TYPE Specialty Grand Challenge

PUBLISHED 15 September 2022

DOI 10.3389/fpace.2022.1027943

Grand challenges in aerospace

OPEN ACCESS propulsion
EDITED AND REVIEWED BY
Ramesh K. Agarwal,
Washington University in St. Louis, Matthew A. Oehlschlaeger*
United States
Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute,
*CORRESPONDENCE Troy, NY, United States
Matthew A. Oehlschlaeger,
oehlsm@rpi.edu
KEYWORDS
SPECIALTY SECTION
This article was submitted to Energetics aerospace propulsion, aeropropulsion, space, decarbonization, aviation, hypersonics
and Propulsion,
a section of the journal
Frontiers in Aerospace Engineering Introduction
RECEIVED 25 August 2022
ACCEPTED 29 August 2022
Aerospace propulsion technologies are well established and commercialized for low-
PUBLISHED 15 September 2022
speed to supersonic air flight, payload launch to space, and missions within space.
CITATION
Oehlschlaeger MA (2022), Grand
However, present aerospace propulsion systems have a number of shortcomings,
challenges in aerospace propulsion. including their environmental impact, performance, and mission capabilities, which
Front. Aerosp. Eng. 1:1027943. represent grand challenges to the aerospace engineering research and development
doi: 10.3389/fpace.2022.1027943
communities. These and other shortcomings will need to be addressed through
COPYRIGHT
fundamental and applied research that seeks to improve current technologies and
© 2022 Oehlschlaeger. This is an open-
access article distributed under the develop understanding of the underlying physics, new engineering methods, and new
terms of the Creative Commons aerospace propulsion concepts and technologies.
Attribution License (CC BY). The use,
distribution or reproduction in other With increased public interest in aerospace engineering, resulting from the wide
forums is permitted, provided the access to air travel and increased number of space launches per year, and the increased
original author(s) and the copyright
economic activity and opportunity for scientific discovery that these activities have
owner(s) are credited and that the
original publication in this journal is provided, the field of aerospace propulsion has a bright future. The grand challenges
cited, in accordance with accepted that our field faces, described in part here, offer great opportunities for the current and
academic practice. No use, distribution
or reproduction is permitted which does
future generation of researchers. The Energetics and Propulsion section of Frontiers in
not comply with these terms. Aerospace Engineering looks forward to supporting and disseminating research that
addresses the current and future challenges in aerospace propulsion and energetics.

Decarbonization of aeropropulsion
The aviation sector will be one of the most difficult sectors of the global economy to
decarbonize, due to the high energy density and other advantageous characteristics of
conventional hydrocarbon fuels for aviation, which made the original development in
aeropropulsion systems and their advancement over the last century possible. Aviation
accounts for 2.5% of global CO2 emissions. Additionally, due to contrails and factors
relating to emissions and flight at altitude, aviation results in 3.5% of the effective radiative
forcing on the earth’s surface (i.e., 3.5% of the warming) (Lee et al., 2021).
Over the last several decades, there has been a massive increase in greenhouse gas
emissions from aviation, due to the decreasing cost of air transport. The decreasing cost of
aviation has resulted from both the decreasing capital costs of aircraft, due to
technological development and innovation, and decreased fuel consumption, due to
improved aeropropulsion systems that have become more efficient with increasing gas
turbine pressure ratios and turbofan bypass ratios. The increased availability and

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Oehlschlaeger 10.3389/fpace.2022.1027943

accessibility of aviation has resulted in a ten-fold increase in the hydrogen (Hoelzen et al., 2021) and ammonia (Boretti and
annual CO2 emissions from the aviation sector since 1950. While Castelletto 2022), both of which offer the potential for net-zero
during that period, the CO2 emissions per passenger mile have carbon emissions. Hydrogen can be generated from water via
decreased by twenty-fold, owing to the improvements in electrolysis, and be net-zero carbon provided the electricity
aeropropulsion systems and aircraft design (Lee et al., 2021). source for electrolysis is renewable. Net-zero carbon (green)
To decarbonize the aviation sector, ultimately the fuel or ammonia can be generated from hydrogen and nitrogen
energy sources used for aeropropulsion will have to be separated from air, using renewable electricity. For the
decarbonized and supplied by non-fossil sustainable resources utilization of hydrogen as an aviation fuel, several
(Nelson and Reddy, 2018). In addition, new propulsion engineering challenges need to be addressed, including
technologies may be required to utilize new fuels or energy storage on aircraft (i.e., cryogenic or high pressure),
sources and the continued improvement of existing handling and transport of hydrogen, and stable combustion
aeropropulsion architectures for increased efficiency, stability, in gas turbine engines. Of course, the wide-scale production of
and other improved performance characteristics is necessary. green hydrogen via electrolysis is a considerable engineering
The development of sustainable jet fuels, that can be challenge in itself. Ammonia, like hydrogen is a gas at standard
directly utilized in exiting and future gas turbine temperature and pressure conditions; however, ammonia may
aeropropulsion engines, is a challenge that is the subject of be easier to implement into aviation infrastructure than
current research efforts (Nelson and Reddy, 2018; hydrogen, given its higher boiling point (−33°C
Chiaramonti 2019). These fuels may include sustainable versus −253°C for hydrogen). Many of the same engineering
aviation fuels (SAF) synthesized from sustainable biological challenges exist for ammonia, including synthesis, storage,
resources, such as crops, waste oils, algal oils, and others, transport, handling, and combustion in engines.
provided that these feedstocks do not have massive land and Additionally, a major concern with ammonia is its toxicity
water requirements (Holmatov et al., 2019), such as those (exposure limit of 50 ppm for humans) and standards will be
required by many first-generation biofuels (e.g., corn ethanol). required to ensure that those involved in the fuel supply chain
Synthetic biology offers promise for the sustainable are not exposed.
production of hydrocarbon fuels with similar characteristics In addition to decarbonizing aviation fuels, advances in gas
to conventional fossil-derived jet fuels (Scown and Keasling turbine jet engines must also continue to be pursued. For
2022.). Synthetic biology makes it is possible to engineer example, extreme gas turbine pressure ratios may be possible
microorganisms that can process carbon feedstocks to in the future that result in supercritical or transcritical
generate fuels directly, with potentially lower input costs, combustor conditions. Fuel injection, mixing, and
land requirements, and faster than competing processes. combustion under supercritical conditions is still not well
Electrofuels, or e-fuels, also offer the potential for understood, presenting a research challenge that requires
sustainable aviation fuels (Goldmann et al., 2018). fundamental research as well as technological developments
Electrofuels are typically synthesized from hydrogen, to enable and control supercritical combustion in
formed by the electrolysis of water using a renewable aeropropulsion systems (Oefelein 2019). Another
electricity source (wind or solar), and a carbon source (e.g., fundamental phenomenon that is worthy of continued
biological carbon feedstock or CO2 captured from an exhaust fundamental and applied research is pressure gain
stream or the atmosphere). While electrofuel fuel production combustion (Kailasanath 2020). Pressure gain combustion
is energy intensive, massive amounts of renewable electricity, implemented in gas turbines or other aeropropulsion
that may be available in decades to come, could make systems could allow for increased efficiencies of the order of
electrofuels viable for transportation applications that 10%–20%.
require high energy density fuels, such as aviation. The The electrification of aeropropulsion is in its nascent stage
development of sustainable fuels that attempt to synthesize (Bowman et al., 2018); at this point, electric propulsion has mostly
hydrocarbons that are in or similar to those in current fossil- only be implemented in the form of pure electric propulsors for
derived jet fuels (i.e., SAFs, fuels derived using synthetic low thrust systems (small aircraft and UAVs) (Pelz et al., 2021).
biology, and electrofuels) will require research Electrification, both in the form of pure electric and hybrid electric
developments in biology, chemistry, and chemical aeropropulsion systems (Finger et al., 2020), is an area that will be
engineering. Additionally, aerospace and mechanical of significant importance to development of new aircraft
engineers will need to optimize aeropropulsion engines for technologies and concepts in the coming decades, representing
these new fuels and consider new modes of engine operation a grand challenge in need of new research efforts. Topics that will
for increased efficiency and other improved performance need continued research efforts include the development and
characteristics. integration of high energy density batteries (or other energy
Non-hydrocarbon fuels that have received considerable storage elements, e.g., super capacitors) for aeropropulsion and
attention for use in aeropropulsion applications include the development and optimization of electric and hybrid electric

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Oehlschlaeger 10.3389/fpace.2022.1027943

propulsion systems for various aircraft configurations. Electric for increase regression rate (Pang et al., 2021). Solid energetic
propulsion will also allow for the development of new formulations that take advantage of nanomaterial features (Van
distributed propulsion systems (De Vries et al., 2019) that will der Heijden 2018), or are additively manufactured, may allow a
allow entirely new aircraft designs to be envisioned. new paradigm in fuel design (Fleck et al., 2020). Green
monopropellants for space propulsion can offer reduced
environmental impact and address safety concerns, an
Hypersonic air-breathing propulsion important challenge that is being addressed (Nosseir et al.,
2021). In addition, space launch propulsion systems are under
Significant research and development efforts are underway development by SpaceX (Raptor), Blue Origin (BE-4), and others
towards the development of hypersonic vehicles that rely on air- that use liquid methane fuel (Neill et al., 2009; Boué et al., 2018)
breathing supersonic-combustion RAMJET (SCRAMJET) rather than liquid RP fuels, indicating that further innovation in
engines (Choubey et al., 2019). The primary limiting conventional liquid-propellant space launch systems is coming in
phenomenon in these engines is combustion at supersonic the near future.
conditions where the residence times are of the order
10–4–10–3 s (Liu et al., 2020). Combustion in these systems is
complex, analogous to lighting and holding a match in a Computational modeling and design
hurricane. It can take place in a diffusive or premixed mode
and involves extremely fast mixing processes and chemical Over the last several decades, computational methods for
reactions. Hence, the design of SCRAMJET and other high- analysis and design of propulsion systems have been widely
speed propulsion systems is dependent on, and limited by, flame adopted by the commercial sector for the rapid development
stabilization at extreme conditions (Huang et al., 2019; Liu et al., of aeropropulsion and space propulsion engines (Spalart and
2020). Given the limiting nature of combustion processes in Venkatakrishnan 2016). These tools, primarily based on
high-speed propulsion systems, understanding the fundamental computational fluid dynamics (CFD), have resulted in
phenomena and developing innovative flame stabilization tremendous cost reductions, improved performance, and
schemes represent important research challenges for the rapid engineering design cycles. Today, CFD analysis for all
development of hypersonic air-breathing propulsion. parts of aero and space propulsion systems is possible to
various degrees, using Reynolds Averaged Naiver Stokes
(RANS), Large Eddy Simulation (LES), and hybrid
Space propulsion turbulence modeling methods and submodels and/or
simplifications for chemistry (e.g., flamlets) and other
The development of electric propulsion devices for satellite complicating physics. As computing power continues to
station keeping have seen rapid growth over the last several expand, grand challenges towards the development of the
decades, including Hall, ion, and electrothermal thrusters. next-generation computational modeling and design tools
However, research challenges exist in the development of new for aero and space propulsion include the greater
electric propulsion concepts that improve specific impulse, implementation of LES models, towards full gas turbine jet
efficiency, scalability, longevity, and reliability of these systems engine transient modeling with LES (Anand et al., 2021).
(Dale et al., 2020; Holste et al., 2020). Examples of electric Additionally, the continued development of direct
propulsion concepts for space that are under research and numerical simulation (DNS) for special use cases (e.g.,
development include electrospray arrays, radio frequency- or combustion) is a challenge worth pursing (Gruber et al.,
microwave-systems coupled with magnetic nozzles, pulsed 2021), such that source terms in sub-grid models used of
inductive thrusters, magnetoplasmadynamic (MPD), and engine component analysis and design can be better
nuclear thermal (NT) propulsion systems, among others approximated. Developments in artificial intelligence and
(O’Reilly et al., 2021). Research challenges for future electric machine learning (AI/ML) should also be viewed as an
space propulsion systems will address the need to improve the opportunity to adopt an integrate novel data and AI/ML
specific impulse and longevity of high-thrust systems and the approaches to classical physics-based simulations such that
efficiency and reliability of low-thrust systems (Dale et al., 2020). those simulations can be accelerated and used for design
Solid and liquid fuels and energetics for chemical propulsion optimization (Vinuesa and Brunton 2022).
for space launch, other space applications, and munitions have
been continually developed for about a century; however, there
are still significant research challenges in the development of new Concluding remarks
fuels/energetics with improved properties and reduced
environmental impact. These include the development of solid The field of aerospace propulsion has a bright future, as the
propellants and solid hybrid propulsion fuels utilizing additives demand for air and space transport continues to expand. As

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Oehlschlaeger 10.3389/fpace.2022.1027943

highlighted here, there are several important scientific and Conflict of interest
technological challenges that will require focused research
efforts for decades to come. The Energetics and Propulsion The author declares that the research was conducted in the
section within Frontiers in Aerospace Engineering seeks to absence of any commercial or financial relationships that could
advance research and development in the areas of air and be construed as a potential conflict of interest.
space propulsion, power, energy, fuels, and energetics by
publishing high-quality contributions in these areas that
addresses challenges at the forefront of aerospace engineering. Publisher’s note
All claims expressed in this article are solely those of the authors
Author contributions and do not necessarily represent those of their affiliated organizations,
or those of the publisher, the editors and the reviewers. Any product
The author confirms being the sole contributor of this work that may be evaluated in this article, or claim that may be made by its
and has approved it for publication. manufacturer, is not guaranteed or endorsed by the publisher.

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