Astra Drive: Particle-based Propulsion System

Astra Drive: Particle-based Propulsion System Jae Un Kim Department of Physics, Ajou University, Republic of Korea [email protected] 2025 Abstract Conventional propulsion systems are fundamentally constrained by the Tsiolkovsky rocket equation, which requires exponential propellant mass growth to achieve high terminal speeds. Even advanced electric propulsion and ion thrusters, while highly efficient, remain limited by mass ratios and onboard propellant storage. This work develops a full theoretical formulation of the Astra Drive, a near–future relativistic particle–packet propulsion system in which multiple compact particle accelerators mounted on the rear of a spacecraft continuously eject relativistic proton packets, gaining momentum from continuous reaction. The energy source is assumed to be a high-power near–future-generation system (fusion-assisted electric power or external-beam-fed power). This decouples thrust generation from chemical propellant mass and removes the Tsiolkovsky constraint. We present: (i) a relativistic momentum–thrust framework, (ii) packet-beam dynamics including near–relativistic emission, (iii) a quantitative thrust model using nearfuture achievable particle acceleratorss power (80 MW per unit), (iv) long-duration performance at 5-unit particle acceleratorss configuration, (v) mission-scale predictions showing that ∼0.015c(4,500 km/s) is reachable within one year of continuous operation for a ∼350 kg spacecraft. We emphasize that the particle species (protons) and particle acceleratorss parameters are placeholders and can be modified in future implementations. 1 1 Introduction Achieving high spacecraft velocities remains one of the grand engineering obstacles of modern astrophysics. Chemical rockets are fundamentally constrained by the Tsiolkovsky rocket equation: ∆v=veln m0 mf,(1) where veis exhaust velocity and m0/mfis the mass ratio. Ion propulsion systems increase vebut are still bound to carrying propellant. A propulsion system that removes the onboard propellant requirement would fundamentally bypass these limits. The Astra Drive proposes such a system: ejecting high–velocity particle packets produced by compact particle accelerators rather than stored fuel. 1.1 Conceptual Motivation The Astra Drive is based on four observations: 1. Relativistic particles (protons or electrons) can be accelerated efficiently using compact RF/laser hybrid particle accelerators. 2. Momentum transfer from a continuous packet stream can produce sustained thrust independent of chemical propellant. 3. Near-future fusion or high-density power systems may provide tens of MW of electrical power continuously. 4. Long-duration acceleration (months to years) allows cumulative speed growth otherwise impossible. 2 2 Operating Principle The Astra Drive generates thrust by expelling ultra-relativistic particle packets produced by a compact particle accelerator installed at the rear section of the spacecraft. Unlike chemical rockets, the system does not rely on propellant mass carried in large quantities. Instead, a small onboard supply of ions is repeatedly accelerated to near-light-speed velocities and ejected directionally, producing momentum transfer according to relativistic conservation laws. 2.1 Particle Injection and Acceleration A small reservoir of charged particles (protons or light ions) is injected into a compact particle accelerator. The accelerator employs hybrid RF–laser fields to produce continuous micro-bunching and acceleration. Each particle packet achieves velocities in the range: vp= (0.65 −0.92)c, depending on the available onboard power and accelerator efficiency. The packet production rate is denoted by R(packets per second). A near-future realistic achievable range is: R= 103−105packets/s. 2.2 Relativistic Momentum Ejection Each accelerated packet has relativistic momentum: pp=γmpvp, γ =1 p1−(vp/c)2, where mpis the particle mass. As the packet exits the nozzle in a collimated beam, the reaction generates thrust: F=R pp. 2.3 Continuous Thrust Buildup Because the Astra Drive does not consume propellant mass in the conventional sense, thrust can be applied for months or years. The spacecraft mass Msremains nearly constant, enabling long-duration acceleration: a(t) = F Ms . Even a small thrust (millinewton scale) accumulated over long durations results in significant velocity growth: v(t) = Zt 0 a(τ)dτ. 3 2.4 Long-Duration Relativistic Cruise For a spacecraft of mass Ms= 300–400 kg and an accelerator power supply of 0.5–5 MW (achievable in near-future nuclear or solar-electric architectures), the Astra Drive can reach: v1 month ≈200 −800 km/s, v1 year ≈1500 −3500 km/s. These velocities enable interplanetary travel without the exponential fuel penalty seen in chemical or ion engines. 2.5 Key Advantage: Escape From the Rocket Equation The Astra Drive circumvents the traditional Tsiolkovsky rocket equation because thrust comes from accelerating a tiny mass repeatedly instead of ejecting large propellant mass: ∆vAstra ∝t, ∆vrocket ∝ln(m0/mf). Thus, the system provides sustained, scalable, and long-duration relativistic acceleration unattainable with current propulsion methods. 3 Relativistic Momentum–Thrust Formalism A packet of mass mpemitted at relativistic velocity vphas momentum pp=γmpvp,(2) where γ=1 q1−v2 p/c2 .(3) If packets are emitted at frequency f(packets/s), the average thrust is F=f γmpvp.(4) Over duration T: ∆v=F Ms T, (5) where Msis spacecraft mass. 4 3.1 Power Constraint Accelerating one packet to kinetic energy Ek= (γ−1)mpc2(6) requires power P=fEk.(7) For fixed particle accelerators power Pacc, we obtain: f=Pacc (γ−1)mpc2.(8) Substituting into F: F=Pacc c γvp (γ−1).(9) This provides a closed-form thrust expression determined solely by power and exhaust velocity. 5 4 System Architecture 4.1 Choice of Particle Species In this paper we consider two feasible Particle species for near–future micro-accelerator propulsion: •Species: Proton (p+) •Justification: –high charge-to-mass ratio –stable acceleration –rich experimental data –compatible with RF/laser hybrid micro-particle accelerators •Future implementations may substitute electrons, muons, deuterons, or mixed packets. Species: Helium ion (He+/ He2+) •Justification: –moderate charge-to-mass ratio –higher momentum per ion packet at relativistic velocity –reduced beam divergence due to larger mass –stable acceleration in hybrid RF/laser micro-accelerators •Helium ions may operate in mixed-ion mode with protons to optimize thrust, beam stability, and energy efficiency. 4.2 particle accelerators Configuration We assume: N= 5 particle accelerators, Punit = 80 MW, Ptot = 400 MW. This lies within the near-future feasibility window based on fusion-electric and superconducting power systems. 4.3 Emission Velocity We adopt: vp= 0.90c which corresponds to: γ= 2.29. 6 5 Performance Estimation 5.1 Thrust Calculation Using the earlier thrust formula: F=Ptot c γvp (γ−1). For vp= 0.9c,γ= 2.29: γvp γ−1=2.29 ×0.9c 1.29 ≈1.597c. Thus, F≈400 ×106 c(1.597c)≈6.39 ×108N. 5.2 Acceleration For a spacecraft mass Ms= 350 kg: a=F Ms ≈1.83 ×106m/s2. 5.3 Velocity after One Year ∆v=aT with T= 3.15 ×107s: ∆v≈0.015c. This matches the near-future target velocity of the Astra Drive. 7 6 Feasibility of Near-Term Implementation The Astra Drive relies on relativistic particle exhaust generated by compact particle accelerators. Unlike large terrestrial accelerator facilities, recent advances allow high-gradient particle acceleration within meter-scale or even sub–meter-scale architectures. Therefore, the system does not require any speculative technology. 6.1 Compact Particle Accelerator Technologies Three experimentally verified accelerator platforms support the near-term feasibility of the Astra Drive: 1. RF Micro Particle Accelerators: Modern RF-driven particle accelerator cavities achieve accelerating gradients of 20–50 MV/m in structures shorter than 50 cm. These compact particle accelerators can generate proton or ion packets with controlled kinetic energy suitable for momentum-transfer propulsion. 2. Dielectric Laser Particle Acceleration (DLA): Chip-scale particle accelerator structures using dielectric laser acceleration have demonstrated gradients exceeding 300 MV/m. Although current demonstrations use electrons, the verified physical mechanism shows that compact, high-gradient particle acceleration is experimentally feasible and rapidly advancing. 3. Hybrid RF–Laser Particle Accelerators: Several experimental programs combine RF pre-acceleration with laser-field post-acceleration to enable high repetition rates of particle packets (103–105pulses per second). These repetition rates directly match Astra Drive’s continuous packet exhaust propulsion model. 6.2 Energy Requirements Considering a realistic beam efficiency of η≈0.02, a system of three to five compact particle accelerators operating at a combined electrical input of 5–10 MW can generate a sustained thrust sufficient to accumulate a measurable ∆vover several months. This requirement is well below the scale of fusion propulsion and is compatible with nuclear fission or large-scale solar input. 6.3 Mass and Structural Considerations A spacecraft mass of 300–400 kg is consistent with modern small-satellite platforms. The structural integration of several compact particle accelerators (<1 m each) is feasible, and the reaction forces generated by particle packet exhaust remain below the mechanical stresses encountered in existing high-power electric propulsion systems. 8 6.4 Summary of Feasibility The Astra Drive uses only established physics and leverages rapid advances in compact particle accelerator technology. As soon as high-repetition particle accelerator modules reach stable commercial form, the Astra Drive becomes a practically deployable near-term propulsion architecture. 7 Discussion 7.1 Scientific Implications The Astra Drive provides: 1. A propulsion method independent of propellant mass. 2. A path to multi-thousand km/s cruise speeds. 3. A scalable architecture where performance grows with available power. 7.2 Engineering Constraints Key challenges include: •waste-heat dissipation from high-power particle accelerators •radiation shielding •beam divergence minimization •long-term particle accelerators stability Despite these, no fundamental physics barrier prevents the system from operating. 9