weapon. Given your rules of thumb, I'd bring the detonation point down to 200 meters or even 100 meters, and use the spreading of the blast along the ground to knock the tanks out of the way. No big deal for a supersonic cruise missile, and does not change the arguments about why Akash appears to have been designed with ramjet cruise. <P>Johann: is the critical damage-threshold figure 4 psi or 4 psf? A supersonic airliner, designed to minimize shocks, still breaks windows if it travels at low altitudes. Is a nuclear blast not a whole lot worse? <P>I still doubt the 0.2kT max yield of the warhead, but will leave that to the warhead weight vs. yield experts. <BR><p>[This message has been edited by narayanan (edited 09-07-2000).]
Narayanan, I certainly would not understimate a tactical nuclear weapon. The blast, flash, EMP and radiation effects from even a single weapon will dreadfully tax the resources of an armoured Division. It takes an enormous investment in equipment and training to mitigate nuclear effects, and it makes fighting and sustaining operations under such conditions exponentially more difficult than any daisy cutter. <P> I've given you what the textbooks say. In the real world your casualties will be somewhat higher (esp. with infantry) because the wider war will always get in the way. You will find yourself forced to make hasty defences in an adequate time, or you will find yourself with inadequate Engineer support, people will have kit failures, or worst of all that may be no warning of an impending attack! The infantry scenario involves 2/3 rds of a battalion (the rest stand guard) + brigade or posibly higher Engr. assets working for two-three days. Defences are not merely holes in the ground, but embankments, overhead cover with soil over prefab sheets, command vehicles dug in, trenches shored up to handle blast, etc,etc. All of this while wearing NBC kit because you anticipated an attack in the first place! Though in the Indo-Pakistani context this might be dispensed with as chem. attacks are unlikely, and you don't have much of a chance if you're standing in the open and a nuke happens to come by. All of this tremendous amount of work will still leave you with ~ 20% casualties. This is before their tanks or Mech. Infantry come through and you actually have to do any fighting while your men are still trying to recover from the shock of having been nuked. They've managed to considerably erode your defensive advantage. To give you some perspective- 50% attrited units can not engage in unsupported offensive operations. 30% attrited units can not engage in any offensive operations, and units with less than 30% of their original strenght must be withdrawn or face complete distruction.<P> Tanks face less difficulty than infantry, but the same truths regarding attrition hold true. Losing a company means losing 25-30% of your Battalion strength right away. If you are engaged in offensive operations against enemy armour or infantry in prepared positions (this means designated kill zones, tank traps, etc) the defenders advantage has been strengthened considerably. Of course it may be that you have lots of Battalions, and your enemy has only a few nukes. <P> Tanks are amazingly sturdy things on the other hand; you might remember the chap in the US who pinched an old M-60 from an Army base a few years ago and tore though a mobile home several cars and almost demolished several buildings without a scratch on itself. Unfortunately for himself, said managed to get himself stuck on a highway divider in his drunken stupour. He failed to secure his hatch and the police rather predictably shot him when they yanked it open and he refused to surrender. So with radiation liners and an NBC overpressure and filtration system, your chances of survival in the region of a nuclear blast are probably the best outside any purpose built bunker.<P> However not all of your tank is made from solid steel. The important thing to remember is that a significant percentage of your remaining tanks have had their combat ability degraded by various levels. Their laser rangefinders or thermal sights may not work, impairing both gunnery and situational awareness, the latter which is already degraded because the tank commander can not poke his head out. Their mobility may be impaired, and almost certainly radios will malfunction. In such conditions it is only the highest level of training and team work of every officer and crewman already established that can cope with let alone overcome such handicaps. That and brutal determination on your Commanders part to win at any cost. <p>[This message has been edited by Johann (edited 09-07-2000).]
Thanks, Johann. <P>For comparison, a 4psf overpressure corresponds to the effect of being in and out of a 40mph wind gust in an extremely short time, though the effect is somewhat nastier. A 4psi overpressure..well, I don't want to be there: its 144 times as bad. <P>Its a curious discovery that the nuke blast effects are computed as overpressure scaled straight as radial dissipation (cube root dissipation), from a single source, and scaled straight from TNT blasts. I hope some of the nuke effects gurus get hooked into this discussion and tell us about it. <P>Here's my layman's dramatized view of a nuclear blast: <P>Somehow I thought the extreme temperatures would cause some dramatic effects not associated with other types of explosions. For example, simultaneous flash heating of huge volumes of air over great distances, and simultaneous ignition of a great many sources just due to the extreme temperature of the first blast. <P>The intense radiant heating is one of the unique features of a nuclear weapon compared to a chemical explosive. I don't know if the high-energy particles or gamma rays and X-rays actually set anything on fire. <P>Now, following the initial shock wave which may travel at some big multiple of the speed of sound, there will be a very very strong, very hot wind of expanding gases. This first blast would expand radially outward, too fast for buoyant effects to drive them upwards. The blast will probably encounter the shocked earth and spread out radially on the ground, with nasty effects on anything in the way. A 300mph (just guessing) tornado at 2000 deg. C? This would probably superheat every object around, for a few seconds, but it may be already oxygen-depleted. <P>Added bonus. During this time, the expanding "fireball" of extremely hot, ionized gases above would continue to radiate very intense heat downwards and sideways, until buoyancy slowly accelerates it upwards. <P>The temperatures of the fireball are far, far worse than those of a smoky TNT fireball. This means that there will be lots of radiation at all kinds of wavelengths. Microwaves, ultra-violet all included. Some of those, I expect, will indeed go through metal and everything else. <P>Here's where I think the camouflage-painted tanks will not only ablate but also absorb a lot of heating, with interesting implications for the fuel tanks, crew etc. This will go on for quite a few seconds, maybe even minutes. <P>Then all this air would get sucked up into a rising vortex ring (the mushroom), and be displaced by a downward blast of colder, oxygen-rich air which of course would light up all the fires. <P>Objects in the general area would thus see a couple of super-hurricanes or tornadoes, with extreme changes in temperature. From opposite directions. <P>So the initial radiant heating which simultaneously heats up a huge volume, and the extreme temperatures which cause strange atmospheric phenomena, are the two effects which may not scale according to the TNT rules. The TNT blast also produces hot air flows etc., but the scale is much lower because the peak temperatures are so much lower. I'm not sure if the nuke transfers energy more efficiently to air and thus gets its kind assistance in doing damage. Since there is a lot of short-wavelength radiation, and this stuff gets absorbed by air, its quite likely. <P>Somewhere in my lurid past I've done some technical translation of interesting discussions from Russian authorities on how to optimize the destructive effects of chemical detonations, depending on whether you wanted to kill people or buildings <P>But of course we know that every nuke test since the first one has had spectators at some distance or other, so I probably have a very sensationalized idea of what its really like. <P><p>[This message has been edited by narayanan (edited 09-07-2000).]
Sorry yaar I am busy at SEMICON-West(@SFO)for these 3 days.<P>Can someone help me get data on weight of Akash's Booster and the final booster velocity ? That would be one critical information that I need to hunt down for the simulation.<BR> <BR>Cheers -Arun<p>[This message has been edited by Arun_S (edited 10-07-2000).]
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR><BR>PS: Narayanan did you do any work Study in Sivakasi on Solid Propulsion Rockets!!! <BR><HR></BLOCKQUOTE><P>LOL!!. Spinster. You are a riot!!. I couldn't stop laughing for a few minutes.<P>No offence Narayanan .<P>Kya andaaz hai!!<P>Rahul<P>[This message has been edited by rahulm (edited 12-07-2000).]<p>[This message has been edited by rahulm (edited 12-07-2000).]
Sorry. Classified. On the other hand, Spinster makes a very important point. Solid propellant IS very cheap compared to other kinds of propellant, and even if the rocket becomes a lot bigger and bulkier than an airbreathing system, its still going to be cheaper to mass-produce (guesstimate pending Arun's detailed computations). <P>The technology for sending multi-stage rockets with multiple warheads is indeed perfected by the people at Sivakasi etc., and they manage to get 95% of these working quite well without any feedback control or telemetry, even when the missile installation and launching is done by non-expert personnel. Such as at festivals all over India. <p>[This message has been edited by narayanan (edited 12-07-2000).]
<BR> Akash was first publicly tested in 1990.The recent launch(last week) was the 14th or 15th(more?).Yet,it is still not inducted.So it does look like Akash represents many development projects rolled into one as Narayanan says.Rocket structure,both design and casing,fuel type and efficiency,stage separation and control plus guidance and homing are getting validated.Ten years is a long time to test a surface to air missile,but that's not the only likely goal.A future supersonic cruise missile may be in the works.<BR> Akash is 6.5 m in length,weighing 650 kg.It does not use HTPB as fuel for the solid motor; the fuel,I believe,is nitrogen cellulose with ammonium perchlorate as oxidiser(?)
Arun, excellent, of course. <P>"Does it leave room for doubt" No, because: <P> <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Energy imparted to the payload when all fuel is expended.<HR></BLOCKQUOTE> <P>is the metric that you used. <P>The net energy added to the payload is the total work done on it, minus the work expended to overcome drag, etc. which you can't recover.<P>Work done = force x distance that you moved the point of application of the force. <P>(Spinster: Isn't that what Eesaac Nootan taught us at Sivakasi?)<P>So you just proved that the rocket-ramjet Akash is an outstanding way to deliver a payload over a very long distance. Supersonic cruise missile, traveling at not-so-high alitude so that you have plenty of work to spare for the air drag at lower altitudes. <P>How does this translate into: "fastest acceleration", or "deliver payload to the greatest altitude and speed ASAP so you can coast from there", both of which are characteristics of SAMs which you specified?<P>And this is without figuring the huge cost and technology risks of having the machined inlets etc. (and the added drag due to the different geometry) of the ramjet. My question, to repeat, was: "why did the designers go to all this trouble just to get a SAM"? <P>A better comparison for this issue might be to see why a rocket-based SAM could not do the same mission; i.e., reach 15km altitude fast with the same payload, still traveling at some respectable speed. Aren't there several examples of SAMS elsewhere? For a country that did not have other indigenously-designed medium-range SAMs ready , and facing such neighbors, why did the Akash make sense as the best way to spend development resources? You just proved that a pure SAM mission is not the answer, beyond all doubt. <P>Thanks, Arun! <P><p>[This message has been edited by narayanan (edited 14-07-2000).]
I am sure we understand both perspectives .<P>Just a minor comment: Solid fueld SAM will certainly give fastest accleration but not velocity (as shown). At the end of the day, fast acceleration without fast end velocity is not very useful attribute of a SAM. Much like a Diesel Engine (has limited RPM range) with 2 gears (very fast initial acceleration but not very fast top speed). Vice-versa is also true. Conversly there is lot to choose from, and Buddha's Golden Middle Path seems to be the sweet spot.<P>As for the alluded strategic objective, very possible. Eventually sub-sonic cruise missile is the first step to a supersonic missile.<P>Cheers -Arun<p>[This message has been edited by Arun_S (edited 14-07-2000).]
) All rockets. Those even have to travel much further out into the atmosphere than Akash does, still they don't try to use ramjets for any stage. Reason can't be cost: its the fact that you can't accelerate nearly as fast. If the SHAAFT was so great for acceleration, hey, don't you think it would be the ideal intermediate stage for the NMD kill vehicle? Does Lockheed or the US DOD have less resources and experience in high-speed flight than DRDO?<P>Its true that rockets have to carry their own oxidizer. But in a solid rocket, the oxidizer and the "fuel" are mixed together nicely, so that when the stuff melts and vaporizes, its all ready to react. Very fast reaction, efficient heat transfer etc. Much simpler than trying to get solid propellant to burn in a fast airstream. <P>I did not understand your statement about nitrogen compression aiding the jet, etc. Please explain.<P>2. The "Trimode" web page, with all due respect, is rather speculative, to put it VERY mildly. A pneumatic launch (air-gun) to send a projectile at full liftoff weight to Mach 2 or at least supersonic speed, so that the ramjet fires up. Very interesting, but I doubt whether the thermodynamics and gas dynamics hold up. The first minor problem is that you come out the end of the tube at Mach 1+, so that the whole vehicle, including its first-stage fuel tank, has to be designed to take the acceleration of that phase: 0 to 300m/s within, what, 10 feet? 30 feet? How long is the launch tube going to be, considering that it has to withstand all the pressure and point up at 45 degrees or so? OK, I'd like to see the fuel tank weight on that, storing enough propulsion for all but the Space portion of its propulsion! Its true that the modified Bofors artillery shell does have base-burning propulsion, which is in some ways similar to a supersonic ramjet, but I think that is a small part of it, meant mainly to reduce the drag at the base and add a little bit of range. Its not primary ramjet propulsion.<P>Further, the "Trimode" author describes this system as a viable and efficient idea for "launching space hotels" etc. Wow! I'd like to welcome him to my Alien Invasion Theory (AIT) Group. The unfortunate difference is that whatever I might write on BRF, I don't post web-pages claiming to be serious, and my name on it, with that kind of stuff. <P>On a previous post, I suggested that TSPAF will now develop a ramjet SAM which will start from rest in ramjet mode, to go "India+1. Sorry, I was kidding. To produce enough thrust from a ramjet to overcome gravity, you need to be going at least Mach 0.6, probably faster. <P>When you run your "rocket simulator", try using material from textbooks on gas dynamics and propulsion, please; not hype web pages. In summary, the SHAAFT web page conclusions are not relevant to the Akash problem; the Trimode web page is sheer nonsense dressed up with the usual "I am an engineer. If you use my Valuable info to design weapons I will report you (!!!) NASA uses these concepts."<P>The question still remains: Why did DRDO go to all the trouble of an integrated rocket-ramjet just to be able to field a medium-range SAM? Answer: nuclear-capable supersonic cruise missile.<P>Johann: Interesting rules of thumb. The more I read these rules from the nuke weapon effect prediction community (like that article on Mumbai after a 15kT nuke explosion) the less terrible nuclear weapons sound. Tank columns moving through with no worry after a tactical nuke burst at 6000 feet above, hey? So all I need to do is buy a Humvee look-alike and I can cruise around Atlanta with no more worry about nuclear incineration. <P>Does it make sense that a tactical nuke is no worse than a daisy-cutter bomb; in fact much less destructive if the soldiers are sitting in trenches?<P>Somehow these numbers do not make sense. But of course I am too lazy, unlike you guys, to actually get out a piece of paper and my 15-year-old TI-30 Solar calculator. 
.<P>Kya andaaz hai!!<P>Rahul<P>[This message has been edited by rahulm (edited 12-07-2000).]<p>[This message has been edited by rahulm (edited 12-07-2000).]