Hello, and welcome back to Beyond NERVA! Today’s blog post is a special one, spurred on by the announcement recently about the Transport and Energy Module, Russia’s new nuclear electric space tug! Because of the extra post, the next post on liquid fueled NTRs will come out on Monday or Tuesday next week.

This is a fascinating system with a lot of promise, but has also gone through major changes in the last year that seem to have delayed the program. However, once it’s flight certified (which is to be in the 2030s), Roscosmos is planning on mass-producing the spacecraft for a variety of missions, including cislunar transport services and interplanetary mission power and propulsion.

Begun in 2009, the TEM is being developed by Energia on the spacecraft side and the Keldysh Center on the reactor side. This 1 MWe (4MWt) nuclear reactor will power a number of gridded ion engines for high-isp missions over the spacecraft’s expected 10-year mission life.

First publicly revealed in 2013 at the MAKS aerospace show, a new model last year showed significant changes, with additional reporting coming out in the last week indicating that more changes are on the horizon (there’s a section below on the current TEM status).

This is a rundown of the TEM, and its YaDEU reactor. I also did a longer analysis of the history of the TEM on my Patreon page (patreon.com/beyondnerva), including a year-by-year analysis of the developments and design changes. Consider becoming a Patron for only $1 a month for additional content like early blog access, extra blog posts and visuals, and more!

TEM Spacecraft

The TEM is a nuclear electric spacecraft, designed around a gas-cooled high temperature reactor and a cluster of ion engines.

The TEM is designed to be delivered by either Proton or Angara rockets, although with the retirement of the Proton the only available launcher for it currently is the Angara-5.

Secondary Power System

Both versions of the TEM have had secondary folding photovoltaic power arrays. Solar panels are relatively commonly used for what’s known as “hotel load,” or the load used by instrumentation, sensors, and other, non-propulsion systems.

It is unclear if these feed into the common electrical bus of the spacecraft or form a secondary system. Both schemes are possible; if the power is run through a common electrical bus the system is simpler, but a second power distribution bus allows for greater redundancy in the spacecraft.

Propulsion System

The Primary propulsion system is the ID-500 gridded ion engine. For more information about gridded ion engines in general, check out my page on them here: http://beyondnerva.com/electric-propulsion/gridded-ion-thrusters/

The ID-500 was designed by the Keldysh Center specifically to be used on the TEM, in conjunction with YaEDU. Due to the very high power availability of the YaEDU, standard ion engines simply weren’t able to handle either the power input or the needed propellant flow rates, so a new design had to be come up with.

The ID-500 is a xenon-propelled ion engine, with each thruster having a maximum power level of about 35 kW, with a grid diameter of 500 mm. The initially tested design in 2014 (see references below) had a tungsten cathode, with an expected lifetime of 5000 hours, although additional improvements through the use of a carbon-carbon cathode were proposed which could increase the lifetime by a factor of 10 (more than 50,000 hours of operation).

Each ID-500 is designed to throttle from 375-750 mN of thrust, varying both propellant flow rate and ionization chamber pressure. The projected exhaust velocity of the engine is 70,000 m/s (7000 s isp), making it an attractive option for the types of orbit-altering, long duration missions that the TEM is expected to undertake.

The fact that this system uses a gridded ion thruster, rather than a Hall effect thruster (HET), is interesting, since HETs are the area that Soviet, then Russian, engineers and scientists have excelled at. The higher isp makes sense for a long-term tug, but with a system that seems that it could refuel, the isp-to-thrust trade-off is an interesting decision.

The initial design released at MAKS 2013 had a total of 16 ion thrusters on four foldable arms, but the latest version from MAKS-2019 has only five thrusters. The new design is visible below:

• 

The first design is ideal for the tug configuration: the distance between the thrusters and the payload ensure that a minimal amount of the propellant hits the payload, robbing the spacecraft of thrust, contaminating the spacecraft, and possibly building up a skin charge on the payload. The downside is that those arms, and their hinge system, cost mass and complexity.

The new design clusters only five (less than one third) thrusters clustered in the center-line of the spacecraft. This saves mass, but the decrease in the number of thrusters, and the fact that they’re placed in the exact location that the payload makes most sense to attach, has me curious about what the mission profile for this initial TEM is.

It is unclear if the thrusters are the same design.

Lovtsov, A.S., and Selivanov, M. Y. “FIRE TESTS OF HIGH POWER ION ENGINE FOR PERSPECTIVE TRANSPORT MODULES” 2014 http://naukarus.com/ognevye-ispytaniya-ionnogo-dvigatelya-vysokoy-moschnosti-dlya-perspektivnyh-transportnyh-moduley

Thermal Management

This may be the most interesting thing in in the TEM: the heat rejection system.

Most of the time, spacecraft use what are commonly called “bladed tubular radiators.” These are tubes which carry coolant after it reaches its maximum temperature. Welded to the tube are plates, which do two things: it increases the surface area of the tube (with the better conductivity of metal compared to most fluids this means that the heat can be further distributed than the diameter of the pipe) and it protects the pipe from debris impacts. However, there are limitations in how much heat can be rejected by this type of radiator: the pipes, and joints between pipes, have definite thermal limits, with the joins often being the weakest part in folding radiators.

The TEM has the option of using a panel-type radiator, in fact there’s many renderings of the spacecraft using this type of radiator, such as this one:

However, many more renderings present a far more exciting possibility: a liquid droplet radiator, called a “drip refrigerator” in Russian. This design uses a spray of droplets in place of the panels of the radiator. This increases the surface area greatly, and therefore allows far more heat to be rejected. In addition it can reduce the mass of the system significantly, both due to the increased surface area and also the potentially higher temperature, assuming the system can recapture the majority of its coolant.

Work has been done both on the ground and in space on this system. The Drop-2 test is being conducted on the ISS, and multiple papers were published on it. It began in 2014, and according to Roscosmos will continue until 2024. http://www.tsniimash.ru/science/scientific-experiments-onboard-the-is-rs/cnts/experiments/kaplya_2/

Here it is being installed:

Here’s an image of the results:

A patent for what is possibly the droplet collection system has also been registered in Russia: https://yandex.ru/patents/doc/RU2607685C1_20170110

This system was also tested on the ground throughout 2018 (https://ria.ru/20181029/1531649544.html?referrer_block=index_main_2), and appears to have passed all the vacuum chamber ground tests needed. Based on the reporting, more in-orbit tests will be needed, but with Drop-2 already on-station it may be possible to conduct these tests reasonably easily.

I have been unable to determine what the working fluid that would be used is, but anything with a sufficiently low vapor pressure to survive the vacuum of space and the right working fluid range can be used, from oils to liquid metals.

For more on this type of system, check out Winchell Chung’s incredible page on them at Atomic Rockets: http://www.projectrho.com/public_html/rocket/heatrad.php#liquidradiator I will also cover them in the future (possibly this fall, hopefully by next year) in my coverage of thermal management solutions.

Of all the technologies on this spacecraft, this has to be the one that I’m most excited about. Some reporting (http://trudymai.ru/upload/iblock/a26/teploobmen-izlcheniem-dispergirovannykh-potokov-teplonositeley-kosmicheskikh-letatelnykh-apparatov.pdf ) says that this radiator can hit between 0.12 and 0.2 kW/kg system specific power!

Reaction Control Systems

Nothing is known of the reaction control system for the TEM. A number of options are available and currently used in Russian systems, but it doesn’t seem that this part of the design has been discussed publicly.

Additional Equipment

The biggest noticeable change in the rest of the spacecraft is the change in the spine structure. The initial model and renders had a square cross section telescoping truss with an open triangular girder profile. The new version has a cylindrical truss structure, with a tetrahedral girder structure which almost looks like the same structure that chicken-wire uses. I’m certain that there’s a trade-off between mass and rigidity in this change, but what precisely it is is unclear due to the fact that we don’t have dimensions or materials for the two structures. The change in the cross-section also means that while the new design is likely stronger from all angles, it makes it harder to pack into the payload fairing of the launch vehicle.

The TEM seems like it has gone through a major redesign in the last couple years. Because of this, it’s difficult to tell what other changes are going to be occurring with the spacecraft, especially if there’s a significant decrease in electrical power available.

It is safe to assume that the first version of the TEM will be more heavily instrumented than later versions, in order to support flight testing and problem-solving, but this is purely an assumption on my part. The reconfiguration of the spacecraft at MAKS-2019 does seem to indicate, at least for one spacecraft, the loss of the payload capability, but at this point it’s impossible to say.

YaEDU Architecture

The YaEDU is the reactor that will be used on the TEM spacecraft. Overall, with power conversion system, the power system will weigh about 6800 kg.

Reactor

The reactor itself is a gas cooled, fast neutron spectrum, oxide fueled reactor, designed with an electrical output requirement rather than a thermal output requirement, oddly enough (choice in power conversion system changes the ratio of thermal to electrical power significantly, and as we’ll see it’s not set in stone yet) of 1 Mwe. This requires a thermal output of at least 4 MWt, although depending on power conversion efficiency it may be higher. Currently, though, the 4 MWt figure seems to be the baseline for the design. It is meant to have a ten year reactor lifetime.

This system has undergone many changes over its 11 year life, and due to the not-completely-clear nature of much of its development and architecture, there’s much about the system that we have conflicting or incomplete information on. Therefore, I’m going to be providing line-by-line references for the design details in these sections, and if you’ve got confirmable technical details on any part of this system, please comment below with your references!

Fuel

The fuel for the reactor appears to be highly enriched uranium oxide, encased in a monocrystalline molybdenum clad. According to some reporting (https://habr.com/en/post/381701/ ), the total fuel mass is somewhere between 80-150 kg, depending on enrichment level. There have been some mentions of carbonitride fuel, which offers a higher fissile fuel density but is more thermally sensitive (although how much is unclear), but these have been only passing mentions.

The use of monocrystalline structures in nuclear reactors is something that the Russians have been investigating and improving for decades, going all the way back to the Romashka reactor in the 1950s. The reason for this is simple: grain boundaries, or the places where different crystalline structures interact within a solid material, act as refractory points for neutrons, similarly to how a cracked pane of glass distorts the light coming through it through internal reflection and the disruption of light waves undergoing refraction in the material. There’s two ways around this: either make sure that there are no grain boundaries (the Russian method), or make it so that the entire structure – or as close to it as possible – are grain boundaries, called nanocrystalline materials (the preferred method of the US and other Western countries. While the monocrystalline option is better in many ways, since it makes an effectively transparent, homogeneous material, it’s difficult to grow large monocrystalline structures, and they can be quite fragile in certain materials and circumstances. This led the US and others to investigate the somewhat easier to execute, but more loss-intensive, nanocrystalline material paradigm. For astronuclear reactors, particularly ones with a relatively low keff (effective neutron multiplication rate, or how many neutrons the reactor has to work with), this monocrystalline approach makes sense, but I’ve been unable to find the keff of this reactor anywhere, so it may be quite high in theory.

It was reported by lenta.ru in 2014 (https://lenta.ru/news/2014/07/08/rosatom/ ) that the first fuel element (or TVEL in Russian) was assembled at Mashinostroitelny Zavod OJSC.

Reference was made (http://www.atomic-energy.ru/news/2015/07/01/58052 ) in 2015 to the fuel rods as “RUGBK” and “RUEG,” although the significance of this acronym is beyond me. If you’re familiar with it, please comment below!

In 2016, Dmitry Markov, the Director of the Institute of Reactor Materials in Zarechny, Sverdlovsk, reported that full size fuel elements had been successfully tested (https://xn--80aaxridipd.xn--p1ai/uchenye-iz-sverdlovskoj-oblasti-uspeshno-zavershili-ispytaniya-tvel-dlya-kosmicheskogo-yadernogo-dvigatelya/ ).

Coolant

The TEM uses a mix of helium and xenon as its primary coolant, a common choice for fast-spectrum reactors. Initial reporting indicated an inlet temperature of 1200K, with an outlet temperature of 1500K, although I haven’t been able to confirm this in any more recent sources. Molybdenum, tantalum, tungsten and niobium alloys are used for the primary coolant tubes.

Testing of the coolant loop took place at the MIR research reactor in NIIAR, in the city of Dimitrovgrad. Due to the high reactor temperature, a special test loop was built in 2013 to conduct the tests. Interestingly, other options, including liquid metal coolant, were considered (http://osnetdaily.com/2014/01/russia-advances-development-of-nuclear-powered-spacecraft/ ), but rejected due to lower efficiency and the promise of the initial He-Xe testing.

Power Conversion System

There have been two primary options proposed for the power conversion system of the TEM, and in many ways it seems to bounce back and forth between them: the Brayton cycle gas turbine and a thermionic power conversion system. The first offers far superior power conversion ratios, but is notoriously difficult to make into a working system for a high temperature astronuclear system; the second is a well-understood system that has been used through multiple iterations in flown Soviet astronuclear systems, and was demonstrated on the Buk, Topol, and Yenesiy reactors (the first two types flew, the third is the only astronuclear reactor to be flight-certified by both Russia and the US).

In 2013, shortly after the design outline for the TEM was approved, the MAKS trade show had models of many components of the TEM, including a model of the Brayton system. At the time, the turbine was advertised to be a 250 kW system, meaning that four would have been used by the TEM to support YaEDU. This system was meant to operate at an inlet temperature of 1550K, with a rotational speed of 60,000 rpm and a turbine tip speed of 500 m/s. The design work was being primarily carried out at Keldysh Center.

The Brayton system would include both DC/AC and AC/DC convertors, buffer batteries as part of a power conditioning system, and a secondary coolant system for both the power conversion system bearing lubricant and the batteries.

Building and testing of a prototype turbine began before the 2013 major announcement, and was carried out at Keldysh Center. (http://osnetdaily.com/2014/01/russia-advances-development-of-nuclear-powered-spacecraft/ )

As early as 2015, though, there were reports (https://habr.com/en/post/381701/ ) that RSC Energia, the spacecraft manufacturer, were considering going with a simpler power conversion system, a thermionic one. Thermionic power conversion heats a material, which emits electrons (thermions). These electrons pass through either a vacuum or certain types of exotic materials (called Cs-Rydberg matter) to deposit on another surface, creating a current.

This would reduce the power conversion efficiency, so would reduce the overall electric power available, but is a technology that the Russians have a long history with. These reactors were designed by the Arsenal Design Bureau, who apparently had designs for a large (300-500 kW) thermionic design. If you’d like to learn more about the history of thermionic reactors in the USSR and Russia, check out these posts:

https://beyondnerva.com/2018/11/08/nuclear-electric-propulsion-history-part-1-the-soviet-astronuclear-program/
https://beyondnerva.com/2019/03/10/topaz-international-part-1-enisy-the-soviet-years/
https://beyondnerva.com/2019/03/22/topaz-international-part-ii-the-transition-to-collaboration/

This was potentially confirmed just a few days ago by the website Atomic Energy (http://www.atomic-energy.ru/news/2020/01/28/100970 ) by the first deputy head of Roscosmos, Yuri Urlichich. If so, this is not only a major change, but a recent one. Assuming the reactor itself remains in the same configuration, this would be a departure from the historical precedent of Soviet designs, which used in-core thermionics (due to their radiation hardness) rather than out-of-core designs, which were investigated by the US for the SNAP-8 program (something we’ll cover in the future).

So, for now we wait and see what the system will be. If it is indeed the thermionic system, then system efficiency will drop significantly (from somewhere around 30-40% to about 10-15%), meaning that far less electrical power will be available for the TEM.

Radiation Shielding

The shielding for the YaEDU is a mix of high-hydrogen blocks, as well as structural components and boron-containing materials (http://www.atomic-energy.ru/news/2015/12/24/62211).

The hydrogen is useful to shield most types of radiation, but the inclusion of boron materials stops neutron radiation very effectively. This is important to minimize damage from neutron irradiation through both atomic displacement and neutron capture, and boron does a very good job of this.

Current TEM Status

Two Russian media articles came out within the past week about the TEM, which spurred me to write this article.

RIA, an official state media outlet, reported a couple days ago that the first flight of a test unit is scheduled for 2030. In addition:

Roscosmos announced the completion of the first project to create a unique space “tug” – a transport and energy module (TEM) – based on a megawatt-class nuclear power propulsion system (YaEDU), designed to transport goods in deep space, including the creation of long-term bases on the planets. A technical complex for the preparation of satellites with a nuclear tug is planned to be built at Vostochny Cosmodrome and put into operation in 2030. https://ria.ru/20200128/1563959168.html

A second report (http://www.atomic-energy.ru/news/2020/01/28/100970) said that the reactor was now using a thermionic power conversion system, which is consistent with the reports that Arsenal is now involved with the program. This is a major design change from the Brayton cycle option, however it’s one that could be considered not surprising: in the US, both Rankine and Brayton cycles have often been proposed for space reactors, only to have them replaced by thermoelectric power conversion systems. While the Russians have extensive thermoelectric experience, their experience in the more efficient thermionic systems is also quite extensive.

It appears that there’s a current tender for 525 million rubles for the TEM project by Roscosmos, according to the Russian government procurement website, through November 2021 (https://zakupki.gov.ru/epz/order/notice/ok504/view/common-info.html?regNumber=0995000000219000115 ), for

“Creation of theoretical and experimental and experimental backlogs to ensure the development of highly efficient rocket propulsion and power plants for promising rocket technology products, substantiation of their main directions (concepts) of innovative development, the formation of basic requirements, areas of rational use, design and rational level of parameters with development software and methodological support and guidance documents on the design and solution of problematic issues of creating a new generation of propulsion and power plants.”

Work continues on the Vostnochy Cosmodrome facilities, and the reporting still concludes that it will be completed by 2030, when the first mass-production TEMs are planned to be deployed.

According to Yuri Urlichich, deputy head of Roscosmos, the prototype for the power plant would be completed by 2025, and life testing on the reactor would be completed by 2030. This is the second major delay in the program, and may indicate that there’s a massive redesign of the reactor. If the system has been converted to thermionic power, it would explain both the delay and the redesign of the spacecraft, but it’s not clear if this is the reason.

For now, we just have to wait and see. It still appears that the TEM is a major goal of both Roscosmos and Rosatom, but it is also becoming apparent that there have been challenges with the program.

Conclusions and Author Commentary

It deserves reiterating: I’m some random person on the Internet for all intents and purposes, but my research record, as well as my care in reporting on developments with extensive documentation, is something that I think deserves paying attention to. So I’m gonna put my opinion on this spacecraft out there.

This is a fascinating possibility. As I’ve commented on Twitter, the capabilities of this spacecraft are invaluable. Decommissioning satellites is… complicated. The so-called “graveyard orbits,” or those above geosynchronous where you park satellites to die, are growing crowded. Satellites break early in valuable orbits, and the operators, and the operating nations, are on the hook for dealing with that – except they can’t.

Additionally, while many low-cost launchers are available for low and mid Earth orbit launches, geostationary orbit is a whole different thing. The fact that India has a “Polar Satellite Launch Vehicle” (PSLV) and “Geostationary Satellite Launch Vehicle” (GSLV) classification for two very different satellites drives this home within a national space launch architecture.

The ability to contract whatever operator runs TEM missions (I’m guessing Roscosmos, but I may be wrong) with an orbital path post-booster-cutoff, and specify a new orbital pat, and have what is effectively an external, orbital-class stage come and move the satellite into a final orbit is… unprecedented. The idea of an inter-orbital tug is one that’s been proposed since the 1960s, before electric propulsion was practical. If this works the way that the design specs are put at, this literally rewrites the way mission planning can be done for any satellite operator who’s willing to take advantage of it in cislunar space (most obviously, military and intelligence customers outside Russia won’t be willing to take advantage of it).

The other thing to consider in cislunar space is decommissioning satellites: dragging things into a low enough orbit that they’ll burn up from GEO is costly in mass, and assumes that the propulsion and guidance, navigation, and control systems survive to the end of the satellite’s mission. As a satellite operator, and a host nation to that satellite with all the treaty obligations the OST requires the nation to take on, being able to drag defunct satellites out of orbit is incredibly valuable. The TEM can deliver one satellite and drag another into a disposal orbit on the way back. To paraphrase a wonderful character from Sir Terry Pratchett (Harry King)“They pay me to take it away, and they pay me to buy it after.” In this case, it’s opposite: they pay me to take it out, they pay me to take it back. Especially in graveyard orbit challenge mitigation, this is a potentially golden opportunity financially for the TEM operator: every mm/s of mission dV can potentially be operationally profitable. This is potentially the only system I’ve ever seen that can actually say that.

More than that, depending on payload restrictions for TEM cargoes, interplanetary missions can gain significant delta-vee from using this spacecraft. It may even be possible, should mass production actually take place, that it may be possible to purchase the end of life (or more) dV of a TEM during decommissioning (something I’ve never seen discussed) to boost an interplanetary mission without having to pay the launch mass penalty for the Earth’s escape velocity. The spacecraft was proposed for Mars crewed mission propulsion for the first half of its existence, so it has the capability, but just as SpaceX Starship interplanetary missions require SpaceX to lose a Starship, the same applies here, and it’s got to be worth the while of the (in this case interplanetary) launch provider to lose the spacecraft to get them to agree to it.

This is an exciting spacecraft, and one that I want to know more about. If you’re familiar with technical details about either the spacecraft or the reactor that I haven’t covered, please either comment or contact me via email at beyondnerva@gmail.com

We’ll continue with our coverage of fluid fueled NTRs in the next post. These systems offer many advantages over both traditional, solid core NTRs and electrically propelled spacecraft such as the TEM, and making the details more available is something I’ve greatly enjoyed. We’ll finish up liquid fueled NTRS, followed by vapor fuels, then closed and open fueled gas core NTRs, probably by the end of the summer

If you’re able to support my efforts to continue to make these sorts of posts possible, consider becoming a Patron at patreon.com/beyondnerva. My supporters help me cover systems like this, and also make sure that this sort of research isn’t lost, forgotten, or unavailable to people who come into the field after programs have ended.