Rocket vs Low Subsonic Ramjet

[Waleed A.]

I have a question . Speaking from point of view of fuel efficiency . Is it better to have a rocket system flying at 0.8 Mach or a ramjet started with rocket .

 

I am doing my final year Project on Ramjet and now having problem in selecting which to prefer either a rocket or a rocket+ Ramjet.

 Thanks

The fuel is JP-4

[SinBad]

Had a quick look at wikipedia. The use you describe (efficiency at low altitude subsonic flight = run as long as you can as little fuel as possible) sounds like looking at cruise missiles for inspiration. Most of the subsonic ones use turbofan or turbojet engines and get a couple of thousand miles range out of not a lot of fuel. The old german V1 cruise missiles used a type of valved pulse jet that got reasonable subsonic efficiency.

As far as i know, ramjets only get efficient at supersonic speeds. Like mach 2+, in subsonic flight (even ones designed for it) they are often less efficient than rockets.

[MarvinKitFox]

You want efficiency, in sustained 0.8 mach flight?

rocket < ramjet < propeller < turboprop(ducted) < turbojet

[Bill Phil]

Ramjets don't give you much thrust until somewhere around Mach 0.5. And even then there thrust is still pretty bad until you get supersonic.

[KerikBalm]

Pretty much anything that uses the air as working mass is going to be more efficient than something that doesn't, at least for those speeds.

If we were talking mach10, then things might change... might....

So ramjet > rocket

[wumpus]

3 hours ago, MarvinKitFox said:

You want efficiency, in sustained 0.8 mach flight?

rocket < ramjet < propeller < turboprop(ducted) < turbojet

And to a large extent, the prices (for single use anyway) go the other way (except for propellers).

[magnemoe]

10 hours ago, SinBad said:

Had a quick look at wikipedia. The use you describe (efficiency at low altitude subsonic flight = run as long as you can as little fuel as possible) sounds like looking at cruise missiles for inspiration. Most of the subsonic ones use turbofan or turbojet engines and get a couple of thousand miles range out of not a lot of fuel. The old german V1 cruise missiles used a type of valved pulse jet that got reasonable subsonic efficiency.

As far as i know, ramjets only get efficient at supersonic speeds. Like mach 2+, in subsonic flight (even ones designed for it) they are often less efficient than rockets.

Yes, the V1 used shutters in front to close the chamber during pulses, this let them run at 6-700 km/h, as I understand the WW2 fighter planes had to patrol high and dive towards them to intercept effectively. Not an fuel efficient engine compared to an jet engine but cheap to make, an piston engine and propeller would make it so slow it would be easy to intercept at least unless they used an huge and expensive fighter plane engine in it. 

Russia has many ramjet missiles they all uses an booster rocket to get up in speed. You will need it anyway to get airborne and making it an bit larger is no huge issue. 

[SinBad]

6 hours ago, magnemoe said:

Russia has many ramjet missiles they all uses an booster rocket to get up in speed. You will need it anyway to get airborne and making it an bit larger is no huge issue. 

But do they have any subsonic missiles with ramjets?

[magnemoe]

4 minutes ago, SinBad said:

But do they have any subsonic missiles with ramjets?

No they are all supersonic, back in the 60 they had surface to air missiles with ramjets, it was dropped as they had to work in various attitudes from treetops to stratosphere, however for seaskimers and cruise missiles it would work well. Far less fuel efficient than turbojet but twice as fast and much cheaper engine. 
Think the only subsonic one used a lot was V1, and only because an lack of experience with supersonic flight and to keep cost down. 
 

[wumpus]

23 hours ago, magnemoe said:

Yes, the V1 used shutters in front to close the chamber during pulses, this let them run at 6-700 km/h, as I understand the WW2 fighter planes had to patrol high and dive towards them to intercept effectively. Not an fuel efficient engine compared to an jet engine but cheap to make, an piston engine and propeller would make it so slow it would be easy to intercept at least unless they used an huge and expensive fighter plane engine in it. 

Russia has many ramjet missiles they all uses an booster rocket to get up in speed. You will need it anyway to get airborne and making it an bit larger is no huge issue. 

I would assume that a few of them have a version without the boosters that are strapped onto jets and simply released at > mach .8.  I know the US (Navy?) had an air-air ramjet missile that downed another aircraft during the Viet Nam war, I'd be surprised if they bothered with a booster for that (although what do I know, in all that turning do you really expect the pilot to care if his relative air velocity is >mach .8?  All he cares about is his velocity relative to his enemy).

Subsonic ramjets sound pretty silly.  First, efficiency increases with speed in ramjets (although I'm not sure if it increases fast enough to overcome the drag).  Second they are so inefficient you typically want to replace them with normal aircraft engines, so they are exclusively used on [obviously single use] missiles (I think Kelly Johnson once designed one into a target practice drone, obviously also single use).  Running subsonic seems like a means to make it easier for enemy anti-aircraft (perhaps you are using many cheap bombs with a goal of using up anti-aircraft missiles).  I suppose if your pilots/anti-aircraft need to practice against subsonic aircraft, you might make a targeting drone run that speed (or more likely pull a glider at high altitude and let it maintain mach .8 by controlling the descent).

[magnemoe]

4 hours ago, wumpus said:

I would assume that a few of them have a version without the boosters that are strapped onto jets and simply released at > mach .8.  I know the US (Navy?) had an air-air ramjet missile that downed another aircraft during the Viet Nam war, I'd be surprised if they bothered with a booster for that (although what do I know, in all that turning do you really expect the pilot to care if his relative air velocity is >mach .8?  All he cares about is his velocity relative to his enemy).

Subsonic ramjets sound pretty silly.  First, efficiency increases with speed in ramjets (although I'm not sure if it increases fast enough to overcome the drag).  Second they are so inefficient you typically want to replace them with normal aircraft engines, so they are exclusively used on [obviously single use] missiles (I think Kelly Johnson once designed one into a target practice drone, obviously also single use).  Running subsonic seems like a means to make it easier for enemy anti-aircraft (perhaps you are using many cheap bombs with a goal of using up anti-aircraft missiles).  I suppose if your pilots/anti-aircraft need to practice against subsonic aircraft, you might make a targeting drone run that speed (or more likely pull a glider at high altitude and let it maintain mach .8 by controlling the descent).

Yes, forgot about air launched ones, they don't need the booster. 

[Nicus]

Probably too late to make a difference, but... Back in the late 40s the French flew a subsonic ramjet powered research aircraft called the Leduc 0.10.   It was air launched from the back of a propeller driven airliner at about 200 mph.  At half power the aircraft accelerated to about 420 mph.  Later flights at full power expanded the speed envelope to M=0.85. 

The tip-jet powered "Hiller Hornet" helicopter developed by Stanley Helicopter during the 50s usually lit its little ramjet engines when they were spinning at about 150 fps.  Tip speed in flight was generally about 600 fps.  At that speed the ramjets had an Isp of about 400 sec.  The same engines were tested at higher velocities on ground based rotor test stands.  At 900 fps (M=0.8) they had an Isp of 1,500 sec.