Space Sunday: propulsion, planets and pictures

An artist’s impression of a lunar base using the micro nuclear reactor (NMR) currently being developed by Rolls Royce. Three of the reactors can be seen in the right foreground, casting palm tree like shadows (the “palm frond” shadows are actually the reactors’ radiator panels). Three more of the reactors can be seen in the centre of the image. See below for more. Credit: Rolls Royce Aerospace

In my previous Space Sunday, I covered some of the renewed interest in nuclear propulsion for space missions – and it certainly is a hot topic (no pun intended). Just 24 hours after that article was published, NASA and the US Defense Advanced Research Projects Agency (DARPA) announced they had signed an interagency agreement to develop a nuclear-thermal propulsion (NTP) concept.

Referred to as the Demonstration Rocket for Agile Cislunar Operations (DRACO), the three-phase programme will look to develop and enhance an NTP propulsion system capable of operating between Earth and the Moon and eventually Earth and Mars, potentially enabling fast transit times to the latter measured in weeks rather than months. Nor is this simply a computer modelling exercise: the agencies plan to fly a demonstrator of the propulsion unit in early 2027.

As I noted in my previous piece, NTP uses a nuclear reactor to heat liquid hydrogen (LH2) propellant, turning it into ionized hydrogen gas (plasma) channelled through engine bells similar to those seen in chemical rockets to generate thrust. As I also noted, NTP for space vehicle propulsion is not new; both the US and the former Soviet Union both pursued NTP projects in the early days of the space race – most notably for the US with the Nuclear Engine for Rocket Vehicle Application (NERVA) project, successfully tested on the ground in 1963/64.

A conceptual rendering the DARPA-NASA nuclear thermal propulsion (NTP) test vehicle the agencies hope to fly in 2027

Per the agreement, NASA’s Space Technology Mission Directorate (STMD) will lead the technical development of the nuclear thermal engine, which will be integrated into a vehicle built by DARPA, with that agency leading the overall programme as the contracting authority. Both agencies will collaborate on the overall design of the engine.

DARPA and NASA have a long history of fruitful collaboration in advancing technologies for our respective goals, from the Saturn V rocket that took humans to the Moon for the first time to robotic servicing and refuelling of satellites. The space domain is critical to modern commerce, scientific discovery, and national security. The ability to accomplish leap-ahead advances in space technology through the DRACO nuclear thermal rocket program will be essential for more efficiently and quickly transporting material to the Moon and eventually, people to Mars.

– DARPA director Dr. Stefanie Tompkins

Meanwhile, on January 27th, 2023, the UK’s famed Rolls Royce teased details of its own foray into the space-based nuclear power / propulsion systems: the micro-nuclear reactor (MNR), an extremely robust, self-contained nuclear fission plant which could be used to supply power to bases on the Moon or Mars, or used as a core element in vehicle propulsion systems either individually or as multiple units to provide both thrust and system redundancy, if required.

Images suggest the Rolls Royce MNR is roughly 3 metres in length (excluding the heat radiators). Credit: Rolls Royce

Development of the MNR started as a result of a 2021 agreement between the United Kingdom Space Agency (UKSA) and Rolls Royce (RR) to study future nuclear power options in space exploration. However, the design for the unit builds on RR’s decades-long expertise in developing power plants for the Royal Navy’s nuclear submarine squadrons and, more particularly a project the company has been developing since 2015 to develop and build small modular reactor (SNRs) to meet the UK’s energy needs (SNRs are self-contained, less complex and lower cost alternative to current nuclear reactors).

Precise details of the size of the unit and its output have not been revealed, although images released by RR suggest a single MNR is around 3 metres in length. In discussing the system, the company indicated its designs have reached a point where it plans to have a full-scale demonstrator / prototype running by 2028.

The MNR forms a part of a broader space strategy from Rolls Royce, which also includes systems for high Mach propulsion systems (e.g. ramjets) which could be combined with rocket propulsion to reach orbit, and a new generation of radioisotope thermal generators (RTGs) for power generation on robotic explorer craft and surface system on the Moon and Mars. The overall aim of the strategy is to offer space agencies and the private sector the ability to easily integrate selected elements of RR’s product offerings into their space projects and programmes.

A rendering of a crew-carrying vehicle entering Mars orbit and using a series of Rolls Royce MNRs (outlined in blue) as a part of its propulsion and power system. Credit: Rolls Royce

Returning to NASA, as well as considering the nuclear option, the US agency has been researching the next generation of rocket engines – the rotating detonation rocket engine (RDRE) – and on January 24th, carried out a series of sustained ground tests of a prototype unit.

In a conventional rocket motor, fuel is expended by deflagration combustion – fuel and oxidiser are burnt to produce an energetic gas flow which is then directed through exhaust bells to generate subsonic thrust. With rotating detonation, fuel and oxidiser are injected into a circular channel (annulus). An igniter within the annulus then detonates the initial incoming mix, generating a shockwave which travels around the channel, returning to the point of injection.

At this point, more fuel is injected into the channel to be detonated by the existing shockwave. This increases the shockwave’s speed and force, and the cycle repeats over and over, the shockwave accelerating to supersonic speed, generating high pressures which can be constantly be directed out of the channel to form thrust through an exhaust system even as the shockwave maintains its momentum within the channel.

A still from NASA’s tests of its rotating detonation rocket engine. Note the “stepped” nature of the engine’s exhaust, a unique property of this type of motor. Credit: NASA

Whilst this may sound complicated, the upshot is that rotating detonation engines (RDREs) theoretically generate around 25% more thrust than conventional rocket motors, which directly translates to greater delta-V being imparted to vehicles departing Earth, so reducing flight times to the Moon and Mars and elsewhere in the solar system. RDEs could also be inherently less complex than subsonic brethren, reducing the mass of a launch vehicle’s propulsion system.

However, there are drawbacks; for example, the very nature of containing the growing force of the shockwave puts an RDRE under tremendous stress and they have been known to explode. They are also incredibly noisy when built at scale.

Both Russia and Japan have experimented with RDRE technology; in 2018, former Roscosmos chief Dmitry Rogozin claimed Russia had successfully developed the first phase of a 2-tonne class of liquid-fuelled RDRE, although this has yet to be substantiated. In 2021, Japan successfully tested a small-scale (112.4 lbf) RDRE in space, using it to propel the upper stage of a sounding rocket.

The NASA test, carried out at the Marshall Space Flight Centre, Alabama, is the first verified test of a full-scale RDRE. The demonstrator motor operated for a total of 10 minutes, reaching peak thrusts of some 4,000 lbf. This is fairly lightweight by rocket standards, but the aim of the test was not just to generate thrust, but to test the engine’s ability to withstand multiple firings and confirm that a copper alloy referred to as GRCop-42 developed by NASA specifically for use in RDRE engines, was up to the task of reducing the stress on the motor by more efficiently carrying the heat generated by the shockwave away from the annulus structure.

While tests with this motor will continue, NASA is now also moving to the construction of a large unit capable of a sustained 10,000 lbf – the same as mid-range rocket motors – to better understand the potential for RDREs to out-perform “traditional” rocket motors. If successful, it could pave the way for RDRE motors capable of match the output of large-scale engines like the RS-25 used by the Space Launch System (SLS) rocket (418,000 lbf).

Timelapse: Observing Four Exoplanets Orbiting their Star

Four faint exoplanets orbit star HR8799. The star is represented by an icon, having been removed from the image (hence the black area around the icon) as its glare would hide its planets. Credit: Jason Wang/Northwestern University.

The image above recorded the motion of four Jupiter-class planets around their parent star. It is made up of images captured of the system over a period of 12 years and then played in sequence.

The four exoplanets were the first – alongside of Fomalhaut b – whose motion around their parent could be confirmed via direct imaging. They are located 133 light-years from Earth and orbit a star called HR 8799, located within the constellation of Pegasus.

The star itself is rather unique. It is relatively young main-sequence star – just 30 million years old (compared to the Sun’s 4.6 billion years). It is roughly 1.5 times the Sun’s mass and 5 times its luminosity; it is also the only known star to simultaneously be a Gamma Doradus variable star, a Lambda Boötis star and a Vega-like star (a star with excess infrared emission caused by a circumstellar disk).

All four planets are exceptionally young and still super-hot form their formation. All are thought to be just under the limit (13 Jupiter masses) for deuterium fusion to be initiated which would have seen them classified as brown dwarfs. As they cool, they will contract in size. Like the outer gas giants of our own solar system, all four orbit a fair distance from their parent, with HR8799 b (the outermost) orbiting at a distance equitable to that of Neptune and with a period of around 460 years, the innermost, HR8799 e orbits at a distance equitable to Jupiter’s from the Sun, with a period of around 45 years, The c and d planets orbit at distances comparable to Uranus and Saturn, with periods of roughly 190 and 100 years respectively.

Overall, the system is very similar to our own – marking it as another rarity (thus far) within our galaxy. Not only do the four giant planets orbit their star at distances very similar to the four gas giants in the solar system, leaving room between them and their parent for smaller, rocky planets to possibly form, the system also has an asteroid belt in much the same position as our own, and its Kuiper belt equivalent which might be the nursery for the formation of dwarf planets like Eris, Pluto, Haumea, Makemake, etc., within our own Kuiper belt.

Webb Suffers Glitch and Recovers

The James Webb Space Telescope (JWST) suffered a communications glitch on January 15th, 2023, causing the Near Infrared Imager and Slitless Spectrograph (NIRISS) to go off-line.

NIRISS has two capabilities: wide-field spectroscopy, which involves capturing the overall spectrum of a wide field of view such as a field of stars, part of a nearby galaxy, or many galaxies at once for analysis; and single-object spectroscopy, allowing it to capture the spectrum of a single bright object like a star in a field of view and reveal its secrets.

The problem began when the instrument was struck by a galactic cosmic ray (GCR), a high-energy particle originating outside of the solar system. As well as their potential to do enormous damage to the human body over time, GCRs can also serve to disrupt / seriously damage electrical systems. In this case, the GCR caused the flight software on the NIRISS to time-out and the main computer on JWST shut it down for safety reasons, alerting controllers on Earth of a possible problem.

While GCRs strikes are not uncommon with space vehicles, it took a couple of weeks for mission engineers to confirm this had been the case, rather than the NIRISS going off-line for another reason. Once the problem had been diagnosed, the engineering team were able to develop a series of steps to bring the instrument back on-line and re-synchronise its communications with the main computer.

The NIRISS is the latest is a small series of incidents to affect JWST. These have included the observatory going into a safe mode for 18 days in December 2022, and an incident in September with the Mid-Infra-Red Instrument (MIRI). Whilst these have interrupted operations, then have not seriously impeded JWST in its first year of operations.

South Korea Images Earthrise over the Moon

The Earth “rises” over the limb of the Moon in a monochrome image captured by South Korea’s Danuri satellite. Credit: Korea Aerospace Research Institute (KARI)

The image above was captured by the Korea Pathfinder Lunar Orbiter (KPLO) at the end of 2022. The image presents a monochrome view of Earth “rising” over the lunar horizon as the satellite orbits the Moon.

The orbiter – also called Danuri – is South Korea’s first-ever lunar orbiter, and was launched on August 4th, 2022, by a SpaceX Falcon 9. It was placed in a low-energy transfer orbit to the Moon, arriving in early December 2022. It is seen as a statement of intent by South Korea to play an active role in the US-led Artemis programme.

With a total mass of 550 Kg, the satellite carries a 40 Kg scientific payload which includes ShadowCam, a high-sensitivity imager designed to search for ice deposits within the Moon’s permanently shadowed regions (PSRs) at the Moon’s poles – seen as a critical part of helping to determine locations near to where a lunar base might be established in order to leverage the ice as a source of water.

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