Scientists at the U.S. Naval Research Laboratory are
developing the next generation of solid-fuel ramjet propulsion,
addressing one of the field's most persistent challenges: understanding
and predicting what happens inside an operating combustor.
NRL scientists have figured out how to "see inside" one of the most
extreme engines ever built, turning guesswork into knowledge and making
future long-range, high-speed flight more achievable than ever before.
A solid-fuel ramjet is an air-breathing engine that uses solid fuel
rather than liquid, offering high energy density and mechanically simple
propulsion by burning the fuel with oxygen from the air to produce
thrust. By drawing oxygen from the atmosphere rather than carrying an
oxidizer on board, solid-fuel ramjets can carry more fuel in the same
volume and fly farther than traditional rocket systems.
"If you replace all the oxidizer and instead use oxygen from the air
to burn your fuel, you can increase range by up to 200 to 300% in the
same form factor," said Brian Bojko, a combustion scientist at NRL.
Despite that promise, widespread adoption has been slowed by the
extreme internal environment of solid-fuel ramjets, where high
temperatures, soot and rapidly evolving flow structures prevent
traditional probes from accessing critical data. Unlike liquid or
gaseous fuels, solid fuels release energy through surface regression and
often produce a complex mixture of combustion products, making it far
more difficult to control burning rates and predict performance. This is
why understanding and predicting what happens inside an operating
combustor is so important.
"In solid-fuel ramjets, you don't have direct control over the mass
flow rate like you do with liquid systems," Bojko explained. "The heat
from combustion actually drives the gasification of the solid fuel, so
pressure, temperature and airflow all feed back into how the engine
behaves."
Without detailed measurements of flame temperature, fuel regression
and fuel-vapor transport, designers have historically relied on
trial-and-error approaches.
"A lot of the design has been kind of Edisonian," Bojko said. "You
take a guess, test it and iterate. But without seeing the physics inside
the combustor, it's hard to know if you're getting the right answer for
the right reason."
At the same time, computational approaches such as Reynolds-Averaged
Navier–Stokes and Detached Eddy Simulation have been limited by a lack
of high-quality experimental data for validation.
RANS, DES and`Large Eddy Simulation represent increasing levels of
physical realism in turbulence simulation, where more turbulent
structures are directly resolved rather than modeled. Moving from RANS
to DES to LES brings simulations closer to the true flow physics,
especially for unsteady flows, but at a significantly higher
computational cost. Reynolds-Averaged Navier–Stokes models capture most
of the turbulence and are computationally efficient but less accurate
for unsteady flows. Detached Eddy Simulation resolves large turbulent
structures while modeling smaller ones, balancing accuracy and cost. LES
resolves most turbulent motion directly, offering the highest accuracy
at the highest computational expense.
"With only a few pressure or temperature points, you can match a
simulation to an experiment and still be wrong," Bojko said. "Optical
access lets us validate the flame structure, recirculation zones and
combustion species directly."
Seeing Flame Temperature in Real Time
To address these gaps, researchers employed optical diagnostics
capable of operating in the harsh, particle-laden environment of a
solid-fuel ramjet combustor. Measuring flame temperature is especially
important, Bojko said, because models often assume combustion efficiency
rather than measure it.
"These diagnostics give us new data we simply didn't have before,"
said David Kessler, a senior computational scientist at NRL. "They allow
us to measure gas-phase species and temperatures in an environment
where traditional probes just don't work."
The chemistry behind how solid fuels decompose and feed the flame is
just as important as measuring the flame itself, according to
researchers. As heat from the flame feeds back into the fuel surface,
the solid polymer undergoes phase change and chemical breakdown,
releasing a complex mixture of gaseous hydrocarbons that sustain
combustion.
"You have this continuous feedback loop," said Brian Fisher, a
combustion research engineer at NRL. "The flame heats the fuel, the fuel
decomposes into gas-phase species, and those species then mix with the
air and keep the flame going. It's a coupled thermal, chemical and
fluid-dynamic process, and that's what makes solid-fuel ramjets both
powerful and challenging to predict."
Mapping Fuel Regression and Validating Models
Understanding how quickly the solid fuel surface recedes, known as
fuel regression, is critical because it directly governs thrust and
performance. The team combined experimental diagnostics with
high-fidelity simulations to resolve heat feedback to the fuel surface, a
key driver of regression.
"One of the biggest things you need to capture is the heat transfer
back to the solid fuel," Bojko said. "RANS can give you an OK answer,
but it doesn't resolve the fundamental processes as well as DES or Large
Eddy Simulation. Those higher-fidelity approaches cost more
computationally, but they give you a much better picture of what's
happening."
Visualizing Fuel Vapor Before It Burns
For the first time, the researchers also visualized fuel vapor
released from the solid surface before ignition, revealing how complex
hydrocarbon species mix and evolve prior to combustion. Solid-fuel
ramjets commonly use hydroxyl-terminated polybutadiene, a long-chain
polymer that breaks down into many different gaseous species.
"When HTPB decomposes, you don't know what species are coming off the
surface, and those species dictate the combustion mechanism," Bojko
said. "They change with temperature, pressure and heat flux, so being
able to characterize them is critical to understanding the underlying
mechanisms across different flight conditions."
In parallel, NRL researchers are investigating advanced composite
fuels designed to increase the energy density of solid fuel in the same
volume.
"We're interested in adding energetic additives, like metal
particles, into polymer fuels to increase their energy density," said
Clayton Geipel, a combustion research engineer at NRL. "As the fuel
burns, those particles are released into the flame and ignite, giving
you more energy from the same volume of fuel. That directly translates
into greater potential range for future systems."
"You want to jam as much energy content into that block of fuel as
you can while still having a reasonable rate of combustion; that's the
challenge," said Albert Epshteyn, materials scientist at NRL.
Although metals can have slightly lower energy per unit mass than
hydrocarbons, their much higher density allows more total energy to be
packed into the same volume, a critical advantage for compact,
long-range systems.
Reducing Risk and Accelerating
Together, these diagnostics and simulations transform solid-fuel
ramjet combustion from a largely inferred process into a measurable,
predictable system. The validated models allow researchers to conduct
design iterations computationally before moving to costly experiments.
"Our main objective is to reduce risk," Bojko said. "If we have
validated computational models, we can do design iterations much more
efficiently in terms of cost and time and narrow down the physics before
we ever go to full-scale testing."
Kessler emphasized the broader impact.
"NRL is developing technologies that help accelerate the transition
of solid-fuel ramjets, technology that can significantly increase the
range of next-generation high-speed systems," he said.
Building on that foundation, the team is now focused on bridging the
gap between small-scale laboratory experiments and real-world propulsion
systems.
"All of our work right now happens at small-scale facilities in
idealized, optically accessible geometries," Geipel said. "That's what
allows us to make detailed measurements, but there are still important
questions about how those results apply to a full-scale, enclosed
ramjet."
While small-scale experiments reveal detailed physics, scaling those
results to full-size engines remains a central uncertainty in the field.
The next phase of the research will focus on extending these validated
tools and models to larger, more representative test configurations.
This intermediate step preserves diagnostic access while introducing
greater geometric and physical realism. That progression is designed to
ensure the physics and chemistry observed in the lab translate reliably
to operational propulsion systems.
By integrating optical diagnostics, detailed chemistry and validated
simulations across multiple scales, the research provides the propulsion
community with tools to reduce uncertainty, shorten development
timelines and enable future high-speed air-breathing propulsion
technologies.
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