ISS Propulsion Module

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ISS Propulsion Module (NASA)

The ISS Propulsion module was proposed as a backup to functions performed by the Zvezda Service Module and Progress spacecraft. Critical ISS functionality such as guidance, navigation, control and propulsion are provided only by Russian (Zvezda and Progress) and the European (ATV) spacecraft.[1] A Propulsion Module would have been needed for ISS altitude maintenance and reboost, debris avoidance maneuvers, attitude control and propellant supply in the event the Zvezda Service Module was not available (launch failure, etc.) to the International Space Station. If the Zvezda had not been available, the Interim Control Module would have been used at first. It only had a lifetime of three years; then the Propulsion Module would have been necessary.

The Propulsion Module was ultimately cancelled and never built. Instead, the ISS has relied on a combination of visiting vehicles for propulsion and reboost capabilities, including Russian Progress spacecraft, the European ATV (2008-2014), and Northrop Grumman Cygnus spacecraft (since 2013).

History

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The ISS requires an average 7,000 kg of propellant each year for altitude maintenance, debris avoidance and attitude control. A Propulsion Module would have provided reserve propellant for one year of ISS orbit life in case of supply interruption. A Propulsion Module would have been attached to the Unity node of the ISS.

Multiple supply vehicles are required to satisfy the ISS's 7,000 kg annual average propellant need. The then-current plan for six Progress M1 spacecraft per year met that need.

The Propulsion Module would hold 9,808 kg fuel. Progress M holds 1,100 kg; Progress M1 holds 1,950 kg. ESA ATV holds 4,000 kg. The cancelled U.S. Interim Control Module holds 5,000 kg of fuel. A Shuttle Orbiter ISS generic reboost had 232 kg of fuel available. An Orbiter Max reboost mission had 1,626 kg of reboost fuel available. Zarya FGB [Russian: ФГБ - Функциональный Грузовой Блок, English: Functional (or Operational) Cargo Block (or Module)] holds 5,500 kg and the Zvezda Service module holds 860 kg, however, it is generally preferred to keep the main thrusters on Zarya and Zvezda in reserve, as they have finite lifespans.

The Propulsion Module, besides being part of the backup plan for if the Service Module was not available, was intended to be an American-owned propulsion system for the station and planned as a late addition. However, the original design was over budget and late. An alternative design, the "Node X" design, which was built around an improperly fabricated hull intended for Node 2 or 3, was then proposed and tentatively included in the plan, designed with two detachable fuel modules that could be carried up and down in a Shuttle cargo bay for replacement to avoid the problems of transferring propellant between tanks in space. It has subsequently been deleted from the plans, instead requiring the station to rely on the ATV and Progress spacecraft for reboost. Furthermore, in the wake of the Shuttle's retirement, further redesign of the Propulsion Module would have been necessary.

Current ISS Propulsion Systems

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Rather than building the Propulsion Module, the ISS has successfully operated using visiting spacecraft for propulsion and reboost functions. As of 2024, several spacecraft provide these capabilities:

Russian Progress Spacecraft

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Progress spacecraft remain the primary method for ISS reboost operations. The current Progress MS variant (since 2015) can carry approximately 1,950 kg of propellant and performs regular reboost maneuvers.[2] Recent missions demonstrate typical operations:

  • February 2024: Progress 87P performed an 18-minute 25-second burn with a delta-V of 1.725 m/s[3]
  • June 2024: Progress 87P conducted a 23-minute 25-second burn with a delta-V of 2.0 m/s[4]

Progress missions typically occur every 2-3 months, with about four to six spacecraft per year providing both cargo delivery and propulsion services.

European ATV (Retired)

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The Automated Transfer Vehicle (ATV) operated from 2008 to 2014, providing 4,000 kg of propellant capacity for ISS operations. The ATV demonstrated record-setting reboost capabilities, including a 20-minute burn that raised the ISS altitude by approximately 7 km with a 4.05 m/s delta-V.[5] Five ATV missions were completed before the program ended with ATV-5 Georges Lemaître in February 2015.

Northrop Grumman Cygnus

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Cygnus spacecraft have been servicing the ISS since 2013 and possess limited reboost capability. While primarily cargo vehicles, Cygnus can perform altitude maintenance and orbit adjustment maneuvers using IHI BT-4 thrusters that provide 450 N of thrust.[6] Current Enhanced Cygnus variants can carry up to 3,500 kg of cargo and remain docked for extended periods.

Other Visiting Vehicles

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  • SpaceX Dragon: While Dragon 2 spacecraft (both Crew and Cargo variants) have 46 Draco thrusters, they are not typically used for ISS reboost operations[7]
  • Russian Soyuz: Primarily used for crew transport with limited propulsion capability for ISS operations
  • Future vehicles: Sierra Space Dream Chaser and JAXA HTV-X are in development to provide additional cargo and potential propulsion services

ISS End-of-Life Planning

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SpaceX US Deorbit Vehicle

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In June 2024, NASA awarded SpaceX an $843 million contract to develop the United States Deorbit Vehicle (USDV) to safely deorbit the ISS at the end of its operational life around 2031. The USDV represents the ultimate solution to ISS propulsion needs for the station's final mission.[8]

The USDV specifications include:

  • Based on SpaceX Dragon spacecraft with enhanced capabilities
  • 46 Draco thrusters (compared to 16 on standard Dragon)
  • 30,000 kg total mass including 16,000 kg of propellant
  • Enhanced trunk section twice the length of standard Dragon trunk
  • Capability to provide approximately 57 m/s delta-V for controlled deorbit[9]

The USDV will dock to the ISS approximately 18 months before deorbit and remain dormant while the station naturally decays to 220 km altitude, then perform the final deorbit burn to ensure safe disposal in a remote ocean area.

Technical Specifications Comparison

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ISS Propulsion Capabilities (Historical and Current)
Vehicle Propellant Capacity Thrust Capability Operational Period Status
Proposed ISS Propulsion Module 9,808 kg N/A Permanent Cancelled
Progress MS 1,950 kg 300 N (main thrusters) Per mission Active
ESA ATV 4,000 kg 490 N (main engines) Per mission Retired (2014)
Cygnus Enhanced Limited 450 N (BT-4 thrusters) Per mission Active
SpaceX USDV (Planned) 16,000 kg 46 × Draco thrusters Single use In development
Zvezda Service Module 860 kg 300 N (main thrusters) Permanent Active (reserve)
Zarya Module 5,500 kg N/A Permanent Active (reserve)

Current Operations

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As of 2024, the ISS maintains its orbit through regular reboost operations averaging once per month, with atmospheric drag causing the station to lose approximately 100 meters of altitude per day. The station's current operational altitude is approximately 400-420 km, and reboost operations typically provide 1-4 m/s delta-V per maneuver.[10]

The Russian segment continues to control all propulsion operations, with Progress spacecraft docked at the Zvezda aft port serving as the primary propulsion system. Cygnus spacecraft provide supplementary reboost capability when docked, though this is used less frequently than Progress operations.

Future Outlook

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The cancellation of the ISS Propulsion Module proved to be the correct decision, as the combination of visiting vehicles has successfully maintained ISS operations for over two decades. The current system provides:

  • Redundancy through multiple vehicle types
  • Regular resupply of propellant and supplies
  • Cost effectiveness compared to a dedicated permanent module
  • Operational flexibility for various mission requirements

NASA's ISS operations are planned to continue until 2030-2031, after which the station will be safely deorbited using the SpaceX USDV. Commercial space stations currently under development by companies including Axiom Space, Blue Origin, and Sierra Space are expected to provide continuity for low Earth orbit operations.

See also

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References

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  1. ^ "ESA - Human Spaceflight and Exploration - International Space Station - DMS-R: ESA's Data Management System for the Russian Segment of the ISS". Esa.int. 2009-01-16. Retrieved 2012-01-26.
  2. ^ NASA. "45 Years Ago: Progress 1 Begins the Era of Space Station Resupply". September 2023.
  3. ^ NASA. "ISS Daily Summary Report – 2/26/2024". February 2024.
  4. ^ NASA. "ISS Daily Summary Report – 6/17/2024". June 2024.
  5. ^ ESA. "Record boost for ATV to raise ISS orbit". 2008.
  6. ^ eoPortal. "ISS: Cygnus". 2024.
  7. ^ Wikipedia. "SpaceX Dragon 2". January 2025.
  8. ^ SpaceNews. "Enhanced Dragon spacecraft to deorbit the ISS at the end of its life". July 2024.
  9. ^ Space.com. "SpaceX has dreamed up a Dragon ship on steroids to drag the ISS out of space". July 2024.
  10. ^ Space Exploration Stack Exchange. "How often does ISS require re-boosting to higher orbit?". 2024.
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