SLPA Stage 4 — The STIP Thruster (Stacked Thermal Input Propulsion)

Stage 3 established the thermal core as the heat reservoir that drives SLPA, decoupling energy generation from propulsion. Stage 4 introduces the STIP thruster as the mechanism by which that stored heat is converted into controlled thrust. Rather than relying on a single propulsion mechanism, the STIP thruster uses a staged energy architecture—layering thermal, chemical, and electrical processes—to transform heated working gas into propulsion. This approach defines a new class of thruster: modular, multi-mode, inherently multifuel, and designed to scale through replication rather than redesign. Because the STIP thruster is defined by how energy is staged and applied, rather than by a specific fuel or power source, it remains compatible with a wide range of working gases and energy inputs. As with classical steam propulsion, which was inherently compatible with in-situ resource use, the STIP architecture supports ISRU by design and can incorporate additional energy layers in the future as new coupling methods or power sources become available—while still remaining within the same STIP class.

1. What Is a STIP Thruster?

A STIP thruster is a panel-integrated, multi-mode gas propulsion system that converts stored thermal energy into thrust through staged expansion. In its simplest form, it operates as a pure thermal thruster using heated working gas. Higher-performance modes introduce controlled chemical augmentation and electrical or plasma excitation within the same thruster geometry, allowing thrust, efficiency, and operating regime to be varied without changing hardware.

Each STIP propulsion panel integrates:

• a gas manifold supplying one or more working gases,
• a sun-facing outer skin with embedded thermal and/or gas channels,
• multiple high-temperature micro-thruster tubes,
• isolation and control valves allowing individual or grouped tube operation
• a controllable thermal interface to the spacecraft hot-gas bus and thermal core.

In this configuration, the spacecraft hull itself becomes a functional propulsion element—simultaneously acting as a solar collector, a controllable radiator, and a scalable thrust array.

2. Mode 1 — Basic Gas Thruster (Emergency / Station-Keeping)

This is the simplest configuration:

• Gas: CO₂, steam, or other inert working gases,
• Heating: none beyond ambient panel temperature,
• Performance: low Isp, low thrust,
• Purpose: station-keeping, attitude control, emergency thrust.

This mode is fail-safe: as long as gas exists, the ship can still manoeuvre.

3. Mode 2 — Sun-Warmed/Solar Panel Warmed Gas (Black-Body Panel Mode)

In Mode 2, the STIP panel itself acts as the heater or can use a solar panel with resistors built into the panel. The outward-facing surface is treated as a dark, high-emissivity coating that absorbs sunlight like a black-body. Shallow gas channels just beneath this surface pick up the stored heat before the gas enters the STIP tube.

Gas path:

• Gas bottle,
• Sun-warmed panel channels (black-body surface under illumination),
• STIP tube,
• Exhaust.

This improves exhaust velocity, with performance limited by:

• available panel area,
• surface temperature rating,
• structural and material limits.

Mode 2 is used for fine manoeuvring, short burns, and cases where it is desirable to conserve core heat.

When the thermal link to the core is partially opened, the same panel channels can be fed with heat from the hot-gas bus, providing mild pre-heating even for “soft” manoeuvres. When the link is fully opened and the ship is oriented to minimise new solar input, the panels act as radiators: excess core heat is conducted to the outer skin and radiated to space, giving a built-in thermal control mode in addition to propulsion.

4. Mode 3 — Thermal Core Coupled Mode (Primary SLPA Propulsion)

This is the main propulsion mode.

Gas path:

• Gas bottle,
• Hot CO₂/steam bus,
• Thermal core channels,
• High-temperature gas to STIP tubes,
• Expansion and thrust.

Advantages:

• Much higher heating capacity than panel-only modes,
• Independent of momentary sunlight,
• Supports long burns,
• Ideal for inner-system transfers.

Core temperatures in this mode typically range from 600–1200 °C depending on material. This is the default long-range SLPA mode.

5. Mode 4 — Stacked Thermal + Chemical Heating

Mode 4 adds a controlled chemical heating stage on top of thermal heating.

Gas flow:

1. Thermal core stage – gas is heated in the core.
2. Mixing stage – small, controlled injections of H₂ and O₂.
3. Chemical heating stage – micro-combustion or dissociation raises temperature further.
4. STIP expansion stage – high-energy gas expands through the tubes to generate thrust.

This stacked approach reaches higher temperatures than thermal-only operation and is used for medium/high-Isp burns, planetary transfer arcs, and high thrust-to-mass manoeuvres.

6. Mode 5 — Hybrid Thermal–Chemical–Electric Plasma Mode

Mode 5 combines thermal, chemical, and electrical inputs to achieve the highest specific impulse.

Process:

1. Thermal core preheating – gas absorbs hundreds of degrees of heat from the core.
2. Chemical pre-conditioning – micro-dosed H₂/O₂ injection produces partial combustion or dissociation, raising temperature and creating radicals that reduce ionization energy.
3. Electrical ionization – the hot, partially dissociated gas enters the STIP plasma region where electrical power is applied to ionize it.
4. Plasma expansion – ionized gas expands through the nozzle region, achieving very high Isp.

This staged approach:

• reduces the electrical power needed to reach plasma temperatures,
• allows smaller or more distant solar arrays,
• enables plasma operation on CO₂, steam, or mixed gases.

Mode 5 is suited to long-duration interplanetary cruise and high-efficiency transfer trajectories.

7. Parallel vs Sequential Tube Firing

STIP tubes can be operated in different patterns:

• Parallel firing – many tubes fire together to provide high thrust for departure burns and major manoeuvres.
• Sequential firing – tubes fire one at a time or in small groups, handing off heat load along the panel. This limits peak temperature in each tube and supports long continuous burns in high-temperature modes.

This flexibility gives STIP excellent thermal resilience and operational control.

8. Hull-Integrated Modularity

STIP panels are integrated directly into the spacecraft hull:

• no large external engine bells,
• no complex gimbals,
• scalable from small tugs to heavy transports,
• built-in redundancy through many microthrusters,
• modular replacement or upgrade of panels.

The spacecraft effectively becomes a distributed propulsion surface combined with a distributed solar and thermal system. In radiator mode, the same panels provide controlled heat rejection from the thermal core and internal volume.

Stage 4 Summary

The STIP thruster is the engine of the SLPA architecture:

• Mode 1: cold-gas emergency and station-keeping,
• Mode 2: sun-warmed panel heating with optional core coupling and radiator mode,
• Mode 3: thermal core coupled propulsion,
• Mode 4: stacked thermal + chemical heating,
• Mode 5: hybrid thermal–chemical–electric plasma mode.

STIP replaces the traditional external rocket engine with a multi-mode, hull-integrated propulsion and thermal-control system using simple gases and stored solar heat, spanning low-power manoeuvring up to high-Isp deep-space cruise.