Space nuclear propulsion can support longer missions and make U.S. satellites more resilient and maneuverable.
America’s national security satellite constellations were designed at a time when space was an uncontested domain. Their design maximized mission efficiency, life span and reliability, while providing only limited maneuverability and countermeasures. These mostly large, monolithic systems deliver tremendous mission functionality but fly predictable orbital paths, making them easy targets for enemy attacks. Like U.S. B-52 bombers over Hanoi during the Vietnam War, flying the same altitudes and flight paths day in and day out makes them sitting ducks for an enemy seeking to stop the overflights.
In today’s increasingly contested operational environment, the United States must revamp its space force design and warfighting strategy so it can conduct maneuver warfare in orbit and beyond. Doing so would enable the U.S. military to take deliberate measures to deter, avoid, and defeat threats—to field an active defense in space—instead of simply allowing its passive constellations to absorb attacks until they fail.
The U.S. satellites supporting civilian and national security missions today employ chemical or electric propulsion to maintain their orbits and make limited maneuvers to steer out of the paths of incoming objects. Because satellites carry only small amounts of chemical propellant, fuel must be used judiciously, as with aircraft that must limit speed to increase range. Satellites typically use chemical-powered thrusters to maintain orbit, adjust their position, or deorbit after mission completion. Electric propulsion, while more efficient than chemical propellants, is too slow for the kinds of maneuver operations the U.S. Space Force needs to ensure operations in the face of threats in space.
Christopher Stone is the Senior Fellow for Space Studies at the Mitchell Institute Spacepower Advantage Center of Excellence. Download the entire report at https://mitchellaerospacepower.org/maneuver-warfare-in-space-the-strategic-mandate-for-nuclear-propulsion/
To better maneuver in space, a more powerful and fuel-efficient means is needed, and nuclear energy offers a compelling solution. Space Nuclear Thermal Propulsion (SNTP) is a high-thrust system that heats hydrogen as a propellant. It is the nuclear equivalent of a chemical rocket but more efficient, enabling the spacecraft to fly longer missions with less propellant. Space Nuclear Electric Propulsion (SNEP) is a low-thrust alternative that consists of a nuclear reactor to generate electricity to power the spacecraft and a slow, but fuel-efficient propulsion system. Nuclear electric power systems could also power space weapons, such as lasers.
Both technologies are safe and could provide a maneuverable satellite force that is more survivable and capable, with both defensive and offensive benefits.
THE TECHNOLOGY
SNTP technology was developed and matured from the 1960s to the 1980s, but never operationalized. Absent a threat to make it necessary, there was no need to rapidly maneuver on orbit. Today, however, China’s strategy of maneuver warfare in space, built on both space- and ground-based weapons, changes the dynamic. By 2040, in fact, China is planning to deploy space vehicles powered by nuclear thermal propulsion. Just as mechanized armor transformed the battlefields in World War I, rendering horse cavalry obsolete, the ability to maneuver in space will be transformational.
Nuclear thermal propulsion will be critical to counter China’s anti-satellite weapons. While SNTP can’t match the thrust generated by chemical rockets, it can conduct longer, more efficient engine “burns,” producing higher velocity and more rapid maneuvers. SNTP can support longer, more complicated missions from a single vehicle and operate for years in space without needing to be refueled.
SNTP engines can also deliver the velocity and maneuverability needed to conduct maneuver operations in space with great efficiency—bottom line, they can operate with less “propellant” than their chemical counterparts and therefore can operate for longer mission times. SNTP engines use fission to generate heat. The higher the engine’s temperature, the greater the thrust and propellant efficiency (or specific impulse). Advanced ceramic composites under development may be able achieve even greater impulse and thrust-to-weight ratios that are already possible today.
When all factors are considered, nuclear thermal propulsion systems are more than twice as fuel efficient as chemical propulsion systems. Uranium-235 has an energy density 4 million times greater than hydrazine, a common chemical propellant for satellite thrusters. While the mass of the hydrogen propellant is comparable to the mass of a chemical rocket’s propellant, the combined mass of SNTP’s hydrogen propellant plus its nuclear reactor is less than that of the chemical propellant plus its combustion chamber. At the end of the day, nuclear thermal propulsion systems are more than twice as fuel efficient as chemical propulsion systems, able to generate the same thrust with half the mass. How much thrust? More than 100,000 Newtons, or enough to accelerate an automobile from 0 to 60 miles per hour in 0.3 seconds. This is the kind of responsiveness necessary to maneuver in Earth orbit, between orbits, and in cislunar space.
Safety is, of course, a primary concern. Unlike nuclear weapons, SNTP reactors are essentially a heater; they contain no explosives and remain in a “cold, subcritical state” until the reactor is turned on for a prolonged period in space. The relatively low radioactivity of un-fissioned Uranium-235 is comparable to radioactivity found in natural sources on Earth such as soil, rocks, and water. Once deployed above 750 km, the reactor poses no hazard to Earth and runs only during thrust operations—typically only several minutes at a time. SNTP engines generate no radioactivity when not in use, and whatever fission products do escape from the reactor during those short bursts are harmlessly dispersed into the vast expanse of space.
Concerns about an SNTP reactor plunging back to Earth in a failed launch are mitigated by launching the nuclear space vehicle from conventional rockets over water and following a launch path that minimizes risk. Further, the reactor’s design ensures that inadvertent criticality events cannot occur—even in the event of a crash into the ocean.
The Defense Advanced Projects Research Agency’s Demonstration Rocket for Agile Cislunar Operations (DRACO) is testing the propulsion efficiency of low-enriched uranium (LEU) reactor engines, which do not require presidential authorization. But even if high-enriched uranium (HEU) cores must be used in a given application, that extra step provides a final safety check.
THE THREAT
Both China and Russia have long recognized the vulnerabilities of conventional satellite constellations. To exploit those weaknesses, China is developing a multi-layered counterspace architecture. Starting with radio-frequency jammers and illumination lasers that can temporarily debilitate satellites, its approach adds additional threats: weapons that can permanently degrade and even destroy satellites, such as ground-launched ASAT missiles and directed energy weapons like high-power lasers. Russia is developing similar capabilities and recently demonstrated its ability to strike a satellite in orbit. Gen. James H. Dickinson, who heads U.S. Space Command, said Russia’s November 2021 ASAT demonstration made clear that it is “deploying capabilities to actively deny access to and use of space by the United States and its allies and partners.”
Understanding how limited fuel affects spacecraft operations, adversaries have designed strategies to degrade U.S. satellite mission life spans by forcing operators into defensive maneuvers that deplete onboard chemical propellant. Even though a satellite may still function in every way, once it’s out of fuel, it can no longer maintain its orbit and becomes operationally useless.
China’s strategy in space differs greatly from the U.S. approach. While the U.S. perspective bases its deterrence on the threat of force, China has made clear that it intends to preemptively use force to coerce and prevent adversaries from intervening against its operations. China’s “attack to deter” concept, which appears in some of their space doctrine, such as The Science of Military Strategy, among others, relies on rapidly maneuvering to exploit an adversary’s weak points and achieve psychological and physical effects:
- Disruption. This could include pre-conflict operations such as jamming and blinding an adversary’s intelligence satellites with lasers. In a more advanced state of crisis, China could escalate to include simultaneous kinetic strikes.
- Preemption. China’s doctrine seeks to “create psychological fear … and have an influence on … national decision-makers” to achieve its strategic objectives—before war is officially declared.
- Dislocation. If an attack to deter fails to achieve its desired result, China’s strategy calls for “destructive strikes to the enemy [in space] … in order to fight rapidly, conclude the operation rapidly, and to withdraw from the confrontation.”
According to publicly available sources, China continues to expand its operational counterspace weaponry, including ground-launched missiles carrying ASAT kinetic kill vehicles and space electronic warfare capabilities. Its People’s Liberation Army (PLA) has demonstrated kinetic ASAT weapons that threaten U.S. space systems in low-Earth orbit (LEO), medium-Earth orbit, and geosynchronous Earth orbit (GEO) and has operational units equipped with radio-frequency jamming to disrupt satellite communications, precision navigation and timing, missile warning, and other vital space systems. The PLA is developing and testing weapons that can rendezvous with orbiting U.S. satellites and observe or attack them electronically or with on-board robotic arms.
WHY NOW?
Military and commercial space operators are already experiencing the contested space environment. Purposeful jamming of space-based assets and their communication links to ground stations is now routine. Space-faring allies, including France, have experienced adversary spacecraft approaching within visual range or closer, without warning or coordination. While these reconnaissance activities could be benign, it is more likely they are preparatory efforts for more aggressive actions. Like the posturing of naval craft at sea or aircraft near sovereign airspace, such maneuvers can be intended to intimidate or incite a defensive response. Such threats alter U.S. military operating assumptions and demand new capabilities in response.
The Department of Defense’s 2020 Defense Space Strategy describes China as the “most immediate and serious threat” to U.S. national security objectives in space. This strategy argues that a more resilient national security space architecture is needed to counter emerging threats. Resiliency measures include the development of satellite constellations that can absorb limited kinetic and nonkinetic attacks and continue to provide critical services to U.S. air, land, and sea forces worldwide—in other words, constellations with enough nodes that there is no single point of failure. Most current constellations include just a few large, monolithic satellites, which can be easily targeted. Enemy attacks that eliminate a relatively small number of satellites in these constellations could greatly disrupt the overhead surveillance, global communications, and other capabilities they provide.
Proliferated LEO satellite constellations offer an alternative by deploying hundreds or thousands of small satellites to form a “mesh” network above the atmosphere. Having so many satellites means none can become a single point of failure, making the system more resilient to attack. Denying enemies the ability to inflict a quick, knockout blow is exactly what force designs like this are intended to achieve. Yet this alone does not solve the problem. First, some missions do not lend themselves to this approach. And second, even small satellites follow predictable orbits. China asserts that both traditional and proliferated constellations are “easy to attack and difficult to defend.” Without enhanced maneuverability, DOD’s push to field larger numbers of satellites per constellation may simply provide more targets, rather than targets that are harder to destroy. As Chinese and Russian military space and counter-space operations continue to mature, the ability to rapidly maneuver across orbits and even into cislunar space—the region between GEO and the moon—will become increasingly critical to U.S. security interests.
Today, satellites with limited chemical propellants can take weeks to months to maneuver across orbital regimes. The USSF must address its maneuver disadvantages, change its forces, and alter the way they operate to get ahead of emerging threats, rather than wait for adversaries to fully mature them. This will require the Space Force to field new space vehicles with SNTP technologies. Otherwise, China’s pursuit of nuclear thermal and nuclear electric propulsion vehicles and other weapons systems will give them a major advantage in space maneuver warfare.
Maneuverable space forces must be part of a multi-tiered force design that also includes proliferated constellations and hardened systems. Even in a proliferated constellation, there is a tipping point, beyond which operations are seriously degraded; likewise, hardening against radiation, lasers, or the limited use of nuclear weapons in the upper atmosphere and in space protects against certain threats. Adding maneuver expands the options available to commanders and increases U.S. flexibility in space. This is especially important in satellite constellations that are critical national resources and can increase defensive and offensive options, such as GPS and ISR satellite constellations. Whether guiding precision munitions or keeping power plants operating, GPS provides precision navigation and timing data used throughout our economy. ISR satellites operating in LEO and GEO are similarly vital, providing critical information used by military operations and farmers alike. Nuclear thermal propulsion can ensure those capabilities are always available when needed.
CONCLUSION AND RECOMMENDATIONS
Traditional resiliency measures are no longer sufficient to protect and defend against adversaries that believe rapid and destructive space warfare will be part of future great power conflicts. Following these six steps will help ensure U.S. forces in space are capable of defensive and offensive maneuver operations in the future:
- DOD should adopt a new space force design capable of decisive maneuver warfare in space. Without the ability to rapidly maneuver, DOD’s disaggregated and proliferated LEO systems will only provide additional targets for Chinese and Russian kinetic and nonkinetic counterspace weapons systems. DOD’s 2020 Defense Space Strategy is a good start to address changing threats, but it does not go far enough.
- DOD, in partnership with NASA and the Department of Energy, should develop and field SNTP and other technologies that will increase their ability to deter and defeat threats against the U.S. national security space architecture. After nearly 70 years of development, experimentation, and testing, now is the time to operationalize SNTP space systems.
- Beginning in fiscal 2024, the Biden administration and Congress should move DARPA’s DRACO program from science and technology development to a full acquisition program of record. Doing so will help DOD operationalize space maneuver warfare before America’s strategic competitors.
- DOD should deploy ground-based and space-based kinetic ASAT weapons systems capable of holding Chinese and Russian targets at risk. This will provide U.S. leadership with near-term options to deter and defend against anti-satellite threats. DOD could achieve this objective by repurposing existing initiatives, including its standard missile and ground based mid-course missile defense interceptor programs.
- DOD should hedge against risk by deploying the mission extension vehicle (MEV) to provide GPS and other vital satellite constellations the ability to conduct limited defensive maneuvers while preserving their onboard chemical propellant.
- The U.S. Space Force must educate the public and Congress on the growing threat to U.S. space systems and the need to create a more robust force design that will enhance deterrence. SNTP can help create a much-needed agile maneuvering force capable of generating a wide range of defensive and offensive effects in, from, and to space at a time and place of our choosing.