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Subject: Recent American developments in VFDR missile propuslion - 13.5, 10, 7 diameter motors
dwightlooi    6/10/2008 11:23:25 PM
VFDR is the acronym for Variable Flow Ducted Rocket. It is different from the Ramjet not only because the fuel is solid but also because the sustainer fuel is lit and burned using an oxidizer blended into the solid propellant. This mixture burns very rich and completes the combustion of the fuel material using air from an inlet duct system. This is unlike a Ramjet which burns its fuel using solely air from the atmosphere. The variable flow denotion indicates that the fuel burn rate, or intake air flow, or both may be throttled via some metering mechanism. The advantages of the VFDR is that the fuel itself is ignitable and is solid making the weapon more reliable and simpler. The disadvantage is that VFDRs have an IpSec* in the 600 sec bracket compared to a pure Ramjet which is around 1200 sec. For comparison a solid rocket is around 250 sec and a turbojet is about 3000 sec. A VFDR missile tends to have 2 to 3x the sustainer performance of a solid rocket sustainer.  However, because a VFDR incurs additional weight and drag from the intake system, valving interstage, and has to live with a relatively inefficient nozzleless integrated booster, overall performance tends to be only about 1.5~2x better. This nonetheless is still a very tangible improvement. The Meteor, for instance, is a VFDR missile.

http://img136.imageshack.us/img136/6100/ipsecvc1.jpg">

In the early 2000s, the USAF/USN awarded various contracts to Aerojet for the development of practical VFDR solutions applicable to tactical missiles as a safeguard against the transformation technology thrust such as a hydrocarbon Scramjet and Dual Combustion Ramjet not panning out in time for next generation US weapons. This resulted in three VFDR solutions. A 13.5" (MARC-282) has since been put into production for use on the GQM-163 Coyote Mach 2.5 sea skimming target for the USN. The 10" VFDR (MARC-290) was recently test fired with HARM front end. Finally, a 7"Aerojet booster has now surfaced with an AMRAAM front end.

http://img512.imageshack.us/img512/9981/2007072013325417ed1yd7.jpg">
GQM-163A Sea Skimming Supersonic Target (SSST) with MARC-282 13.5" VFDR sustainer motor.

http://img362.imageshack.us/img362/7205/080109f0725k009bb3et5.jpg">
http://img512.imageshack.us/img512/6115/080109f0725k017nk8fh0.jpg">
MARC-290 10" VFDR integrated with AARGM (AGM-88E) front end for test firing.

http://img388.imageshack.us/img388/1243/jrakzrakraketahn3.jpg">
7" VFDR (unknown designation) with AIM-120 front end.

* 1 IpSec = 1 lb of thrust for 1 lb of carried expendables (fuel and/or oxidizer) burned. And IpSec of 250 seconds means that 1 lbs of propellant gets you 250 lbs of thrust for 1 second,1 lbs of thrust for 250 secs, or somewhere in between.
 
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VelocityVector       6/11/2008 12:39:48 AM

That now we can place a large BFW on the lap of an enemy pilot with confidence at distance boggles my mind.  Notwithstanding the simplicity of my mind, we know how to deliver in circumstances even when FedEx cannot.  Beware, enemy.  Reliable delivery, overnight and in any weather, positively on-time and coming to your neighborhood soon enough ;>)

v^2

 
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Herald12345    Note the NAME.   6/11/2008 1:06:51 AM
http://img388.imageshack.us/img388/1243/jrakzrakraketahn3.jpg">

RAYTHEON

http://www.fitness-bellach.ch/attachments/Image/LOGOS/Smiley_Face.gif">

Herald



 
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doggtag    something else Herald and I agree on...   6/11/2008 7:20:17 AM
...Raytheon ain't all that bad (esp. when compared to others).
 
ITAS RULES! (heh heh)
 
No, but seriously,
I was under the impression that VFDRs drew in their oxidizer from the atmosphere, hence why Meteor has those intakes.
But the opening article here suggests otherwise.
 
Solid propellant rocket motors already have an oxizider mixed into the propellant grain,
but I apparently then was completely mis-understanding when I read elsewhere that VFDRs used a richer, minimal-oxidizer propellant grain and drew in air, via the variable flow ducts (not wholly unlike how jet fighters have variable geometry intakes), and mixed atmospheric oxygen with the fuming propellant in a combustion chamber area (also called a ramrocket, whereas ramjets burn liquid, or even gel, fuels).
 
Are there different kinds of VFDRs then?
Or am I totally offbase confusing ramrockets with VFDRs?
 
 
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dwightlooi       6/13/2008 9:10:05 AM

...Raytheon ain't all that bad (esp. when compared to others).

 

ITAS RULES! (heh heh)

 

No, but seriously,

I was under the impression that VFDRs drew in their oxidizer from the atmosphere, hence why Meteor has those intakes.

But the opening article here suggests otherwise.

 

Solid propellant rocket motors already have an oxizider mixed into the propellant grain,

but I apparently then was completely mis-understanding when I read elsewhere that VFDRs used a richer, minimal-oxidizer propellant grain and drew in air, via the variable flow ducts (not wholly unlike how jet fighters have variable geometry intakes), and mixed atmospheric oxygen with the fuming propellant in a combustion chamber area (also called a ramrocket, whereas ramjets burn liquid, or even gel, fuels).

 

Are there different kinds of VFDRs then?

Or am I totally offbase confusing ramrockets with VFDRs?

 

VFDRs have oxidizers mixed into the propellant grain. They just have less of the solid propellant mixture being oxidizers and more of it being fuel -- enough to support combustion without air but not enough to fully combust the fuel. The fuel rich exhaust of its solid propellant burn is then mixed with intake air to burn completely.

A typical modern solid propellant mixture for pure rockets is approximately 20% fuel (usually powdered metal such as Aluminum, Magnesium, Boron or a mixture thereof), 65~70% oxidizer (usuallly Ammonium Perchlorate) and about 10~15% binder (usually PBAN, HTPB or HTPE).

A typical VFDR propellant mixture is approximately 50% fuel (usually a heavier metal like Boron), approximately 35% oxidizer and 15% binder. The oxidizer ratio is not sufficient to burn the fuel on its own and the remaining oxygen has to come from air ingested via the intake ducts. However, the oxidizer loading has to be high enough such that the propellant burns in the gas generator can prior to being throttled pass a valving assembly and mixed with the intake air, otherwise you have a dead missile that won't go anywhere.

A true Ramjet only carries fuel and no oxidizer. The fuel -- usually a hydrocarbon liquid like kerosene -- is turbopump or pressure fed to the combustion chamber and burned using solely air from the intakes.

The advantage of the VFDR is that for a given amount of internal volume (and weight) dedicated to the carriage of sustainer fuel, the VFDR carries twice to 2.5x as much fuel as the equivalent volume filled with a pure rocket sustainer. As such, if a volume is sufficient to provide the desired thrust level for 20 seconds as a pure rocket it'l provide 40~50 seconds of similar thrust as a VFDR. That's the theoretical upper bound for VFDR performance gain over a traditional rocket. Real world gains tend to be lower for a multitude of reasons. The top few being as follows:-

(1) A VFDR incurs Ram drag from its intakes, whereas a pure rocket does not. Ram drag is the drag induced by the intake system ingesting air and slowing it down to subsonic speeds before introducing it to the combustion chamber. All VFDRs and Ramjets do this. An engine that does not is technically a SCRamjet which is a really tricky beast. Net thrust of a pure rocket is simply Mass x Acceleration of its exhaust products. Net thrust of a VFDR is M x A - Ram Drag.

(2) A VFDR wastes internal volume on devices such as the valving intterstage between the sustainer and the booster/combustor.

(3) A VFDR is necessarily heavier due to the added structure of the intakes and the valving interstage.

(4) A VFDR tends to have a lower specific impulse booster because the booster has to be integrated into the VFDR combustor where air and the fuel rich sustainer exhaust is burned. For optimal function, a VFDR combustor needs to have very low back pressure which precludes the use of a proper nozzle. The absence of a proper nozzle reduces the specific impulse of the booster stage due to sub-optimal expansion ratios. Most VFDRs (eg Meteor) has to resort to a nozzleless integrated booster and this costs about 20~30 IpSecs on the booster grain.

Because of the above practical shortcomings, the 2~2.5x theoretical improvement in sustainer performance is typically reduced to about 1.5~2x. This is the root of the age old debate as to whether VFDRs are worth the effort because the performance of a VFDR missile can be matched if one simply doubles the sustainer volume while keeping the booster volume, missile size, weight the same. For example, if you take a typical missile like the AMRAAM with an ~47% motor mass fraction and increase that to about 60%, you will
 
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