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Two methods can be used to achieve the Vulcan hypersonic engine

DARPA held an industry day where it outlined some of its plans for hypersonic aircraft of the future. The program being shown off at the industry day was the DARPA Vulcan project. The project centers around developing an aircraft that uses a constant volume combustion (CVC) engine capable of flight at speeds from a standstill to Mach 4 and over.

Aviation Week’s Ares blog reports that the first part of the program was an introduction to the problem the program faces -- how exactly to accelerate an aircraft from a stop to speeds fast enough to activate a supersonic-combustion ramjet.

The program has some interesting slides and information (PDF) from the famed Lockheed Skunk Works HTV-3X flight demonstration vehicle that was conceived as part of the DARPA Falcon program. One of the slides gives an idea of the size of the HTV-3X vehicle by comparing it to the Have Blue aircraft that ended up being about 60% of the F-117 stealth fighter.

The Lockheed HTV-3X vehicle itself has been superseded by the DARPA Blackswift hypersonic program DailyTech has covered before. The engine that DARPA envisions for the Vulcan project is a CVC and turbojet combination.

According to Ares two methods can be used to achieve this type of engine. In one method a common air inlet would be used for both the turbojet engine that is to carry the aircraft from a stop to Mach 4 and higher speeds and the CVC that would take over at propel the aircraft to Mach 6 and over. This method is called turbine-based combined cycle.

The second method to achieve the engine needed is called an annular approach and would embed a turbojet inside a CVC ramjet engine. The big challenge here is that the turbojet would have to be cocooned when the CVC is active to protect it from the high heat produced inside the Vulcan engine over Mach 2.

Because a turbojet capable of propelling a aircraft over Mach 4 would be large and expensive to develop, DARPA instead wants to take a conventional Mach 2 turbojet and combine it with a CVC to get an engine capable of high Mach speeds, but at much cheaper development costs.

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Get your facts straight
By Mclendo06 on 6/26/2008 3:37:36 PM , Rating: 3
The second method to achieve the engine needed is called an annular approach and would embed a turbojet inside a CVC ramjet engine.

Little problem here:

A there is no such thing as a CVC ramjet. By definition a ramjet works on the Brayton cycle, which is constant pressure combustion, not constant volume combustion. A CVC engine would work on the Humphrey cycle, but these are generally (by necessity, to retain the constant volume during combustion) pulse combustion engines which are still in development AFAIK. Here's how it works:

In a ramjet, the pressure of the air remains about the same as it is combusted, but its volume increases which is how you get power out of the cycle.

In a CVC engine, the volume in the combustion chamber is held constant (by sealing it off somehow, which is why there is a "pulse" as the chamber is sealed and unsealed) and the pressure increases as the air is "detonated" instead of just burned. This results in a larger amount of energy being obtained from the cycle which is why DARPA is interested in this sort of an engine.

Aurora Bomber, here we come...

RE: Get your facts straight
By Master Kenobi on 6/26/2008 6:05:43 PM , Rating: 3
Aurora in flight!

RE: Get your facts straight
By phaxmohdem on 6/26/2008 8:39:20 PM , Rating: 2
Best not send that towards my Strategy Center, or I'll send my Burton over to knife ur d00ds :)

RE: Get your facts straight
By Cullinaire on 6/27/2008 1:32:04 AM , Rating: 2
Not if the pathfinders have anything to say about that.

RE: Get your facts straight
By ikkeman2 on 6/27/2008 2:52:16 AM , Rating: 2
V1 (WWII) was powered by PD.

By introducing a shut-off plate in the front section of your (SC)ram-jet, you can use it as a CVC. A ram-jet is nothing more than a shaped tube with a burner in it. As you approach the speed that the RAM starts to work, you can just keep the plate in the open position.

Combined cycle here we come

RE: Get your facts straight
By Mclendo06 on 6/27/2008 6:55:49 PM , Rating: 2
True, it is possible to create an engine that is capable of behaving two different ways (the J58s on the SR-71 come to mind). I was just pointing out that the way the article is worded, it comes across as a ramjet that utilizes constant volume combustion, which is not currently conceivable. Once you start CVC, you aren't operating a ramjet anymore. You can have one or the other, but not both at the same time.

There are some interesting Wiki articles about Pulse Jet Engines, Ramjets, and the V1 (which used a PD engine, not a ramjet). Pretty straight forward and worth a read, and accurate as far as my aerospace engineering BS degree can tell.

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