Nuclear Rockets—An Idea Whose Time Has Gone?

This essay is based on the research for the panel “Nuclear Rockets—An Idea Whose Time Has Gone?” that I was honored to moderate for 2025 Seattle WorldCon. The panelists were Bob Hranek, Charles J. Walther, Mark Olson and Dan Dubrick who brought decades of professional experience and research to bear on the topic. Loath to throw away this work, we constructed this essay to serve as a short primer for the current status of nuclear rocketry.

Manju

Nuclear Rockets—An Idea Whose Time Has Gone?

By

Bob Hranek, Charles J. Walther, Mark Olson,  Dan Dubrick and Manjula Menon

The performance of any rocket is primarily defined by two metrics: Thrust (T) and Specific Impulse (Isp). Thrust is the force generated by a rocket, calculated as the product of the fuel mass flow rate (M) and the velocity of the exhaust (Ve), expressed as T = Ve × M/sec. For example, the Saturn V’s F-1 engine produced a staggering 6.9 million Newtons of thrust, showcasing the immense power needed to escape Earth’s gravity.

Specific Impulse, measured in seconds, quantifies a rocket engine’s efficiency in converting fuel into thrust. It represents the change in momentum per unit of fuel used, calculated as Isp = T / (M × Gc), where Gc is Earth’s gravitational acceleration (9.8 m/s²). Chemical rockets, like the Saturn V’s F-1, achieved an Isp of 263 seconds at sea level and 304 seconds in a vacuum. Modern chemical engines, such as SpaceX’s Raptor 3 (350 Isp at sea level), or Blue Origin’s New Glenn’s BE-4 (340 Isp at sea level), push these numbers up but still face a fundamental constraint: over 90% of a rocket’s launch mass is fuel, making them inefficient for deep-space missions.

By leveraging the energy of nuclear reactions to superheat propellant and generate electrical power, nuclear engines achieve significantly higher efficiency. For instance, the Nuclear Engine for Rocket Vehicle Applications (NERVA) program in the 1960s demonstrated an Isp of up to 869 seconds, nearly three times that of the F-1 engine. This efficiency stems from nuclear reactions’ ability to produce far more energy per unit of fuel than chemical reactions, akin to the difference between nuclear and chemical bombs. Unlike Radioisotope Thermoelectric Generators (RTGs), which are heavy, always-on thermal batteries used when solar power is impractical (for example, deep space probes), nuclear rockets actively harness fission or fusion to propel spacecraft, offering both high thrust and high Isp for missions beyond Earth’s orbit.

NASA’s pursuit of nuclear propulsion began in the mid-20th century, driven by the promise of superior performance for ambitious space missions. One of the earliest concepts, Project Orion (1955–1964), proposed using pulsed nuclear explosions to propel spacecraft. Theoretical studies suggested Orion could achieve an Isp of up to 6,000 seconds with fission bombs or an astonishing 75,000 seconds with fusion bombs. A hypothetical Orion craft, propelled by 800 small (0.35 kt) nuclear bombs detonated at a rate of one per second, could have delivered 6,100 kg to low Earth orbit (LEO), 5,700 kg to a lunar landing, or even 1,300 kg on a three-year round-trip to Saturn. However, the 1963 Partial Nuclear Test Ban Treaty, combined with concerns about nuclear fallout and weaponization, led to the project’s cancellation.

The NERVA program (1955–1973) took a more practical approach, developing a nuclear thermal rocket (NTR) to replace the Saturn V’s third stage. Authorized in 1955 by the Mills Committee for an intercontinental ballistic missile (ICBM) upper stage, NERVA evolved under Project Rover to produce approximately 267,000 Newtons of thrust with an Isp of 869 seconds. The Phoebus 2a test in 1968, the most powerful nuclear rocket engine ever tested, operated for 32 minutes, including 12 minutes at 4,100 MW. Despite these successes, NERVA was discontinued in 1973 due to shifting priorities and insufficient demand for deep-space missions.

Parallel efforts included the Systems for Nuclear Auxiliary Power (SNAP) program (1955–1972), which developed compact nuclear reactors. The SNAP-10A, launched in 1965, operated for 43 days in orbit, generating 590 watts from a 40 cm × 22.4 cm, 290 kg reactor producing 30 kW of heat. Though successful, SNAP’s limited power output restricted its applications. The SP-100 (1983–1994), a successor to SNAP, aimed to develop a more advanced space reactor but never progressed to flight hardware.

More recently, the Demonstration Rocket for Agile Cislunar Operations (DRACO) program (2020–2025) sought to demonstrate an NTR using low-enriched uranium by 2027. With a $500 million budget, DRACO aimed for a thrust-to-mass ratio 10,000 times greater than electric propulsion and 2–5 times greater than chemical propulsion. DRACO’s rapid startup times (as little as 60 seconds) were much faster than other space or terrestrial nuclear power reactors, which could be up to several hours. However, it faced significant challenges, including high reactor temperatures (up to 2,700 K), the need for cryogenic liquid hydrogen storage, long startup & shutdown transients compared to nearly instantaneous chemical engines, and costly modifications to launch infrastructure. In 2025, DRACO was cancelled due to budget overruns and the decreasing cost of conventional launches, such as those by SpaceX.

Despite past setbacks, nuclear propulsion remains a compelling option for future space exploration, particularly for missions where chemical and solar propulsion fall short. The Space Nuclear Propulsion and Power (SNPP) initiative highlights recent advancements that bolster its viability, including improved uranium fuel forms, high-assay low-enriched uranium (HALEU), advanced reactor designs, and high-temperature materials capable of withstanding 3,500 K. These developments, coupled with validated physics modeling and a growing nuclear industrial base, position nuclear propulsion for missions requiring high thrust and Isp, such as crewed deep space missions.

Nuclear propulsion excels in scenarios where solar power is impractical. On the lunar surface, 354-hour nights make nuclear reactors more reliable than battery-stored solar power, especially for underground bases immune to surface damage. On Mars, dust storms can cripple solar arrays, making nuclear a robust alternative. Beyond the asteroid belt (approximately 400 million km from the Sun), solar power becomes less cost-effective, favoring nuclear systems. Additionally, nuclear propulsion offers advantages for spacecraft requiring minimal observability, such as military assets in cislunar space.

Emerging technologies promise to further nuclear propulsion’s potential. The Centrifugal Nuclear Thermal Rocket (CNTR), under study by NASA, could achieve an Isp of 1,500 seconds, enabling 420-day round-trip Mars missions. Pulsar’s Direct Fusion Drive (DFD), with a modest Isp of 105 seconds but minimal fuel consumption, could enable two-year Saturn missions. Howe Industries’ Pulsed Plasma Rocket (PPR), an evolution of Project Orion, proposes 100,000 Newtons of thrust and 5,000 Isp for a two-month Mars trip. Meanwhile, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), with NASA-backed development, offers flexible thrust and Isp configurations, with goal performance ranging from 2,956 to 29,969 Isp.

Looking ahead, the next 30 years could see significant progress, particularly from the Chinese Space Program, which benefits from consistent funding and robust systems integration. Commercial ventures, such as asteroid mining, could also adopt nuclear propulsion for its efficiency. Once one entity successfully implements nuclear propulsion, others are likely to follow.

However, challenges remain. In the U.S., funding hinges on demonstrating return-on-investment, and bureaucratic hurdles, coupled with public stigma against nuclear technology, all posing significant obstacles. The lack of ground testing facilities for nuclear reactors larger than the Kilopower Reactor Using Stirling Technology (KRUSTY) complicates development, as does the need for costly launch infrastructure modifications and lengthy regulatory approvals (12–24 months for FAA licensing). Addressing these requires investing in testing infrastructure and public education to dispel nuclear fears.

By shortening travel times (and perhaps enabling heavier radiation shielding like water belts), nuclear rockets could make crewed deep space missions safer by reducing human exposure to cosmic radiation, as envisioned in concepts like the Hermes cycler from The Martian. While projects like Orion and antimatter-catalyzed propulsion remain science fiction due to safety and cost concerns, continued development of NTRs, fusion drives, and electric propulsion systems could transform deep space exploration.

NOTES:

Below is the handout that Bob created (with input from other panelists) that was used to construct the essay:

Sun, Aug 17, 12:00-13:00, rm 447-448, 2025 Seattle WorldCon Panel SPA20 Nuclear Rockets – An Idea Whose Time Has Gone?: “Development of the DRACO nuclear rocket is facing delays since testing an “open” nuclear reactor in Earth’s atmosphere has become untenable. Are there any options?”

Manjula Menon (mod), Bob Hranek, Charles J. Walther, Dan Dubrick, Mark Olson

[My career began with 6 years of USAF Computer Programming plus 34 more years as an Aerospace Systems Engineer. Since I was a Defense Contractor for the Intelligence Community, I’m usually depicted as representing the Dark Side on panels. Hence my ‘Protogen’ name plate, which fans of The Expanse will understand. That being said, I do NOT speak for ANY of my employers! On this panel I’ll add my aerospace experience in support of nuclear rockets. I OVERprepare for all my panels, so if you’d like my file regarding Nuclear Rockets, then just email me at BobHranek@gmail.com]

Nuclear Rocket Fundamentals:

1. The basic measures used to describe rockets are Thrust (T) and Specific Impulse (I_sp).
   1. Thrust (T) = fuel mass flow rate (M) times the velocity of the exhaust (V_e), or T = V_e x M/sec.
      6. T is only limited by the amount of fuel you can burn. Saturn V F-1’s T = 6.9 million Newtons.
   2. Specific Impulse (I_sp) = how efficiently a rocket engine converts fuel into thrust measured in seconds (s).
      9. Change in momentum per unit of fuel used, I_sp = T/M x G_c), where G_c = 9.8 m/sec².
   3. Rockets with enormous thrust, like the Saturn V’s F-1, had 263 I_sp at sea level & 304 I_sp in vacuum.
      1. Best conventional I_sp are New Glenn’s BE-4’s 340 I_sp at sea level & BE-3U’s 445 I_sp in vacuum.
   4. All chemical reaction rockets have fundamental limits on how efficient they can be, requiring over 90% of launched mass to be the fuel needed to reach Earth’s orbit.
2. Nuclear engines can be much more efficient, such as NERVA’s 1964-1969 tests of up to 869 I_sp (3 times F-1’s).
   1. Use the energy of nuclear reactions, both to superheat propellant & to generate electrical power.
3. Nuclear engines can be much more powerful than Chemical engines, just like nuclear bombs are MUCH more powerful than Chemical bombs.
4. Manjula‘s James and Gregory Benford’s Starship Century includes a nice summary of the need for nuclear rockets.
5. These are NOT Radioisotope Thermoelectric Generators (RTGs), which are just thermal batteries with decaying radioactive elements as the heat source. RTGs are heavy, always “on”, & generally just used when solar is inefficient.
6. ATOMIC ROCKETS is a great site to get an overview of current, planned, & far-out engines for spacecraft of ALL sizes.

Nuclear Rocket History:

7. 1955-1964 Project Orion proposed using pulsed nuclear bombs to provide high thrust AND high I_sp for space travel.
   1. Fascinating theoretical technical specs of up to 6000 I_sp using fission bombs or 75000 I_sp using fusion bombs.
   2. 800x .35 kt bombs (1/second) might have propelled a 10000 kg craft to deliver 6100 kg to LEO, 5700 kg to soft landing on Moon, 5300 kg to Mars & back, or 1300 kg on a 3-year Saturn & back mission.
   3. 1963 Partial Test Ban Treaty was part of reason for shutting down this project.
8. 1955-1973 NERVA (Nuclear Engine for Rocket Vehicle Applications)
   1. 1955/03, Mills committee authorized development for a nuclear rocket upper stage for an ICBM.
   2. Project Rover later designed to replace 3^rd stage of Saturn V, or ~267,000 N thrust, with best test = 869 I_sp.
   3. 1968/06/22, Phoebus 2a , most powerful ever tested, operated for 32 minutes, 12 minutes at 4100 MW.
9. 1955-1972 Even-numbered Systems for Nuclear Auxiliary Power (SNAP) were compact nuclear reactors.
   1. 1965 SNAP-10A US fission reactor operated for 43 days in orbit, generating a maximum of 590 watts.
      22. Reactor 40 cm long x 22.4 cm diameter, 290 kg (unshielded), generating 30 kW heat.
   2. 1973-1978 Project Daedalus proposed a Fusion rocket to reach Barnard’s Star9 light yearsaway in 50 years.
      1. 2-stages 54,000,000 kg initial mass (50,000,000 kg fuel + 3,500,000 craft + 500,000 payload) in Earth orbit.
      2. 1^st stage takes 2 years to reach 7.1% of light speed(c), detaches, & 2^nd stage fire for 1.8 years to 12% c.
   3. 1977-? Variable Specific Impulse Magnetoplasma Rocket (VASIMR) by varying the RF heating energy & plasma, VASIMIR should be able to “shift gears” to provide low-thrust/high I_sp, high-thrust/low I_sp, or anything in between.
      1. 2015 Ad Astra’s VX-200 engine provided 5 N thrust for 200 kW (40 kW/N).
         1. 2009 NEXT’s conventional ion thruster produces 0.327 1.36 e-04N with only 7.7 kW, or 24 kW/N.
      2. 2023 Ad Astra Won 2 NASA Contracts For VASIMR Technology Development, promising, but not mature yet.
      3. ATOMIC ROCKETS reported GOAL performance for 10000 kg VASIMIR engine with 5.9 MW thrust power are:
         1. High Gear= 29969 I_sp,  40 N thrust, mass flow 0.136 g/s, 0.000408 N thrust/kg.
         2. Med Gear= 14985 I_sp,  80 N thrust, mass flow 0.544 g/s, 0.000815 N thrust/kg.

• Low Gear=    2956 I_sp, 400 N thrust, mass flow 10 g/s,      0.00408 N thrust/kg.

19. ALL= 60% thermal & total efficiency, 19.6 MWe input, using liquid H₂, accelerated via magnetic nozzle.

12. 1983-1994 SP-100 (Space reactor Prototype) successor to SNAP, but never advanced to flight hardware.
13. 1992-? Antimatter-catalyzed nuclear pulse propulsion invented at Penn State, antiproton-initiated fusion research performed at Lawrence Livermore in 2004. This remains a speculative technology for the foreseeable future.
    1. ~1 microgram of antihydrogen required per 1 kt yield.
    2. Estimated cost to produce 1 microgram of antihydrogen = $100 million.
    3. Presuming containment can be achieved, fueling a Project Orion-like craft above = $28 billion.
14. 2020-2025 DRACO (Demonstration Rocket for Agile Cislunar Operations) aimed to demonstrate a nuclear thermal rocket(NTR) engine using low-enriched uranium in orbit by 2027. (a.k.a., ROAR = “Reactor On A Rocket”)
    1. 2023, $500 million project to demonstrate NTR with 10,000x greater thrust-to-mass ratio than electric propulsion and 2-5x greater than chemical propulsion in space.
    2. DRACO’s reactor startup time was as little as 60 seconds from zero to full power, much shorter relative to other space or terrestrial nuclear power reactors, which could be up to several hours.
    3. Technological, infrastructure, & regulatory challenges:
       1. High operating power density & reactor temperature necessary to heat propellant to 2700 K.
       2. Need for long-term storage & management of cryogenic, liquid hydrogen (LH2) propellant.

• DRACO’s startup & shutdown transients were still long relative to nearly instant chemical engines.

1. Expensive mods to launch vehicle, pad, & ground systems including support for unique missions.

1. Why cancelled: the decreased cost of conventional launch (like SpaceX) & DRACO cost overruns.
2. 2025/07/24 AWST, S. Grapples With Barriers To Rapid Space Maneuvering, at end of article: “Congress rejects NASA plans to defund nuclear propulsion research”, “Senate Appropriations Committee included “no less than” $110 million for NASA to develop, produce and demonstrate NTP systems in its 2026 spending bill markup as well as $10 million to establish a national nuclear propulsion center of excellence.”
3. 2025/02/06 ESA’s Alumni NTP engine may pick up where DRACO left off.

15. 2022, How to Solve Big Problems: Bespoke Versus Platform Strategies, informative 34-page study on why SpaceX was able to outperform NASA on cost, speed-to-market, schedule, and
16. 2024/12, Overview of Space Nuclear Propulsion & Power (SNPP)
    1. 16 slides that contain excellent summaries of the concepts, histories, & technologies intended for this panel.
    2. “U.S. DoD & civil & commercial space enterprises recognize the need for alternatives to traditional propulsion and power options that may enable novel missions or enhance mission-unique capabilities.”
    3. SNPP has become a viable consideration based on decades of advancements in:
       1. Uranium fuel form development, enrichment, and processing
       2. Reactor design and manufacturing (materials, additive manufacturing)

• Nuclear industrial base growth (new companies, capabilities, facilities)

1. Interests in space applications across government and commercial communities
2. High-assay low-enriched uranium (HALEU)―easier to obtain & nonweaponizable but less efficient
3. Advanced fuel forms: tristructural isotropic, accident tolerant, production history (terrestrial & naval)

• Multiple core configurations—moderator block, tie rod, particle/pebble bed
• High-temp materials & manufacturing methods: additive manufacturing, welding, ceramic composites

1. Validated physics modeling and design tools: reduces cost/risk, improves design

1. General SNPP Challenges & Considerations: (CTE = Critical Technology Element)
   1. HALEU Processing – Significant investment needed to develop facilities for enrichment of low-enriched uranium to HALEU to meet prospective needs
   2. Development of Very High-Temperature Materials and Assembly Processes
      1. Required for NTP where internal reactor temperatures can exceed 3,500 K (~6,000 °F)
      2. Will also benefit SNP systems that have much longer operational lifetimes

• Reactor Ground Testing Not (Currently) Possible
  1. No facilities exist in the U.S. that can support reactor ground testing (larger than KRUSTY)
  2. NTP testing requires that engine exhaust be scrubbed of radiologics before release; could result in very large, prohibitively expensive facilities that take years to build & qualify
  3. SNP systems require reactor testing with partially and/or fully integrated CTEs

1. Complex System Engineering & Integration (SNP)
   1. CTEs must auto-interact to ensure stability & control across the power-demand range
   2. Must be packaged & qualified for space applications
   3. Multiple, lengthy, expensive test/qualification programs
2. Commercial Launch Operator Must Obtain License to Launch Nuclear Material
   1. SNPP asset must undergo several multi-agency reviews to assess safety assurance
   2. FAA license application process can take 12–24 months
   3. License might also cover debris and/or disposal, depending on mission
3. Modifications to Launch Vehicle, Pad, and Ground Systems to Support SNPP
   1. May require requalification, thus driving schedule and cost
   2. May result in substantial investments for unique, infrequent, one-off missions

• Adopting SNPP systems will require careful consideration of their cost, development time, benefits, & challenges when performing mission-level comparisons to current systems

2030. 2025/08/04, Duffy to announce lunar nuclear reactor, soliciting proposals for 100 kW reactor by 2030.

17. 2023/07/26, Pulsar’s Nuclear Fusion Rocket, 1^st half of 12-minute video exploring possibility of 2027 test in space.
    1. Proposed Direct Fusion Drive (DFD) has I_sp = 105 sec, but “uses so little fuel it can burn constantly” to allow ½ travel time to Mars, or 2-year travel time to Saturn!
18. 2024/05/22, Howe Industries’ Pulsed Plasma Rocket (PPR) 1^st 4 min of 12-min video using NASA’s Pulsed Fission-Fusion (PuFF) Propulsion Concept of tiny nuclear bombs for propulsion for a 2-month mission to Mars.
    1. Thrust = 100,000 Newtons & 5000 I_sp. An extension of the old Project Orion
19. 2024/06/02, Scott Manley’s 22-minute Nuclear Rocket summary is great for new & experienced Aerospace fans.
20. 2025/06/30 AWST, p40, Isaacman claimed US States political support for pivoting NASA to nuclear-electric propulsion.
21. 2025/06/06 S. Unleashes Liquid Uranium Rocket to Conquer Mars with Unmatched Nuclear Speed, hyped-up title.
    1. IF this Centrifugal Nuclear Thermal Rocket (CNTR) can be implemented, it could deliver 1500 I_sp.
    2. 2023 NASA’s Centrifugal Nuclear Thermal Rocket Challenges & Potential detailed technical paper for fast (<15 month) round-trip human Mars missions or high delta-V missions in cislunar space.
    3. Neutronic Design of the Centrifugal Nuclear Thermal Rocket for 420-day round-trip human Mars missions.
22. 2025/07/20, Eager Space’s Nuclear Electric Propulsion 30-min video, EXHAUSTIVE details on getting rid of waste heat.
    4. Starlink V2 Ion Thruster Argon 2500 I_sp, 4.2 kW used to produce 170 mN (weight of 3 AAA batteries in 1 G).
    5. Need 240 MW to make an Ion engine with as much thrust as RL-10.
    6. X3 Hall Effect engine = 5.4 N each.
    7. MAT-100 engine = 1.4 N each.
    8. VASIMR VX-200 engine = 2.4 N.
    9. High-Power Brayton Nuclear Electric Propulsion for 2033 Mars Round-Trip Mission using two 5 MW Reactors, 4 Generators, 1361 m2 Reactor Radiators, 182 meters long, & 560 tons total. Liquid H2 5000 m3. Two 2.5 MW Plasma Thrusters, but the depicted heat radiators are WAY TOO SMALL.
23. 2025/07/14 AWST, p56-60, “Hidden Asset” details the Armstrong Test Facility (ATF) history in the development of NERVA & several other government & commercial high-energy Aerospace projects. ATF needs to be saved from short-sighted budget cuts if the use of nuclear power in space is going to progress in the U.S.
24. Chinese Nuclear-Powered Spacecraft Development
    1. 2024/03/20 China tests nuclear-powered engine for Mars spaceship in Scientia Sinica Technologica.
    2. Chinese systems integration is currently the best, they have consistent & reliable funding once they decide on a goal, & they will take the best of all the technologies we’ve discussed & actually implement them.

Panel Agenda: (The answers listed here are Bob’s opinions, and do not reflect the entire panel’s POV.)

25. Why did nuclear propulsion fail before?
    1. Nuclear LAUNCH is a non-starter due to public fears until we’re VERY experienced with matured
    2. Technilogical Readiness Levels (TRL) of too many required systems were too low to warrant development.
    3. Long-lead-times of nuclear propulsion proposals made use of chemical propulsion more cost-effective.
    4. There was insufficient need for deep-space operation to warrant the investment in nuclear propulsion.
26. What must be done to make it succeed next time?
    1. Develop nuclear space propulsion for operations where it outshines solar & chemical implementations.
       1. Deep-space missions, such as people to Mars, where high thrust & I_sp are required to minimize crew radiation exposure by getting them there more quickly & with more radiation protection, such as thicker water belts, which is too heavy for cost-effective chemical propulsion.
       2. Military operations in Cislunar space, where maneuver & observability requirements make nuclear propulsion more desirable than chemical & solar options.
    2. Where beyond Earth is nuclear power better than solar?
       1. Lunar surface, where 354 hours of night make having a continuous source of power better than storing 354 hours of solar power in batteries. If base is completely underground, then no risk due to solar panel damage.
          1. This applies to Mars as well, since dust storms there can severely limit solar power.
       2. For any spacecraft that needs to limit its observable size, if nuclear heat dissipation is < solar panel size.
       3. Depending on the cost-effectiveness of the solar panels, then missions operating around 400,000,000 km from sun (asteroid belt) & beyond.
    3. What technologies do we have now that might make nuclear useful?
       1. Continuing DRACO, SNPP, CNTR, and other projects to implement nuclear propulsion in space
    4. What can we reasonably foresee coming in the next, say, thirty years?
       1. My crystal ball has been fractured by living in the stupidest timeline, but I expect the Chinese to develop cislunar space (including Lunar colonization) before the rest of the world can stop its infighting to catch up.
       2. A few limited commercial ventures will start asteroid mining & nuclear propulsion could be used for that.
       3. Once 1 country or corporation uses nuclear propulsion practically, then several will follow within a decade.
    5. IF small fusion reactors (no ITER in space, please!) come to fruition, might this be a better power source than fission?
       1. Obvious answer is “YES!”, but there are longevity issues with current ITER that would be hard to overcome.
       2. Fusion power outshines fission, just like fission is always inherently more powerful than chemical energy.
    6. One of the biggest issues for exploring space is developing affordable technologies to sustain funding for exploration, even without financial return. How can nuclear propulsion help?
       1. Nuclear propulsion can help by dramatically shortening human exposure to radiation during interplanetary missions by decreasing the travel time & allowing more radiation-absorbing water to be transported.
    7. Any timelines for when we’ll have a nuclear rocket?
       1. In the ‘West’, the main obstacle is funding. Unless a return-on-investment can be shown, nothing gets done.
       2. In the U.S., cutting through bureaucratic barriers may be the next most difficult hurdle. There’s still a HUGE irrational stigma against anything with the word ‘nuclear’ included, so educating people is key.
       3. Next will be building the infrastructure to build & test these technologies at full power, which don’t exist.
       4. Your best bet may be to follow the Chinese Space Program, since they’re the only group actually thinking long-term AND funding long-term space development in a reliable & efficient manner.
          1. 2025/07/20 China Reveals 12,400 MPH Propulsion Tech That Can Orbit the Planet in Under 2 Hours is an example of how far ahead the Chinese are in hypersonic technology.
       5. What is possible in this context?
          1. What is “possible” is only limited by our imaginations. What gets funded is whatever ideas can actually persuade scientifically-illiterate politicians.
       6. What is likely to remain science fiction?
          1. Project Orion, because no one (except maybe the crazy Russians?) is comfortable with that many nukes being made and distributed as a source of propulsion. Way too easy to be used as weapons too.
          2. Antimatter-catalyzed nuclear pulse propulsion, because of the extreme cost of creating antimatter, the extreme technical challenges of 100% containment, and the extreme danger of weaponization.
          3. I LOVE everything about The Expanse TV series & Books (I’ve got 14-pages of notes for it!), but their SUPER efficient Epstein Drive that makes interplanetary travel so easy will likely always remain science fiction.
             1.  ATOMIC ROCKETS lists the performance of Epstein’s fusion-drive yacht as:
                1. Thrust: 1,000,000 N
                2. Specific Impulse (i_sp): 1,100,000 seconds
                3. Exhaust Velocity (v_e): 11,000,000 m/second (~3.7% of light speed)
                4. Mass Flow Rate (ṁ ): 0.09 kg/second
                5. Thrust Power: 5.5 Terawatt
                6. Engine’s Thrust to Weight Ratio: ~140
                7. Propellant mass fraction (yacht’s mass fueled / mass empty): ~4
             2. Alternative set of performance statistics has been suggested by the Tough SF website, based on the performance of the Rocinante (main ship of the show) on-screen:
                1. Thrust: 6,370,000 N
                2. Specific Impulse (i_sp): 1,927,000 seconds
                3. Exhaust Velocity (v_e): 18,900,000 m/second (6.8% of light speed)
                4. Mass Flow Rate: 2.2 kg/second
                5. Thrust Power: 60.2 Terawatt
                6. Total power output: 96.8 Terawatt
                7. Engine’s Thrust to Weight Ratio: Presumably over 3 (Roci has a dry TWR of 2.6)
                8. Fusion type: D-He3 (1:2 mixture ratio)
                9. Fusion pulse rate: “[what we see] can be achieved with as few as 10 pulses per sec, or hundreds if possible“

Manjula Menon

Manju’s essays include Exhuming “They’d Rather be Right” in Tangent Online, Science Fiction and The Shaping of Belief, and Truth Embedded in a Tale, in Sci Phi Journal. Her awards include a Yaddo Fellowship and a Breadloaf Writers Conference “waitership”.

Manju has degrees in astrophysics and electrical engineering and is the co-founder of The Science Fiction and Philosophy Society.

Read more of her work at manjulamenon.substack.com