Burn baby, burn! The technology of the Mars Orbit Insertion burn

This post was contributed by Thomas Ormston, a spacecraft operations engineer here at ESOC, and highlights the science, the engineering, the technology and the incredible teamwork involved in getting ExoMars/TGO captured into orbit around another planet.

The most critical moment so far in the ExoMars Trace Gas Orbiter’s journey will be Wednesday’s Mars Orbit Insertion burn – the long (ca. 134 mins) engine firing that will slow the spacecraft down sufficiently to be captured into Mars orbit. What actually happens during this critical burn, though? Here we thought we’d give you a more detailed rundown of that all-important moment.

The burn will be performed autonomously by the orbiter, based on commands uplinked beforehand by the control team at ESOC in Darmstadt. Around half an hour before the burn starts, currently set for 13:04:47 GMT (15:04:47 CEST) on 19 October, the spacecraft will begin turning around to point its big main engine toward the direction of travel. As this is happening, the large 2.2m-diameter high gain antenna on the Trace Gas Orbiter will be locked into a safe ‘boost position’ [the opposite of what’s shown here – Ed.] for the burn. As this doesn’t point it at Earth, we will temporarily lose contact with the spacecraft. Also at almost the same time, the solar arrays will rotate and also lock into their safe boost position. Finally the orbiter will start reconfiguring its radios to send a beacon signal through its Low Gain Antenna.

When this radio reconfiguration is complete, the orbiter will start sending out this ‘carrier only’ signal. The Low Gain Antenna isn’t powerful enough to send data to Earth, hence why we use this simple beacon signal. The key advantage is that the Low Gain Antenna signal can be picked up almost no matter what orientation Trace Gas Orbiter is in. The signal will be acquired by NASA’s big 70m-diameter dish in Canberra, Australia and that will let the team on ground know that the orbiter is there. Critically it should also show a jump in frequency caused by Doppler shift as the orbiter fires its engine, allowing us to monitor the progress of the burn even without telemetry data.

As the spacecraft reconfigures itself, and points the engine nozzle in the right direction, the clock will tick down to the programmed ignition time. The exact ignition time that will be loaded is being refined all the time by our flight dynamics experts to take place at exactly the right second. At that time the valves above the main engine will open and the Monomethylhydrazine propellant and Mixed Nitrogen Oxides oxidiser will flood into the engine. These are hypergolic – meaning no ignition spark is necessary! The two liquids will ignite on contact and at this moment the big engine will roar into life, generating 424 Newtons [pushing with about the same force as that of a 45-kg weight on Earth’s surface – Ed.] of thrust in the direction of flight – effectively slamming on the brakes as the orbiter hurtles towards Mars.

With the engine firing with all its might, Trace Gas Orbiter will settle into the longest engine burn of its life. Just as our Flight Dynamics team are calculating the exact point of ignition, they are also calculating the predicted moment of shutdown. We currently expect that to be roughly 139 minutes after ignition, but this actually isn’t a time we programme on board the satellite. Trace Gas Orbiter has another trick up its sleeve – sensitive accelerometers will measure by how much the orbiter is decelerating and when exactly the right amount of braking force has been generated, it will shut the engine down. This approach allows the spacecraft to autonomously compensate for any over or under performance of the engine.

But suppose there’s a problem with the accelerometers, and they don’t signal the shut down – the engine won’t simply keep running forever. One hundred and forty seven minutes after the programmed ignition time, Trace Gas Orbiter will reach ‘MOI Timeout’. At this time, it will shut down the engine (if it’s still firing) no matter what and start reconfiguring back to a normal Earth-pointing communication mode. It will safely isolate the main engine, allow the power-generating solar arrays to freely track the Sun once more and turn the big high-gain antenna toward Earth. Finally, it will turn on the main radio data signal and start telling us how the burn went.

Here lies the last bit of the puzzle though – we actually won’t be able to hear Trace Gas Orbiter at the end of burn! This is actually all planned – while the burn is happening, the spacecraft will pass behind Mars (occultation) and we’ll lose all radio contact with it. It will only emerge from behind the Red Planet after the burn is complete and it is reconfigured to talk to us. It will be a tense wait for everyone on ground but, all being well, it will come out of occultation on time and telling us all we need to know about the burn performance. Flight Dynamics will then measure and assess the orbit the spacecraft has actually achieved and provide us final confirmation that Europe has returned to Mars!

 

Comments

17 Comments

  • masanori says:

    Thanks for this detailed blog post!! So far I’m so glad to hear both the separation & the orbit raising manoeuvre went well. The only concern might be the delay of telemetry coming down. Hope you will see the reason why by the next event.

    It will be a long night in my time zone & I’m ready for that. I wish you all the best of luck on #MarsLanding & #MOI !!

  • Anxo says:

    How is the spacecraft turning around? Using engines as well?

  • Harald Maier says:

    One question came to my mind reading this: Will ExoMars/TGO fly through its own exhaust gas?
    My answer is this: As there is no friction force in space (and gravitation can be neglected in this question (my guess :))) the gas will never slow down so every particle leaving Exomars will never hit it, it will always be “in front” of it. Am i right or did i screw something up for example with a reference point of the velocity?

    • William Easdown says:

      Yeah you’ve basically got it. The exhaust gas leaves the engine with a lot of velocity away from the spacecraft and the lack of an atmosphere to slow down the gases mean they retain this velocity. Also, as the spacecraft will be slowing down with respect to Mars, the velocity difference between the spacecraft and exhaust gases will increase.

  • ManuFez says:

    I was thinking that the low gain antenna would be usable to send some low rate telemetry to Earth (as well as to receive telecommands, in case of unavailability of the HGA) instead of a simple beacon tone.
    If something goes wrong with the HGA, is the TGO lost?

    • Dominik says:

      TGO would not be lost, but operating it would be much more problematic. It might be possible (I’m not sure), to transmit through LGA with really low data-rate, using largest DSN antennas.
      What’s more, thanks to Electra SDR, data could be relayed through other orbiters like MEX and MVN, or even MERs, as a last-resort contingency.

  • Fernando Reig says:

    Many thanks to T. Orsmton for this excellent post that allow followers (not experts) of Exomars to know more detailed news without having to wait oficial ESA news. Very interested in knowing details of daily actions, decisions, problems of the spacecraft operations.

  • Dmitri Gorskine says:

    The expedition depends on the size of stones under place of landing.
    If the stone will be big under the spacecraft, it will turn over. What you prepare to do, dear engineers?

    • William Easdown says:

      The trajectory designers will have chosen an area that doesn’t have many large stones. Like you say, if Schiaparelli lands on a big rock it could be damaged by the impact or from falling over, but I’d imagine the centre of gravity of the lander is also low to reduce the chance of it falling over.

  • Raymond says:

    During the MOI burn, with the main engine at full power, how is the engine aligned in the right direction? Reaction wheels or with nozzles too? And how is proper engine alignment during the MOI burn determined, by accelerometers only, or by startrackers?

    • Thomas Ormston says:

      The engine is fixed to the spacecraft, so the whole spacecraft must be pointed in the right direction. The accelerometers only measure how much it is decelerating by.

      To measure the orientation (attitude) of the spacecraft there are two systems – star trackers as you mentioned and ring laser gyroscopes. The gyroscopes provide the rapid fine attitude measurements and any gradual drift in them is corrected by the star trackers.

      To maintain the attitude the reaction wheels are used. The thrusters will actually all be firing during the burn to add a little extra thrust to the main engine.

  • Fernando Reig says:

    Why TGO does not decelarates more, reaching a nearer orbit around Mars instead of having such a huge elongated one? Maybe not enough hypergolic fuel in tanks? or maybe dynamics only allow this way?
    May I also ask what is Moi burn?

    • Thomas Ormston says:

      Good question, but you answered correctly – to go into a circular orbit would require too much fuel. THO will actually go into a circular orbit close to Mars though. Over the months after arrival it will do a procedure called aerobraking. This means it will dip slightly into the Mars atmosphere, using the drag from the atmosphere to slow it down further and drop it into a circular orbit.

      The “MOI Burn” is the shorthand phrase we use at ESOC to describe the big engine burn when we arrive. It means “Mars Orbit Insertion Burn”.

  • cosmo says:

    I think, it’s only fair to note that the Russian 64m antennae in Kalyazin and Medvezh’i Ozera are also going to listen to TGO during MOI.

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