The Transport and Energy Module (TEM) is the newest nuclear electric spacecraft design to undergo significant development. A collaboration between Russia’s Roscosmos and Rosatom, with various design centers contributing to the effort on both the aeronautical and astronuclear aspects of the spacecraft.
The TEM is designed with a variety of mission profiles in mind. Originally proposed as part of a collaborative effort with the US, ESA, and other agencies to power a crewed Mars mission (a mission it may still be able to carry out), today the most common application is as an orbital tug: raising satellites, deorbiting defunct ones, changing inclination, and possible satellite servicing missions.
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).
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!
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.
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
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.
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.
The Future Remains Bright!
While there is a healthy amount of skepticism about the launch date of this spacecraft, it remains one of the most exciting concepts in astronuclear design in the last decade. It offers not only interplanetary mission profiles, but more excitingly is one of the most developed concepts for an inter-orbital tug, an often-neglected but increasingly more important – and valuable – field.