This is quite a complex solution
but as stated, it gets there and we have a craft operating in scramjet mode at
mach 7, having taken off from a runway.
Once in scramjet mode, development to even greater velocities could be
straight forward because we have a configuration that is designed for hyper
velocity up to space entry.
As important, we can envisage a
heavy lifter able to loft a space ready craft up to high elevations and an
initial mach 7 velocity. Such a craft
should be able to achieve low orbit with moderate additional thrust.
However, I think that we will
have magnetic field exclusion vessel in our toolkit long before we build
something that difficult and expensive to operate. The technology is just accelerating far too
fast right now for this option not to be deployed far sooner. It is a little like delivering the new
fighter that everyone is so proud of as the last wrinkles is scrubbed out of
the software for the next generation of fighter drones. The best strategy is to announce support and
stall.
Aerojet Unveils Novel Hypersonics Plan
Jun 10, 2011
By Guy Norris, Graham Warwick
Aerojet is proposing development of a novel combined-cycle propulsion
system for reusable hypersonic vehicles which packages current technology to
achieve a seamless transition from a standing start to Mach 7 plus.
The concept tackles key problems that developers face in trying to
accelerate aircraft to high enough speed for a scramjet to begin operating.
Although rocket boosters have been used to accelerate experimental
scramjet-powered vehicles like the X-51A to the take-over point, this approach
is not suitable for reusable platforms that would operate from a runway.
Major hurdles in the path to successful aircraft-like operation
include producing sufficient thrust to punch through the high drag encountered
at transonic speeds around Mach 1. Even if this can be overcome, designers also
face a “thrust gap” between around Mach 2.5, where current turbine engine power
falls off, and Mach 3.5-4, where the transition to a dual-mode ramjet/scramjet
takes place. To date, all attempts to develop a viable high-speed turbine
engine to bridge this gap have failed.
Aerojet’s TriJet concept builds on the advantages of two traditional
air-breathing propulsion systems extensively studied for this role—the turbine-
and rocket-based combined cycles (T/RBCC). However, in isolation, both have
disadvantages. TBCCs require turbine engines that are often heavy and large,
taking up space for fuel, and have poor transonic acceleration, while RBCCs
have significantly lower fuel efficiency than turbine engine-powered concepts. The
TriJet combines the two options into one by melding a turbine engine and
rocket-augmented ejector ramjet (ERJ) with a dual-mode ramjet (DMRJ) to achieve
the final push to hypersonic flight.
In a basic sense, the vehicle will take off using turbine engine power,
which can be partially augmented by rocket-assisted thrust from the ejector and
dual-mode ramjet. At higher speed, the turbine engine is shut down and closed
off, with the ejector ramjet taking over. The exhaust from the ERJ is ducted
through the nozzle of the DMRJ to form an aerodynamic choke. This enables the
DMRJ to produce more thrust and accelerate the vehicle to greater speeds until
the ERJ can be closed off, and the dual-mode ramjet converted into a scramjet.
The concept is attracting U.S.
As there are no ground-test facilities large enough to run a Mach 6-7
scramjet sized for an ISR platform, the demonstrator could also become its own
flying testbed, says Adam Siebenhaar, Aerojet’s director of hypersonic
propulsion. The vehicle could be flown first with turbojets and ejector ramjets
to Mach 3, before the dual-mode ramjets are installed and the flight envelope
expanded to Mach 6 or beyond, he proposes.
Describing a notional twin-TriJet-powered vehicle more than 100 ft.
long, Bulman says each TriJet would contain a single 40,000-lb.-thrust-class
turbojet, ejector ramjet and dual-mode ramjet. In conjunction with the
turbojets, the combined-cycle system would generate a total of 160,000 lb.
thrust on takeoff. “We’re saying this is practical now. Why wait? We’re at the
point where we can really do something that has a low risk,” says Bulman.
The innovative inlet minimizes spillage of air and enables a degree
of combined thrust to be used throughout mode transitions when, in other
designs, one engine shuts down while another takes over. “Why have two
horses on your cart and only have one working at a time? Having two semi-weak
horses is better than a racehorse with a mule on its back,” Bulman says. The
inlet design has been shown to be “very forgiving,” in tests from Mach 0 to 4.6
and showed the “turbine engine helps start the inlet,” says Siebenhaar.
The vehicle takes off using either just the turbine engines, or the
whole TriJet ensemble, depending on weight. “You may get thrust from the ERJ
and even the ramjet/scramjet,” says Bulman. The ERJ is fitted with small
rockets in the front of the duct and fuel injectors at the rear. On firing the
rockets, likely using hydrogen peroxide as an oxidizer, air is drawn through
the ejector and combined with the rocket plume to generate additional thrust.
Rockets are also embedded in the DMRJ, which “becomes an ejector ramjet at low
speed,” he adds.
Once airborne, the vehicle receives fuel from a tanker. After
refueling, the vehicle’s turbojets and ERJs are throttled up to maximum thrust
to accelerate through the transonic drag region. Passing through Mach 1.5, with
drag coming down, the inlet ramp doors to the turbine engines are partially
closed as the thrust from the ERJs builds up. By Mach 2 the rockets are turned
off, and then at Mach 2.5 the turbine inlets are fully closed and the engines
purged and cocooned, ready for restarting for the return leg.
For the challenging push from Mach 2.5 to Mach 4, Aerojet’s concept now
reconfigures itself to get maximum performance from the DMRJ, which has no
doors in the flowpath. The ramp doors of the ERJ partially close and the
exhaust is directed into the DMRJ nozzle. The flow convergence in the nozzle
forms a sustained aerodynamic choke, causing the flow through the engine to be
wholly subsonic.
With the flow slowed, additional fuel can now be burned in the first
part of the scramjet nozzle, which becomes a ramjet combustor—or a “righteous
ramjet” as Bulman describes it. The fuel is injected by Aerojet’s core burning
device, which was developed to improve combustion in inward-turning, or round
flowpaths. In this device, three cantilevered fuel-injection struts are tied
together at the center, allowing the pilot flame and the center of the
combustion process to be kept away from the walls. The extended combustion flow
area makes thermal balancing easier, and generates more thrust.
“Core burning turns the engine inside out. Flame propagates from the
center of the engine, and only touches the wall at the back, which keeps the
engine cool on the outside. It reduces the heat load by 40-50%,” says Siebenhaar.
The scramjet operates like two ramjets in this mode up to Mach 4.5, at
which point the flow to the ejector ramjet is shut off and the vehicle relies
solely on the scramjet, which is optimized for supersonic combustion to Mach 7
or 8
.
Following completion of the mission, the vehicle decelerates and the
door to the ERJ is opened to provide sustained thrust down to around Mach 4. As
the vehicle decelerates below Mach 3, the inlets to the turbine engines are
opened and at a lower Mach number these are re-started for the final powered
rendezvous with a tanker and/or recovery to a runway.
Although Bulman acknowledges that the development of an ISR or strike
platform remains a longer term option, he says the TriJet concept and
specifically technologies such as the core burner can be applied to missiles.
“We’re working on putting it into our plan with joint Air Force
funding, but core burning will be a key part of future missile work,” he says.
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