Disruptive Propulsive Technologies

If and how interplanetary trips may be shortened?


Missions involve high speed increments beyond the capability of chemical propulsion


SPACE PROPULSION APPLICATIONS

In a long-term vision, the question becomes, what will be future manned interplanetary missions, and what is needed?

NASA and Roskosmos are looking at ‘far’ destinations, i.e., not the Moon,

although the Moon is still seen by most space planners an indispensable stepping stone to Mars and near Earth objects or asteroids (NEO or NEA)

Power Conversion - In principle Stirling Cycle is an attractive option for 30kWe, but there are doubts.

Direct ICR Closed Brayton Cycle (CBC) was narrowly preferred for 30kWe and accepted as an option for 200kWe for good efficiency,

simplicity of design, no freezing of reactor cooling and turbo-alternator operating gas.




Disruptive Propulsive Technologies for European Space Missions  


- the question is how interplanetary trips may be shortened -



NASA favors, or favored, NEO missions as less expensive than going to the Moon, since no lander is needed (currently, no space agency is planning to build a man-rated Moon lander), but the life support system for a NEO mission does not exist yet, and, in fact, NASA is in the process of reassessing NEO vs. Moon goals. Such issues are more and more influenced by the growing consciousness of the health risks posed to crews by solar and cosmic-galactic radiation (SR and GCR).


 These risks are still difficult to quantify, but caution suggests to shorten interplanetary missions, e.g., to Mars as much as feasible. In turn, this means discarding conventional Hohmann trajectories (minimum energy trajectories). A Hohmann-type, To reduce dose there are two logical solutions: either to reduce flux to crew by some form of shielding, or to shorten exposure time (faster missions).

Interplanetary orbits - Any way to speed up interplanetary travel pays off in terms of astronaut’s safety. We have some understanding of the effects of microgravity on bone mass and metabolism, but we are just now beginning to explore the psychological consequences of crew confinement inside cramped quarters for many months (the recently completed Mars 500 ground experiment is the first example).

The question at this point becomes of if and how interplanetary trips may be shortened. Here this question is discussed by using basic physics, leaving details (but no show stoppers) to future engineering. In this context the basic physics of interplanetary travel is presented below, starting from what is current practice.

In fact, all present or planned mission are dominated by the key constraint to save mass to be orbited. Anything to be used for space missions must first be orbited, at a cost, for LEO, roughly between 10 and 20 k$/kg, depending on launcher provider. With CP, most of the mass is propellants, see Eqn (1). Physics tells us that once in LEO, the minimum energy (or the minimum ΔV) required to change orbit, for instance to go to Mars, is that of Hohmann trajectories. These are ellipses osculating the starting orbit (for instance, that of the Earth around the Sun) and the final one (for instance that of Mars), having assumed the two are on the same ecliptic plane. Hohmann trajectories require the least expenditure of energy, thus the least expenditure of propellants, and are realized by applying thrust (F) for a time Δt << of the total interplanetary travel time T. In the limit Δt /T  0 thrust becomes impulsive, and it is this feature that minimizes energy losses due to non-parallel spacecraft F and instantaneous velocity V. If F is impulsive, F and V are parallel, and all thrust is used to increase V by an assigned ΔV. It is always possible for a spacecraft to change its orbit by using slow acceleration (Δt /T finite  low F); however, in significant gravitational fields such as Earth’s, the trajectory becomes initially a spiral where F and V form an angle and thrust work is partly wasted. Low thrust (‘spiraling’) losses may be relatively large, e.g., in a low thrust trajectory from LEO to L1 (theoretical ΔV about 4 km/s) the actual ‘expenditure’ of ΔV may become about 6 km/s; see [Martinez-Sanchez and Pollard, 1998] for a LEO to GEO quantitative example.


Advanced space technologies have been reviewed and analysed in view of heavy interplanetary missions of interest for Europe and European industry capabilities. Among the missions of interest:


o Heavy robotic missions to outer planets,

o Asteroid deflection missions,

o Interplanetary manned mission (at longer term).


These missions involve high speed increments, generally beyond the capability of chemical propulsion (except if gravitational swing-by can be used). For missions beyond Mars orbit, the fission nuclear energy sources become competitive with solar panels.

Two electrical power levels have been considered: 30 kWe and 200 kWe. The lowest power level (30 kWe) is more suited to surface energy source (Moon or Mars manned base) or to relatively small automatic platforms. The 200 kW power level is more suited to heavy robotic missions, including efficient asteroid deflection.

Nuclear Thermal Propulsion (NTP) has been also considered, especially for asteroid deflection. NTP may be compatible with late detection acting by direct impact.

The public acceptance of these new technologies has been analysed, showing the necessity to provide safe ground testing facilities as well as a mission scenario excluding re-entry of an activated space nuclear reactor.

So far the ‘Global Exploration Strategy’ [1] has focussed on a roadmap to explore the inner solar system ultimately aspiring to crewed missions to the Moon and Mars. It is recognised that the next step will be the exploration of the outer solar system and beyond. In particular, manned, missions require significant power for propulsion, to maintain a survivable habitat and to conduct useful operations at their destination. Increasing use is made of electrical power for propulsion, exploiting the very high specific impulse achievable to limit propellant mass to manageable quantities. Within the inner solar system this power can be mostly generated by solar arrays. In the outer solar system nuclear power remains the only practical means of generating the very high power levels identified in mission analysis to deliver significant payload within acceptable timescales [2].

Nuclear power is recognised [3] as a key enabling technology for the Global Exploration Strategy. High power generation is one of the fundamental capabilities which are a common essential requirement for both inner and outer solar system exploration. Mission analysis has consistently demonstrated that nuclear electric propulsion is an enabling technology, for instance for a sample return mission to a Jovian moon or to put a spacecraft into orbit around Neptune. More recently, in the HiPER project [5], mission analysis also identified that space nuclear power generation could benefit a wide range of applications. In the longer term, the power available could also be exploited for high power instrumentation and even for asteroid mining.

Propulsion is one of the main users of the higher power nuclear fission applications. In principle space high power propulsion can be met by nuclear thermal (NTP) or nuclear electric (NEP) propulsion technologies. Most recent studies however have focussed on NEP because, although the systems are more complex, the much higher specific impulse achievable makes the very significant reduction in propellant mass very attractive for long duration missions or the higher thrust achievable can lead to reduce significantly the mission's duration for an equal mass of propellant.

In practice, nuclear electric power generation has a wider range of potential applications such as power for habitats on the Moon and Mars.

Radioisotope thermoelectric generators (RTG) or radioisotope heater units (RHU) do not provide power on the scale of fission nuclear power generators and they are therefore not considered further in this paper. Fusion technology is also excluded. It is still too immature in space applications for now.

Nuclear power has been integral to US and Russian space plans for many years and both countries have experience in orbiting nuclear power generators [4]. Activity in this area lapsed during the last decade because of the focus on the inner solar system and funding constraints. However, the interest on NEP has been revived by the Russian MW-class nuclear power and propulsion system (NPPS) [7] concept combined with a heavy launcher capable of lifting 70 to 130 tons LEO payload, and in US in the area of low power range at LANL (Los Alamos National Lab). Recent studies have shown that Ariane 5 ECA and the Atlas 5 heavy launcher could lift higher power generators of power up to about 200 kWe. Together these developments indicate that space nuclear power will increasingly become part of the plans and policies of the major space-faring nations.


Subsequent studies have drawn heavily on the experience from these projects. Sample return mission payloads including a lander and re-ascent vehicle are likely to be several tons in mass. A 6 year round trip to Mars or a 10 year round trip to a Jovian moon, with a year’s stay time in each case, needs tens or hundreds of kWe depending on specific impulse (Isp) and propellant mass used.

The studies have indicated that for higher power levels closed cycle Brayton thermal to electrical power conversion is significantly more efficient. Although, new materials may help raise thermo-electric energy conversion from 5 to say 10%, the 17 to 20% efficiency claimed for the Brayton cycle still brings significant specific mass benefit. The relative simplicity of gas cooled reactors is an advantage in terms of long life, but experience to date has been with liquid metal cooling. Liquid metal cooling is more complex (needs additional pumps and heat exchanger), and requires significantly more energy to heat the coolant and reactor to an operating temperature for commissioning or for ‘cold’ re-starts.

Fixed, body mounted metallic radiators have high mass and area unless the operating cycle temperature can be raised significantly (radiator size varies with temperature to a fourth power law). Deployable radiators are lower mass but larger area and need an additional heat exchanger. There is the added complexity of packaging a large structure for launch and deploying it safely. Lighter materials such as carbon-carbon composites offer new options for fixed radiators.


Technical Options

A review of technical options for a 30 kWe and a 200 kWe nuclear power generator revealed a fair degree of commonality between the two findings.


Design Constraints

Design constraints (identified in HiPER [5]) are:

o Compatibility with an Ariane 5 ECA launch: >800km in-orbit commissioning altitude, radiator compatible with the Ariane 5 fairing, Launch safety criteria (e g water immersion).

o Ten years of operation within a 15 years’ lifetime,

o Specific mass of 25 kg/kWe for a 200 kWe generated power or better,

o High temperature reactor (fast indirect or epi-thermal direct) and conversion system (Brayton cycle),

o Resilience to sudden load fluctuations,


Reactor Technologies

The preferred options were pin-fuel fast reactors for indirect inter-cooled and recuperated (ICR) Brayton because of compact, low mass features or particle-based fuel reactors for Direct ICR Brayton cycle.


Control Systems

The operating principle is ‘load following’ through negative thermal control, accepting a degree of ‘thermal lag’, and containment with beryllium reflectors. Control rods give a more compact, low mass core but control drums require fewer shield penetrations and are simpler to rotate reliably. Both electrical and pneumatic drive should be compatible with the temperature field and high radiation environment (dry lubrication may be required).


Shielding

A layered shadow shield design (Beryllium, Lithium Hydride, Tungsten or with Beryllium Oxide to overcome Lithium hydride thermal expansion sensitivity) was adopted for both 30kWe and 200kWe; Shadow angles were up to 28° and a 22.5m separation boom was planned.


 Both 30kWe and 200 kWe considered turbine rotation of ~ 45Krpm but for 200kWe turbine blade creep life above 1100 K will require new materials. Indirect ICR CBC is an alternative for both 30kWe and 200kWe (more compact reactor but added complexity of liquid metal pumping, and melting liquid metal for commissioning, cold starts, etc).


Radiators

Both fixed and deployable radiators are options for the 200 kW power level. At very high temperature operation, fixed radiators become compact and mass and area competitive. Also, it should be mentioned that Russia is developing a droplet radiator which allows lower system temperatures but requires a large area when deployed.


Electrical generation

The turbo alternator output may be alternate current (AC) or direct current (DC) if rectified. AC may be used to reduce harness mass but there are potential complications in interfacing with EP or a high power instrument and the DC battery.

Power Management and Distribution (PMAD)

Turbo-alternator output could be tailored to EP or radar tube operating voltage, resulting in a reduced PMAD mass.

Battery size, coupling (DC/AC converter) and functions (commissioning, load ballast, etc) shall be studied.


Summary

The selection of CBC Brayton power conversion for both 30kWe and 200 kWe allows a high degree of focus in the technical options. It is also helpful because of the inherent ‘scalability’ of the technology. The main issues to be resolved are the trade-off between liquid metal and gas cooled reactors and the operating temperatures. Materials which allow higher temperature operation for 10 year lifetimes will tend to make the relative simplicity of gas cooled systems more attractive.


Public Acceptance And Dissemination

The importance of preparing public outreach study/material for nuclear space technology to be developed and proposed to EC / Europe was recognised. A similar approach had been used for the Prometheus programme (using the Keystone Centre in Colorado). The recent launch of RTGs and RHUs in the US still attracted small protest groups. It was essential to assemble a team that both understood the technical issues and the public concerns. This included both traditional concern about nuclear dangers and also whether it was a good way to spend government money (the case for private investment did not look strong). The US experience was that the management of public acceptance could be a relatively small part of the budget if tackled early and effectively.

Uranium enrichment was considered necessary to design a sufficiently compact reactor for space. This is one factor why a Public Acceptance assessment study is an early priority task before to take into account the suited recommendations. The fact that at launch the uranium is "new" and so weakly radio-active as shown in fig. 3 with the handling operations of the fuel is a major fact for enabling a positive Public Acceptance of fission reactor for space applications.

Thus important public considerations are safety and that government will spend tax money wisely. The following need to be established:

o Definition of the economics of the technology,

o The need of sustainability: long term output of the technology.

o Good communication: avoid news like “millions burn down on launch pad crash”.


Safety

The safety guidelines for nuclear power sources in space are provided in numerous documents [10].

Space and nuclear safety experts from “big ESA member states” are drafting a technically sound European framework that:

o Provides a predictable, efficient, "workable" process for ESA missions,

o Addresses the main concerns of participating member states,

o Takes advantage of the existing European nuclear safety expertise and experience gained on the subject in US and Russia,

o Provides a technically sound basis for an early decision on processes, roles and responsibilities.


Sustainability

Europe is unlikely to fund enabling research for a space nuclear fission programme until an application (or range of applications) has been identified which is justified in terms of benefit, credibility and cost. It is difficult to determine benefit, credibility and cost until the enabling research has helped to quantify the performance which may be achieved. In this sense, a dual-mode program, to build compact reactors capable of both industrial power generation and space propulsion and power, could free this impasse. A way to start the iterative process would be a workshop [11] for the space science and exploration, space mission and spacecraft design and nuclear communities with a view to:

o Define specific research and development projects, including cost and schedule, to deliver the performance required for the identified science and exploration objectives based on the initial assessments made in the DiPoP project:

- High temperature reactor (including controls) and fuel materials research (potentially in collaboration with Generation IV civil nuclear power development),

- High temperature turbo-alternator materials research to overcome creep life limitations,

- Low mass, high temperature radiator materials (not-porous to helium) research,

- Low mass shielding configurations compatible with high temperature operation and efficient spacecraft architectures

- Mass efficient power management and distribution and associated safety features,

- In-orbit commissioning and end-of-life disposal,

o Identify trade-offs between objectives, performance, technical development, schedule and cost.

o Propose one or more candidate mission analysis to provide a baseline for evolution of the Technical Roadmap (in practice a family of mission analyses would be a sensible investment to establish a range of potential applications and give confidence of a multi-application program).

o Propose a program to achieve public awareness and secure public acceptance for a European space nuclear fission program.

Either ESA or the EC could sponsor a workshop (EC sponsorship is understood to be proposed). The output of the workshop and mission analysis can then provide a basis to determine specific enabling research projects in the EC Horizon 2020 programme and further mission analysis could be sponsored by ESA as part of the General Studies programme.


Conclusions

Past experience indicates that fission nuclear power generation is technically feasible. Subsequent studies indicate the need for significant technical development in Europe to realise the performance identified in the range of proposed applications.

From the range of applications for which space fission nuclear power is potentially necessary initial candidate selections of European missions are:

o Generating electrical services for a remote planetary outpost and high power instrument was selected for 30kWe.

o Earth threatening NEO deflection or outer planet orbital surveying mission for 200kWe fission nuclear electric propulsion. The performance needed for these applications would also support other identified applications.


Closed Brayton Cycle power conversion with either an indirect liquid metal cooled or direct gas cooled fast reactor is selected for both power levels.

Materials research into the high temperature operation (but with non oxidising gas) needed to achieve optimal mass efficiency for space reactors, Brayton turbo-alternators and radiators. Research is also needed into very high power electrical equipment and switching.

A representative review of the capabilities of European government organisations research centres, industry and universities indicated potential expertise and infrastructure for all aspects of a European space nuclear fission programme.

Generation IV civil terrestrial reactor research includes high temperature liquid metal and gas cooled projects ; there are many useful synergies, particularly in associated materials research..

Potential interest in a European space nuclear fission programme was expressed by many of the organisations contacted in the survey and covered all aspects. Evidence of sustainability of the programme is seen as a pre-requisite for both government and industry.


Acknowledgments

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n°284081 for the Disruptive Technologies for Power and Propulsion (DiPoP) Study.

Koppel, Valentian, Blott, Jansen, Ferrari, Bruno, Herdrich, Gabrielli


References

[ 1] The Global Exploration Strategy: the Framework for Coordination, May 2007.

[ 2] HiP-AST-D-2.7-i1r1 HiPER Consolidated Mission Analysis 8th December 2011.

[ 3] IAC-10-A3.1.1 Assessing Space Exploration Technology Requirements as A First Step To-Wards Ensuring Technology Readiness For International Cooperation In Space Exploration by CSA, NASA, ESA and JAXA October 2010.

[ 4] IAA Commission III SG2 Nuclear Space Power and Propulsion Autumn 2007.

[ 5] HiPER Nuclear Power Generation Concept Design HiP-SEP-D-3.9-i1r0 dated 31st May 2011.

[ 6] HiPER Nuclear Power Generator Roadmap HiP-SEP-D3.8-ill0 dated 6th May 2011.

[ 7] Project of Creation of Heavy Spaceship with Megawatt-class NPPS. A. S. Koroteev, V. N. Akimov, C. A. Popov, 2010.

[ 8] European Working Group on Nuclear Power Sources for Space Report, March 2005. Section 6.2.1

[ 9] European Working Group on Nuclear Power Sources for Space Report, March 2005. Section 6.2.3.

[ 10] Principles Relevant to the Use of Nuclear Power Sources In Outer Space, 1992”, Principle 4 ammended by "Safety Framework for Nuclear Power Source Applications in Outer Space", Jointly published by the United Nations Committee on the Peaceful Uses of Outer Space Scientific and Technical Subcommittee and the International Atomic Energy Agency, Authors: JEG (Joint Expert Group of STSC and IAEA), A/AC.105/934, Printed by the IAEA in Austria 2009.

[ 11] Nuclear Power Sources Final Report, DiP-Sep-RP-002 D30.3 Nuclear Power Sources Final report, 2012, available soon on www.DiPoP.eu.

[ 12] International Atomic Energy Agency (IAEA), The Role of Nuclear Power and Nuclear Propulsion in the Peaceful Exploration of Space, Vienna, 2005.

[ 13] MEGAHIT@esf.org