Research Progress and Preliminary Scheme of Space Transportation System for Human Mars Exploration

WANG Xiaojun,WANG Xiaowei

China Academy of Launch Vehicle Technology,Beijing 100076

Abstract:Human Mars exploration has significant values in terms of exploring extraterrestrial life,interplanetary immigration,promoting science and technology development,and the progress of human society.This paper summarizes the research progress on human Mars exploration and the corresponding proposed space transportation systems in the world,and analyzes the development trends.A preliminary scheme including the human Mars exploration mission architecture and corresponding space transportation system is then proposed,and the related key technologies are identified.The results can be a reference for future further research.

Key words:human Mars exploration,space transportation system,research progress,preliminary scheme

1 INTRODUCTION

As human society develops further,deep space exploration technology and the human Mars exploration mission becomes more and more attractive.Since the USSR sent the first robotic probe to Mars in 1960,47 robotic Mars exploration missions had been conducted,and numerous data has been obtained from these explorations.In fact,concerning the human Mars exploration,the US has conducted specific studies and proposed many schemes.The US government has announced they will send humans into Mars orbit and return safely to Earth by the mid-2030s.Human Mars exploration has significant values for the investigation of alien life,interplanetary immigration,developing science and technology,and promoting the development of human society.As part of the basic infrastructure,the space transportation system required is an important factor related to risk,complexity and cost of the human Mars exploration mission,and thus the relevant technologies should be studied.

2 CURRENT STATUS OF RESEARCH

Table 1 lists human Mars exploration proposals or programs in the world.Some typical programs are further analyzed in the following.

Table 1 Human Mars exploration proposals or programs in the world

2.1 The US

The NASA Mars Design Reference Missions (DRM) consisted of a series of conceptual design studies for human Mars missions,conducted in the 21st century,including DRM1.0,DRM2.0,DRM3.0,DRM4.0,DRA5.0 (Design Reference Architecture 5.0).The latest DRA5.0 proposes SLS launch vehicles,Orion spacecraft,and an interplanetary transportation system with nuclear thermal propulsion.The complete exploration mission assumes about 900 days,of which the duration on the surface of Mars assumes about 550 days.

As shown in Figure 1,the features of the DRA5.0 architecture are as follows:LEO departure,cargo and crew separation,several launches and several rendezvous in orbit,conjunction-class long-surface-stay mission,nuclear thermal propulsion,plus In-Situ Resource Utilization (ISRU).

Figure 1 Sketch of DRA5.0 mission framework [7]

Figure 2 The sketch of Mars transfer vehicle in DRA 5.0 [9]

Both cargo and crewed Mars transfer vehicles use nuclear thermal propulsion and three 11.3 t nuclear thermal engines.The cargo vehicle would have a launch mass of 246.2 t with a payload mass of 103 t.The crewed vehicle would have a launch mass of 356.4 t with a payload mass of 62.8 t.

The Mars Descending and Ascending Vehicle (MDAV) would use LOX/methane as main propellants and two configurations including horizontal and vertical were analyzed.The horizontal configuration can be used for hard shell based aerocapture,while the vertical configuration can be used for inflatable hypersonic aerocapture.

The Trump administration continued the plan aimed to conduct human Mars exploration in the 2030s with three distinct phases and proposed to return to moon by 2024,pushing the Artemis lunar program.NASA says it will use the Artemis lunar program in combination with the Lunar Gateway as stepping stones to make great scientific strides“to take the next giant leap-sending astronauts to Mars”.

In 2016,Lockheed Martin proposed the Mars Base Camp(MBC) Plan aiming to send astronauts into Martian orbit as early as 2028.The MBC plan mainly assumes current technologies(SLS and Orion) and multi-launch configuration to assemble a station in Martian orbit,after flying to Mars,capable of sending six astronauts into Martian orbit for a long-duration mission.

The features of the MBC architecture are as follows:Cis-lunar orbit departure,cargo and crew separation,on-orbit assembly,solar electric and LH2/LOX propulsions,refueling on orbit and water electrolysis.

The MADV would be fully reusable.Any hardware that is required to be removed and replaced between MADV missions would be swapped by the MBC crew using hardware that can be carried on MBC.

In 2016,SpaceX proposed its idea of Martian colonization and the concept of an Interplanetary Transport System (ITS).The latest proposed transportation scheme would utilize the Starship and Super Heavy system with plans for a maiden flight in 2021,human lunar journey in 2022 and human Mars exploration in 2024.

As shown in Figure 5,the mission possesses features including LEO departure,integrated crew and cargo transportation,propulsion based on LOX/methane,on-orbit refilling,aerocapture,and ISRU.

Figure 3 Sketch of human Mars exploration mission framework based on MBC [15]

Figure 4 Sketch of MBC [18]

Figure 5 Sketch of human Mars exploration mission by SpaceX [18]

Figure 6 The variation of Starship and Super Heavy system [18]

The Starship and Super Heavy system will have a take-off weight of 5000 t,capable of sending 100 t payloads or 100 persons to Martian surface.The system will have two stages and would be fully reusable,with a total length of 118 m.The first stage would have a diameter of 9 m,using 37 LOX/methane Raptor engines,while the second stage would have a length of 50 m,using 6 LOX/methane Raptor engines.

2.2 Russia

In 2005,Russia S.P.Korolev Rocket and Space Corporation Energia formulated the Concept for the Development of Russia’s Manned Space Development 2006—2030,planning human Mars exploration in 2025—2030 and aiming at an astronaut activity on Martian surface with a duration of 15—30 days.The Mars probe was reusable and would be maintained in LEO comprising a Mars orbiting spaceship,a take-off/landing spacecraft,a rescue ship to Earth,electric propulsion engines and a solar tug.

As shown in Figure 7,the mission would have the features including LEO departure,integrated crew and cargo transportation,on-orbit assembly,short-duration stay,electrical propulsion,and utilize engine brake.

Figure 7 Sketch of mission flight profile [32]

Figure 8 Mars probe [32]

The mission proposed would use 20 Proton launch vehicles to send the components of the Mars probe to LEO,where it would be assembled into a comprehensive system.After that,a crew of 4 persons would be sent to the Mars probe.The Mars probe would have an initial weight of 480 t.The take-off/landing spacecraft would have a weight of 35 t and a 4-person crew.The probe system would spend 2.5 years flying to Mars and returning to Earth.The Mars probe would require about 400 electric propulsion engines,each of which would have a thrust of 70 N.

The Mars probe would fly in LEO for a period of time,and after the necessary acceleration,it would depart from Earth orbit and fly to Mars.Once the Mars probe enters the Martian orbit,it would brake and finally separate the take-off/ landing spacecraft (TLS) carrying astronauts.After landing on Mars,the astronauts would work on the surface of Mars for 2 to 4 weeks,and then return to the Mars probe in near Martian orbit.When returning to the Earth,the Mars probe would enter the heliocentric orbit,traverse the orbit of Mercury twice,chase the Earth,brake and enter the Earth orbit,and finally return to the Earth.

2.3 Europe

In 2009,ESA announced the Aurora Programme,realizing its human mission to Mars (HMM),which plans to send six astronauts in 2033 on a 963-day mission including a 30-day stay on the Martian surface.

HMM is more conservative,and reflects the idea of mission simplification and selection of mature technologies throughout the scheme.The utilization of mature chemical propulsion modes and existing launchers makes the initial mass in LEO (IMLEO) larger and requires a long assembly time in LEO.An integrated spacecraft system that is fully assembled in LEO is proposed in LEO to avoid the risk of rendezvous and docking near Mars.To reduce the mission scale,a mission of only 30-days on the surface of Mars is proposed;half of the astronauts land on Mars;and only a long-term habitat will be considered during the mission.The engine will be used to decelerate avoiding the risk of a Martian atmosphere assisted orbit transfer.

As shown in Figure 9,the mission features include LEO departure,integrated cargo and crew transportation,on-orbit assembly,short-term human spaceflight,chemical propulsion based on LOX/LH2+traditional propulsion,and engine based braking.

Figure 9 Diagram of HMM mission phases [34]

The transfer vehicle (TV) proposed in the HMM mission will be composed of a transfer habitation module (THM) and a propulsion module (PM).As shown in Figure 10,a complete PM will be composed of a Trans Mars Injection (TMI) system,a Mars Orbit Insertion (MOI) system and a Trans Earth Injection(TEI) system.Each system will be abandoned after use.

Figure 10 The propulsion modules of the TMI,MOI and TEI systems [34]

Figure 11 A typical mission architecture of human Mars exploration

2.4 China

On April 24,2020,the China planet exploration mission was named as the Tianwen Series,and the first Mars probe was named as Tianwen 1.On July 23,2020,a LM-5 launch vehicle was launched at the Wenchang Launch Center,conducting Tianwen 1,China’s first Mars exploration mission.

The Chinese government has not published any specific plan for human Mars exploration,but it regards the first Mars exploration mission as the starting point and the focus,laying the foundation for human Mars exploration.GUO Hailin et al.analyzed the flight principles and launching time for Mars exploration missions.Harbin Institute of Technology has studied the design and optimization of interplanetary low-thrust transfer orbits for years.CUI et al.investigated the optimization of Earth-Mars transfer orbit and proposed the nominal orbit optimization model,upon which the TPBVP problem is further solved by a mixed optimization model combining the Pontryagin minimum principle and SQP method.ZHANG Wenbo et al.optimized the round orbit between Mars and Earth between 2031—2045 via two orbit maneuver strategies.LI Zhen et al.found that aerocapture is maneuverable and has advantages for energy requirements compared with brake based direct flight into orbit,and thus it is a feasible method to enable human spacecraft to fly into the orbit.

ZHOU Xudong et al.proposed a conceptual complete round Earth-Mars flight mission scheme including orbit and spacecraft scheme,taking into account velocity increment and flight duration as the key parameters for orbit design.ZHU et al.proposed a mission scheme for China to conduct human Mars exploration in 2033,where the launching window,orbit and flight process were analyzed,and the key technologies are identified.HONG et al.designed a mission architecture featuring 8 launches,5 LEO dockings and crew/cargo separated transportation and proposed a system scheme with 450 kNthrust nuclear thermal engine propulsion system.The architecture makes full use of the advantages of nuclear thermal propulsion in the light of its high specific impulse,high energy transfer efficiency and low risk.WANG et al.studied nuclear thermal propulsion systems for future human Mars exploration and large cargo transportation.The study compared and analyzed three types of nuclear thermal engines.The study results can be the reference for choosing future nuclear thermal engines.In addition,China has been developing a heavy launch vehicle and plans to complete this by 2030 under the current schedule.

In general,human Mars exploration has been one of hot research items in China’s space sector.Various mission modes,Mars transfer trajectory design,propulsion technologies and system designs have been studied.Meanwhile,based on China’s LM-5 and super heavy-lift launch vehicle,a preliminary space transportation system with nuclear thermal propulsion has been proposed,possessing features including separate cargo and crew transportation,on-orbit assembly,conjunction/opposition trajectory,nuclear thermal/chemical propulsion,propulsion brake+aerocapture.

2.5 Development Trend Analysis

1) Human Mars exploration has been one of the next targets for the space capable nations and relevant studies and schemes have been proposed.The US government has officially announced that they would conduct human Mars exploration mission in 2030.

2) The plan for each country is based on its own launch technology capability and the flying mode is diverse.Specifically,NASA’s DRA would utilize separated cargo and crew transportation,and advanced nuclear propulsion technology;ESA and Russia propose mature and a conservative integrated cargo/crew transportation and chemical/electric propulsion scheme.Some scholars in China also proposed nuclear thermal transfer stages based on super heavy-lift launch vehicles.

3) Relevant studies show that the complexity,system scale and task safety are greatly influenced by factors including mission architecture design,propulsion technology and departure point.Traditional chemical propulsion is theoretically capable for human Mars exploration,but it faces many challenges including huge scale,multiple docking,complex mission,and high risks.Nuclear propulsion has a high specific impulse and thus can greatly reduce the system scale and mission complexity,but relevant key technologies require breakthroughs.

3 AN ARCHITECTURE AND SPACE TRANSPORATION SYSTEM SCHEME FOR HUMAN MARS EXPLORATION

This section presents a typical human Mars exploration mission architecture featuring nuclear propulsion technology,separated cargo/crewed transportation and high Earth orbit (HEO)departure.The flight profile is as follows.

1) A ferry stage (FS) to LEO using a launch vehicle.Then,components for the cargo transfer stage (TS) and payloads (MADV and infrastructure on Martian surfaces)are sent into orbit by multiple launches and are assembled into a cargo TS.

2) After transferring the cargo TS into HEO,the FS returns to LEO and waits for on-orbit refueling.

3) After entering into Earth-Mars transfer orbit,the cargo TS carrying the necessary payload reaches a Martian orbit by aerocapture and waits for crew arrival.Then,the surface infrastructure will be landed on the Martian surface and begin to produce fuel.

4) The FS is refueled and waits for another ferry mission.

5) Components of crewed TS and deep space habitat(DSH) are sent into LEO by launch vehicles and assembled into a crewed TS,which is then transferred by FS to HEO.

6) Once the goods on Mars and the crewed TS in HEO operate well and the open window approaches,the crewed spaceship will be directly sent to HEO by a crewed launch vehicle and docked with the crewed TS in HEO.Then,the crewed TS will transport the crew to enter the Earth-Mars transfer orbit.

7) The crewed TS enters a Martian orbit via braking and docks with the MADV.The crew will then enter the MADV for entering the Martian atmosphere,landing on the Martian surface to conduct work.

8) After finishing working on the Martian surface,the MADV will be refueled.Then,the crew enters into the MADV and returns into Martian orbit.

9) The MADV is docked with the crewed TS in Martian orbit and the crew enters the habitat.

10) The crewed TS enters Mars-Earth transfer orbit,returning to Earth,and the crewed spaceship conducts re-entry and lands on Earth.

The scheme makes a trade-off between mission safety and system scale.Table 2 analyzes the scheme features,providing a reference for further studies.

To accomplish the mission,a detailed scheme of the space transportation system for human Mars exploration is given in Table 3.

Table 2 Analysis of mode and feature of the proposed architecture

Table 3 Introduction of the transportation system

4 KEY TECHNOLOGIES

The human Mars exploration is a complex mission.Compared with traditional launch vehicles,the required transportation system needs a relay flight of multiple launchers,featuring long distance flight,long transfer times,large scale,multiple on-orbit assemblies,all in a harsh flight environment,which brings many new challenges in system design,propulsion,GNC,power and communications,and system structure.Table 4 lists the relevant key technologies.

Table 4 Key technologies of space transportation system of human Mars exploration

5 CONCLUSIONS

1) Human Mars exploration is significant and is an inevitable trend.Plenty of studies have been made around the world,yet international coordination and collaboration is required to make full use of each others advances and to send humankind to Mars as soon as possible.

2) Human Mars exploration involves many factors and has many challenges,so the system scheme should be conducted based on comprehensive study and a trade-off between mission safety,system complexity and system scale.The proposed mission architecture and preliminary scheme for the space transportation system provides an optional solution.

3) The space transportation system for human Mars exploration are much more complicated compared to traditional launch vehicle,and the proposed scheme in this paper includes traditional launch vehicles,ferry stage,transfer stages,and Mars Descending and Ascending Vehicle.Lots of new key technologies need to be solved,involving System design,propulsion,GNC,power and communication and system structure.

4) The study of human Mars exploration brings a unique global value and the relevant studies will definitely benefit all humankind.We hope that our counterparts collaborate with each other and move to together for the future.