Nuclear Thermal and Electric Propulsion
Executive Summary
Nuclear in-space propulsion splits into two physically distinct families. Nuclear thermal propulsion (NTP) heats liquid hydrogen in a fission reactor and expels it through a nozzle, delivering roughly double the specific impulse of the best chemical stages — about 800–900 s versus ~450–465 s — while retaining high thrust [6][7]. Nuclear-electric propulsion (NEP) uses a reactor to generate electricity that feeds ion or Hall thrusters at 3,000–7,000 s Isp, but at vanishingly low thrust [9][12]. The two are not competitors so much as tools for different mission segments: NTP for fast, high-thrust departure burns; NEP for patient, fuel-efficient deep-space cruise.
The physics is favorable; the engineering and the politics are not. In June 2025 DARPA confirmed the cancellation of DRACO — the Demonstration Rocket for Agile Cislunar Operations, the U.S. flagship NTP demonstrator — and the finalized FY2026 budget zeroed both NTP and NEP, with NASA materials stating the programs were terminated for cost savings and because nuclear was "not identified as the propulsion mode for deep-space missions" [1][2][3]. The $499 M DRACO contract (DARPA/NASA, Lockheed Martin prime, BWXT reactor/fuel) ended without a flight [10][14].
NTP's headline weakness is thrust-to-weight (T/W) of only ~3–5, an order of magnitude below a chemical core stage; combined with reactor and radiation-shield mass, gravity losses on a departure burn erode much of the Isp advantage for some trajectories [7]. NEP's weakness is specific mass: NASA Mars architecture studies assume ~20 kg/kWe at megawatt scale [8], meaning a multi-MWe powerplant weighs tens of tonnes before any propellant — and no such system has flown.
Bottom line: After DRACO, the binding constraint on space fission propulsion is no longer the physics — NTP's ~900 s Isp and NEP's ~3,000–7,000 s Isp are real and bounded by well-understood limits — but ground-test infrastructure, HALEU fuel qualification, and sustained funding. Engines of Tomorrow estimates a credible, crewed-relevant NTP flight demonstration now slips to the early-to-mid 2030s at the earliest, with the only near-term public-market exposure being narrow and indirect (BWXT fuel/reactor; LMT and RTX as integrators).
Context and Scope
This report covers two of our core coverage domains — nuclear thermal and electric propulsion and the engineering path to flight / TRL — with a downstream defense and commercial programs and economics lens. The system boundary is the in-space propulsion stage: the reactor, the energy-conversion or direct-heating path, the thruster, and the propellant feed, from low Earth orbit (LEO) outward. Launch from the ground (a chemical problem) is out of scope; reactors are launched cold and started only above a nuclear-safe orbit [14].
The question is timely because the field just lost its anchor program. For two decades, NTP and NEP advanced on study contracts and component work; DRACO (2021–2025) was the first funded path to an actual in-space firing. Its cancellation, confirmed in mid-2025 and ratified by the FY2026 budget, forces a reset: what is physically true about these systems, what was actually demonstrated, what the cancellation removed, and what — if anything — survives to carry the technology to flight [1][2][3].
The comparison set is deliberately broad. NTP and NEP must be judged not in isolation but against the incumbents they would displace: advanced chemical propulsion (the only crewed-rated in-space option today) and solar-electric propulsion (SEP), which already flies high-power Hall thrusters and dominates the efficient-cruise niche inside Mars orbit [9][12].
Technology Landscape and State of the Art
Nuclear thermal propulsion (NTP): the physics
NTP is conceptually a heat exchanger with a fission core. Liquid hydrogen flows through a reactor operating at ~2,500–3,000 K, is heated to high temperature, and expands through a nozzle. Because thrust efficiency scales with exhaust velocity — and exhaust velocity scales with the square root of (temperature / molecular weight) — using hydrogen (molecular weight ~2) rather than chemical-combustion products like water vapor (~18) is the entire trick. A chemical rocket is energy-rich but stuck with heavy exhaust; NTP decouples the energy source (fission) from the propellant, letting the engineer choose the lightest possible working fluid [6][7].
The result is specific impulse of ~800–900 s, roughly double the ~450–465 s of the best chemical upper stages (LH₂/LOX) [6][7]. This is a real, well-bounded number — it is limited not by reactor power but by the temperature the fuel elements can survive before the hydrogen-rich, high-temperature environment corrodes them. The historical U.S. program proved this is the hard problem (see below).
Crucially, NTP retains high thrust — tens to hundreds of kilonewtons — unlike electric systems. But its thrust-to-weight ratio is only ~3–5 [7], compared with ~70+ for a chemical core engine, because the reactor, the pressure vessel, the turbopumps, and the radiation shield are heavy. For an impulsive departure burn from LEO, low T/W means the burn takes longer, the spacecraft spends more time climbing out of the gravity well, and gravity losses can exceed 3,500 m/s, eroding much of the Isp advantage on some trajectories [7]. NTP wins decisively on missions where the burn can be done at high altitude or where total ΔV, not burn duration, dominates.
The historical baseline: Rover/NERVA
NTP is not speculative at the component level — the U.S. ran a serious, well-funded ground program from 1955 to 1973. The Rover program (Los Alamos) and NERVA (Nuclear Engine for Rocket Vehicle Application, from 1963) built and hot-fired a series of reactors [4]:
- The KIWI-B4 design was adopted as the NERVA baseline at ~825 s Isp [4].
- The Phoebus-2A reactor reached ~4,100 MW for 12 minutes and a peak thrust of ~930 kN — still the most powerful nuclear rocket reactor ever operated [4].
- In the summer of 1972, the NF-1 nuclear furnace tested composite uranium-zirconium-carbide fuel elements at 44 MW over multiple 90-minute runs, demonstrating improved corrosion resistance — directly addressing the fuel-durability bottleneck [4].
Rover/NERVA was cancelled in 1973 for budget and mission reasons (Mars was deprioritized), not technical failure [4]. The lesson for 2026 is sobering: the technology has been "nearly ready" for over fifty years, and every revival has died on funding and ground-test logistics rather than physics.
Nuclear-electric propulsion (NEP): the physics
NEP severs heat from thrust entirely. A reactor generates thermal power; a power-conversion system (typically a closed Brayton cycle) turns it into electricity; that electricity drives electric thrusters (gridded ion, Hall, or magnetoplasmadynamic). Because the propellant is accelerated electromagnetically rather than thermally, exhaust velocity is far higher: 3,000–7,000 s Isp for argon-plasma systems, roughly 3–7× hydrogen NTP [9][12].
The penalty is thrust. Electric thrusters produce millinewtons-to-newtons, so NEP accelerates continuously over months. The architecture is dominated by specific mass (α, in kg/kWe) — the dry mass of the powerplant per kilowatt of electrical output. NASA Mars architecture studies assume NEP at ~20 kg/kWe for crewed-relevant systems [8]; the bulk is not the reactor but the heat-rejection radiators, which must dump waste heat from a multi-MWe plant in vacuum. A 2 MWe system at 20 kg/kWe implies ~40 tonnes of powerplant before propellant or payload [8]. Advanced concepts target ~2 kg/kWe above 10 MWe, but those remain paper systems [9].
What actually flies — and the small-reactor state of the art
No fission propulsion system has flown. The nearest hardware is at the kilowatt scale: NASA's surface-power and small-reactor work, and the SR-1 Freedom design targeting ~20 kWe from a HALEU-fueled, heat-pipe reactor with a closed Brayton converter [8]. The Defense Innovation Unit funded Ultra Safe Nuclear and Avalanche Energy for small nuclear power-and-propulsion demonstrations aimed at cislunar maneuvering [11]. These are power demonstrators, not the multi-MWe propulsion reactors crewed Mars NEP would require — a gap of two-to-three orders of magnitude in power.
Competing Pathways
| Pathway | Principle | Isp (s) | Thrust class | T/W | Specific power / mass | TRL | Status |
|---|---|---|---|---|---|---|---|
| Chemical (LH₂/LOX) | Combustion, hot heavy exhaust | ~450–465 [6] | High (MN) | ~70+ | n/a | 9 | Operational; crewed-rated |
| Solar-electric (Hall) | PV power → ion/Hall thruster | ~1,600–1,800 flight; to ~5,000 demo [12] | Low (mN) | <0.001 | ~10–100 kW PV demonstrated [12] | 7–9 | Operational (flight Hall thrusters) |
| NTP | Reactor heats H₂, nozzle expansion | ~800–900 [6][7] | High (10s–100s kN) | ~3–5 [7] | n/a | ~3–5 (component-tested historically) | No flight; DRACO cancelled 2025 [1] |
| NEP | Reactor → electricity → electric thruster | ~3,000–7,000 [9][12] | Low (N) | <0.001 | ~20 kg/kWe assumed; ~2 kg/kWe aspirational [8][9] | ~2–3 | No flight; FY2026 funding zeroed [2] |
TRL is our assessment synthesizing the cited program status; NTP/NEP TRLs reflect that component and subscale work exists but no integrated flight system has been built or flown.
Techno-Economic Analysis
Space fission propulsion has no levelized "cost per unit" in the energy sense; the relevant economics are program cost to first flight and mission-level mass/time savings versus chemical and SEP. The analysis below is built transparently from public program figures and labeled estimates.
Cost Model and Assumptions
| Parameter | Value | Unit | Basis / Source |
|---|---|---|---|
| DRACO program value (Phases 2–3) | 499 | USD M | DARPA/NASA, 2023 [10][14] |
| — NASA commitment | up to 300 | USD M | NASA, 2023 [10] |
| — of which engine design/development | up to 250 | USD M | NASA, 2023 [10] |
| DRACO target demo | FY2027 (cancelled) | year | DARPA [14]; cancelled 2025 [1] |
| FY2026 NTP/NEP appropriation | 0 | USD | NASA FY2026 budget materials [2] |
| NTP reactor temperature | ~2,500–3,000 | K | NTP physics [6][7] |
| NTP Isp | ~800–900 | s | DOE/NASA [6][7] |
| Chemical (LH₂/LOX) Isp | ~450–465 | s | comparison baseline [6] |
| NEP assumed specific mass | ~20 | kg/kWe | NASA Mars architecture [8] |
| Small reactor demonstrator | ~20 | kWe | SR-1 Freedom / HALEU [8] |
| Mars transit time, NTP-class | ~500 (vs ~900 chemical) | days | NTP studies [7] |
Program economics and unit economics
DRACO is the only recent number with a verified price tag: $499 M for a single in-space demonstration, split between DARPA and NASA, with NASA committing up to $300 M [10][14]. That buys one reactor, one fuel load, one integrated stage, and one firing — and it still did not survive to flight. A crewed-relevant NTP stage, with ground-test facilities, multiple fuel iterations, and qualification, is a multi-billion-dollar program (Engines of Tomorrow estimate: a full NTP development-to-crew-rating effort is plausibly $5–10 B over a decade, scaling DRACO's demonstrator cost by the gap from a one-shot demo to a qualified, reusable, crew-rated stage — an order-of-magnitude-plus uplift consistent with historical flagship propulsion programs).
The mission-economic case rests on two levers:
- Transit time. NTP-class propulsion is cited as cutting a crewed Mars transit from ~900 days to ~500 days, reducing crew radiation dose, consumables, and life-support mass [7]. This is the strongest argument and is genuinely hard to replicate chemically.
- Propellant mass / launch count. Higher Isp means less propellant per unit ΔV, reducing the number of heavy-lift launches to assemble a Mars stack — a direct cost saving if launch remains the dominant line item.
Sensitivity
The answer is governed by a small number of drivers. In descending order of leverage on whether nuclear beats the incumbents:
| Driver | Low case | High case | Effect on the nuclear case |
|---|---|---|---|
| Sustained annual funding | $0 (current) | $0.5–1 B/yr | Decisive — at $0 the program does not exist [2] |
| NEP specific mass (α) | ~20 kg/kWe [8] | ~2 kg/kWe aspirational [9] | A 10× α improvement is the difference between unflyable and competitive |
| NTP fuel durability (run time at temp) | minutes (historic) [4] | hours, many restarts | Sets reusability and crew-rating credibility |
| Ground-test availability | none operational | exhaust-capture facility | Gates any U.S. full-scale hot fire |
| Launch cost | high | low | Erodes the propellant-saving argument for nuclear |
The two that dominate are funding (currently zero) and, for NEP, specific mass (currently ~10× too heavy). Neither is a physics problem; both are engineering-and-budget problems.
Market and Demand Outlook
There is no commercial market for space fission propulsion today; demand is agency-driven and program-contingent. The addressable pull comes from three sources: crewed Mars exploration (NASA), responsive cislunar maneuvering (DoD/Space Force, the original DRACO rationale), and outer-planet science where SEP runs out of sunlight [9][14]. With FY2026 funding at zero for both NTP and NEP, near-term U.S. demand is effectively suspended, not merely slowed [2].
our analysis frames the path to a flight demonstration as three scenarios to ~2035:
| Scenario | Probability (Engines of Tomorrow estimate) | Path | First in-space nuclear-propulsion firing |
|---|---|---|---|
| Reauthorization | ~30% | A future budget restores an NTP demonstrator (DRACO-like or successor); HALEU pipeline already exists | ~2031–2033 |
| Quiet continuity | ~45% | No flagship; kilowatt-class reactor and fuel work continues via DoD/DIU and small-reactor lines; propulsion deferred | Power demo possible ~2027–2029; propulsion firing slips past 2035 |
| Dormancy | ~25% | Funding stays zero; NTP/NEP revert to study-only as in 2000–2015 | No firing this decade |
The probability-weighted read is that a full-scale NTP propulsion firing is more likely after 2033 than before, and a crewed-relevant qualified stage is a 2040s proposition (Engines of Tomorrow estimate). The small-reactor power demonstrations (≤20–100 kWe) are far more likely to occur this decade than any propulsion firing, because they ride defense and surface-power demand that survives the propulsion cuts [8][11].
Feasibility, Scale-Up, and Risk
The honest go/no-go: the technology is feasible in principle and was component-validated historically, but the U.S. has just removed the only funded path to flight, and the supporting infrastructure does not currently exist at full scale.
The gating risks, in order:
- Funding (the binding constraint). FY2026 zeroed NTP and NEP [2]. Without a sustained line, no facility gets built and no fuel gets qualified.
- Ground testing. A full-scale NTP hot fire exhausts hot hydrogen that has passed through a reactor; the U.S. has no operational facility to capture and scrub that exhaust to modern environmental standards. This was a real DRACO friction point and is a multi-hundred-million-dollar capital item on its own [1][14].
- Fuel qualification. NTP requires fuel elements that survive ~2,500–3,000 K in flowing hydrogen for the full burn and multiple restarts. NF-1 (1972) proved composite UZrC could resist corrosion, but modern HALEU-based fuel must be re-qualified from a cold start; DOE-supplied HALEU feedstock to BWXT was part of DRACO and the HALEU pipeline itself runs on extended contracts [4][5][13].
- Reactor/shield mass (NTP) and radiator mass (NEP). These are the physics-rooted penalties — T/W ~3–5 for NTP [7], ~20 kg/kWe for NEP [8] — that no amount of money removes, only engineering chips away at.
- Thermal management. NEP's multi-megawatt heat rejection in vacuum is the single largest mass and reliability driver and remains unproven at scale [8].
- Regulatory/launch safety. Launching fissile material (even cold, un-started HALEU) carries an inter-agency safety and approval burden that contributed to DRACO's schedule slips [1][14].
Risk Register
| Risk | Likelihood | Impact | Mitigation |
|---|---|---|---|
| Funding stays zero (no flagship) | High | Critical — no flight path | Ride DoD/DIU small-reactor demand; keep fuel/reactor IP warm |
| No full-scale ground-test facility | High | High — blocks NTP hot fire | Subscale/element testing; international or shared facility |
| HALEU fuel-element re-qualification slips | Medium | High — sets NTP schedule | Leverage NF-1 heritage + existing HALEU contracts [5][13] |
| NEP specific mass stays ~20 kg/kWe | High | High — keeps crewed NEP unflyable | Radiator and Brayton R&D; start kW-class flight data |
| Launch-safety/regulatory delay | Medium | Medium — schedule | Cold-launch, start above nuclear-safe orbit [14] |
| Workforce/IP erosion after DRACO | Medium | Medium — restart cost | Retain BWXT/lab teams via adjacent reactor work |
Market and Equity Implications
The investable surface here is small, indirect, and dominated by diversified primes — not a pure-play opportunity. The thesis (nuclear propulsion is real but funding-gated and post-DRACO dormant) cuts against any near-term revenue narrative and in favor of optionality on franchises that survive on adjacent demand.
| Company (Ticker) | Exposure | Reasoning (tied to the thesis) | Horizon |
|---|---|---|---|
| BWX Technologies (BWXT, NYSE) | Neutral-to-Positive | The closest thing to a pure space-fission franchise: DRACO reactor and HALEU-derived fuel supplier [10][14]. DRACO revenue is gone, but the naval-reactor and microreactor/HALEU-fuel core is intact and benefits from any future restart; the propulsion line is upside optionality, not the base case. | 3–7 yr |
| Lockheed Martin (LMT, NYSE) | Neutral | DRACO prime/integrator [10][14], but the program is immaterial to a company of its scale; cancellation is not a financial event. Optionality only if a successor flagship appears. | 5–10 yr |
| RTX Corp. (RTX, NYSE) | Neutral | Aerojet Rocketdyne (now within RTX) has propulsion/integration heritage cited in space-nuclear teaming [11]. Exposure is diffuse and dominated by its core chemical-propulsion and defense business. | 5–10 yr |
Most of the actual builders are agencies, national labs, or private/non-listed (NASA, DOE, DARPA/DIU, Ultra Safe Nuclear, Avalanche Energy, Blue Origin) [8][11] — there is no clean public-equity way to express a "space nuclear" view. The listed names are diversified primes for whom this technology is a rounding error today.
The Take: The correct way to read BWXT here is not as a space-propulsion stock — DRACO's death proves that line is uninvestable on a near-term basis. Its space-nuclear value is a free option riding on a business (naval reactors, terrestrial microreactors, HALEU-adjacent fuel) that is funded for reasons that have nothing to do with Mars. The non-obvious implication: the power-reactor side of space fission (≤20–100 kWe demonstrators for cislunar and surface use) is where 2026–2030 dollars and contracts will actually land — propulsion is downstream of power, and the market is mispricing how long the propulsion firing is still away. Anyone underwriting "nuclear propulsion by 2030" is, in our read, underwriting a power demonstration and mislabeling it.
Outlook and Strategic Implications
The physics verdict is settled and favorable: NTP roughly doubles chemical Isp at usable thrust, and NEP delivers 3–7× NTP Isp for patient cruise [6][7][9][12]. The program verdict is bleak. DRACO — the first funded path to a firing in fifty years — is cancelled, and FY2026 carries no NTP or NEP line [1][2][3]. The binding constraints are now institutional: money, a ground-test facility, and fuel qualification, in that order.
For an operator or investor, the decision-grade takeaways are:
- Treat a near-term flight as a low-probability event. our probability-weighted estimate puts a full-scale NTP propulsion firing more likely after 2033 than before, and a crewed-rated stage in the 2040s.
- Follow the power, not the propulsion. Kilowatt-class space reactors (≤20–100 kWe) ride defense and surface-power demand and are far more likely to fly this decade [8][11]; they are the leading indicator that the fuel and reactor supply chain stays alive.
- The equity expression is optionality, not exposure. BWXT carries the only meaningful franchise, but as a free option on top of a self-funding reactor business; the primes are immaterial-but-present.
What to watch: 1. FY2027 budget request (early 2026 cycle): any restored NTP/NEP line reverses the dormancy thesis. 2. A successor to DRACO or a DoD cislunar maneuver award: the original responsive-space rationale could resurface under defense funding even if NASA stays out. 3. First kilowatt-class space-reactor power demonstration (~2027–2029): confirms the fuel/reactor supply chain survives and de-risks any later propulsion restart [8][11]. 4. HALEU supply contracts beyond mid-2026: the fuel feedstock pipeline is the quiet enabler; extensions signal continued government commitment [5][13].
Disclosures & Disclaimer
This report is general commentary published for information purposes only. It is not investment advice, a recommendation, or a solicitation to buy or sell any security. Engines of Tomorrow is a research publication, not a registered investment adviser or broker-dealer. Views are the publication's own analytical opinions, are subject to change, and may prove wrong. Readers should do their own research and consult a licensed financial professional before acting. The publication and/or its principals may hold positions in securities mentioned. © Engines of Tomorrow.
Methodology and Assumptions
This report synthesizes public agency, standards, and technical sources (cited inline) into a comparative techno-economic frame for nuclear thermal and nuclear-electric propulsion. Performance figures (Isp, thrust-to-weight, specific mass, temperatures) are drawn from DOE, NASA NTRS, ANS, and peer/industry sources and reported with their conditions. Program facts (DRACO value, schedule, FY2026 status) are taken from agency releases and reputable reporting. Where a figure is our own inference — full-program cost, scenario probabilities, the timing of a first firing — it is explicitly labeled "Engines of Tomorrow estimate" and its basis is shown in the surrounding text or the sensitivity/assumptions tables. No proprietary data was used and no figures were fabricated. The conclusion would change most on two inputs: a restored federal funding line (currently zero), and a step-change in NEP specific mass (currently ~10× too heavy for crewed use). Data vintage spans 1972 (NF-1 fuel testing) to 2026 (FY2026 budget); the latest year is preferred for all program-status claims.
References
- DARPA / Breaking Defense. "DARPA's DRACO nuclear propulsion project ROARs no more." Breaking Defense, June 2025. https://breakingdefense.com/2025/06/darpas-draco-nuclear-propulsion-project-roars-no-more/
- Nuclear Engineering International. "DRACO project cancelled." NEI, 2025. https://www.neimagazine.com/news/draco-project-cancelled/ (FY2026 budget zeroes NTP/NEP).
- New Space Economy. "What was the DARPA DRACO Program?" 2026. https://newspaceeconomy.ca/2026/03/15/what-was-the-darpa-draco-program/
- Beyond NERVA. "NTR Hot Fire Testing Part I: Rover and NERVA Testing." 2018. https://beyondnerva.wordpress.com/2018/06/18/ntr-hot-fire-testing-part-i-rover-and-nerva-testing/ (KIWI-B4 ~825 s; Phoebus-2A 4,100 MW / ~930 kN; NF-1 1972 UZrC fuel).
- Centrus Energy Corp. Form 8-K (HALEU contract extension to June 30, 2026). U.S. SEC EDGAR, 2025. https://www.sec.gov/Archives/edgar/data/0001065059/000106505925000053/exhibit_991x20250625xhal.htm
- U.S. Department of Energy, Office of Nuclear Energy. "6 Things You Should Know About Nuclear Thermal Propulsion." energy.gov. https://www.energy.gov/ne/articles/6-things-you-should-know-about-nuclear-thermal-propulsion (NTP ~800–900 s vs chemical ~465 s).
- "Nuclear thermal rocket." Wikipedia (compiling primary NTP performance: Isp, T/W ~3–5, gravity-loss and Mars-transit figures). https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
- ANS / Nuclear Newswire. "NASA announces plan for space nuclear propulsion by 2028." American Nuclear Society, 2022. https://www.ans.org/news/article-7879/nasa-announces-plan-for-space-nuclear-propulsion-by-2028/ ; NASA NTRS, "Coupled Reactor Multiphysics and Mass Scalability Assessment for Crewed Megawatt-Class NEP," 2022. https://ntrs.nasa.gov/citations/20220016297 (NEP ~20 kg/kWe; SR-1 Freedom ~20 kWe HALEU).
- SpaceNews. "NASA to test nuclear electric propulsion with 2028 mission to Mars." 2021. https://spacenews.com/nasa-to-test-nuclear-electric-propulsion-with-2028-mission-to-mars/ (NEP Isp 3,000–7,000 s; α aspirational ~2 kg/kWe).
- SpaceNews. "NASA and DARPA select Lockheed Martin to develop DRACO nuclear propulsion demo." 2023. https://spacenews.com/nasa-and-darpa-select-lockheed-martin-to-develop-draco-nuclear-propulsion-demo/ ($499 M program; NASA up to $300 M / $250 M engine).
- World Nuclear News. "Contracts to demo novel space propulsion technologies." (DIU awards to Ultra Safe Nuclear and Avalanche Energy; General Atomics / X-energy / Aerojet Rocketdyne teaming). https://www.world-nuclear-news.org/Articles/Contracts-to-demo-novel-space-propulsion-technolog
- NASA / IEPC. "Characterization of a 20 kW-class Hall Effect Thruster," IEPC-2017-381, Electric Rocket Propulsion Society, 2017. https://electricrocket.org/IEPC/IEPC_2017_381.pdf ; "Solar electric propulsion," Wikipedia (flight Hall Isp ~1,600–1,800 s; high-power demos to ~5,000 s). https://en.wikipedia.org/wiki/Solar_electric_propulsion
- ANS / Nuclear Newswire. "BWXT to provide engine, fuel for DARPA space project." American Nuclear Society, 2021. https://www.ans.org/news/article-5215/bwxt-to-provide-engine-fuel-for-darpa-space-project/ (DOE-supplied HALEU metal processed to LEU fuel by BWXT).
- "Demonstration Rocket for Agile Cislunar Operations." Wikipedia (program structure, cold-launch / start above nuclear-safe orbit, schedule slip to indefinite hold Jan 2025). https://en.wikipedia.org/wiki/Demonstration_Rocket_for_Agile_Cislunar_Operations
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