Ingenuity continued its journey towards the river delta at the beginning of this month with Flight 24. This flight took place Sunday, April 3, and the data arrived back later that evening. The flight was the fourth of five sorties Ingenuity will make to cross the “Séítah” region of Jezero Crater and arrive in the vicinity of its delta. This multiflight shortcut across Séítah is being done to keep ahead of the Perseverance rover – which is currently making great time on a more circuitous route to the same area.
The Ingenuity and Mars 2020 teams have big plans for the helicopter at the delta. But they have to get there first, and prior to Flight 24 a crucial decision had to be made on which of three different flight plans offered the best chance of a successful delta arrival.
The three options on the table were:
- Option A: a single, long flight.
- Option B: two shorter flights.
- Option C: a very short Flight 24 to make the long flight out of Séítah slightly easier than option A.
In deciding which of these options to greenlight, the Mars Helicopter team had to consider multiple factors: thermal, atmospheric conditions, flight time, drift, landing sites, and keeping up with the rover. We’ll explore each of these factors and what role they played in the overall risk assessment and selection of our decision.
For spacecraft, “thermal” refers to the management of the temperatures of each component. Every part of Ingenuity has what is called Allowable Flight Temperatures (AFT), which give a range of temperatures at which each part is safe to operate. Even your phone or computer has a recommended temperature range: Too cold or too hot and it will not work as intended. Keeping “within AFTs” is critical for ensuring the health of Ingenuity, which means we are very careful to manage this – for example, by using heaters overnight when it is cold, and limiting activities during the day, when it is warmer. A particular challenge for Ingenuity is managing the temperature of its actuators, the servos and motors that allow it to fly (see some of these here). These components generate a lot of heat during flight, to the extent that the maximum flight time is often limited by the maximum AFT of these actuators.
Atmospheric Seasonal Conditions
If you have been following this blog, you will know that we have been operating with reduced air density since September, requiring an increase in rotor rpm from 2,537 to 2,700. Flight 14, for example, was a checkout flight to confirm Ingenuity could fly in these conditions. For all flights since then, Ingenuity has been successfully operating with 2,700 rpm. Unfortunately, though, using a higher rpm causes the actuators to heat more rapidly and reach their AFTs sooner, limiting maximum flight time. Practically, this has limited us to flights of 130 seconds or less. Thankfully, we are toward the end of the Martian summer, with its low air density, and starting to move into the Martian fall, with higher air densities (see below), meaning we can now return to the 2,537 rpm of our first 13 flights. This change in rpm allows an increase in flight time to approximately 150 seconds. However, atmospheric density isn’t the only factor at play: The main driver of the changes in density is the temperature of the atmosphere, which also has a major impact on – you guessed it – the temperature of Ingenuity.
It is warmer now coming out of the summer than with our earlier flights in the spring. So even though we have been flying at 10:00 a.m. local mean solar time (LMST)- on Mars throughout the summer, Ingenuity has been hotter than flights at 12:00 LMST in the spring. A warmer atmosphere means warmer components, meaning we reach maximum AFTs sooner. This means, flying at 10:00 LMST, we still can’t fly for as long as we did previously, such as during Flights 9, 10, and 12.
Flight Time and Distance
With the current atmospheric conditions at Jezero Crater, the AFTs of the actuators are the limiting factor for the total flight time. Let’s take a more detailed look at the different options for Flight 24 and beyond:
- Option A: The long flight out of the delta requires 170 seconds of flight, the maximum of our previous flights. This is not possible until the atmosphere cools down further.
- Option B: The two shorter flights are operating the same as our previous “summer” flights: 130 seconds of flight time. This flight time is possible without any changes.
- Option C: The first flight, a short hop, is designed to reduce the flight time needed for the second flight to 160 seconds. This is possible if we: i) reduce the rpm to 2,537, and ii) fly earlier in the sol to have lower atmospheric temperatures.
The team determined that by flying 30 minutes earlier, at 09:30 LMST, the flight time could be increased by 10 seconds. However, Ingenuity had never flown at 09:30 LMST before, so this would be a new “first.” And flying earlier brings with it associated risks with the charge state of the helicopter’s batteries: Ingenuity uses power to heat itself overnight and recharges its batteries with its solar panel, meaning the batteries have less charge in the morning. If we choose to fly at 9:30, we would first have to test it out – waking Ingenuity at this time without flying, to check that it would have sufficient charge for a flight.
In summary, the different maximum flight time options available are:
- 130 seconds (baseline)
- 150 seconds (decreased rpm)
- 160 seconds (decreased rpm and earlier flight time)
Flight time is normally equivalent to distance traveled, but it also depends on the maneuvers being performed. For example, rotating in place (called “yawing”), is done (at least at Mars) slowly, taking a handful of seconds with no distance traveled. For that reason, Mars Helicopter flights with more yaw maneuvers don’t travel as far in the same flight time.
All these factors come into play with option C – the short hop. This flight would enable the longer 160 second flight, for several reasons: 1) it is a check-out test for flying back at 2,537 rpm, 2) it is a test for flying at 09:30 LMST, and 3) it reduces the flight time for the subsequent flight by doing the time-consuming yaw maneuvers and moving slightly closer to the target for the second flight. All three of these steps are required to enable a 160-second flight out of the Séítah.
As discussed previously, Ingenuity was a tech demo expecting to fly over flat ground. When flying over “non-flat” terrain such as hills, cliffs, large boulders and large dunes, Ingenuity’s estimate of its position and heading can drift. This drift leads to a wider area where it may land, called the landing ellipse. The farther it flies, the larger the potential drift, and the larger the landing ellipse. The Séítah region has many of these non-flat features (see the dunes and rocks in the image at the top, or on the interactive map), making it riskier for Ingenuity to fly over this region. An additional challenge with the upcoming flights is the presence of hardware from Perseverance’s entry, descent, and landing (EDL), including the sky crane, parachutes and backshell. The green dots (in figure 1) show the predicted locations of this hardware from orbital imagery. Some of these components are under the flight path of option B, which presents a potential for unexpected performance from Ingenuity’s laser altimeter (a laser that measures the helicopter’s height above the surface) and visual odometry system, which could cause more drift.
Each flight of Ingenuity has a planned landing ellipse (or sometimes just a landing region) that has been analyzed to be safe to touch down on, and to be large enough for the expected drift. The challenge is finding a large enough landing area that is free of hazards, such as rocks, large slopes, or even EDL hardware. Finding large landing sites is challenging in Séítah, so shorter flights are preferred, to reduce the potential drift, and hence reduce the required size of the landing ellipse. Outside of Séítah, the terrain is relatively flat and helicopter-friendly, allowing for large landing ellipses and long flights with greater drift. Let’s look at the different options and their landing sites:
- Option A: one landing ellipse outside of the Séítah that is large and safe.
- Option B: The landing ellipse for Flight 24 is within the Séítah, limiting its size, and requires a medium-distance flight, given less margin and making it slightly riskier than landing outside the Séítah.
- Option C: The first landing site (for Flight 24) requires only a short flight, reducing the amount of potential drift, and it remains within the relatively large landing ellipse of the previous flight, 23.
Keeping up With the Rover
Perseverance is making great progress on its drive to the river delta, and it is important that Ingenuity keeps pace to arrive at the delta before the rover does. This is for two reasons: telecommunications and safety. Ingenuity only communicates with the helicopter base station on Perseverance, so it needs to stay close enough to have a good connection. For safety, it is ideal if Ingenuity flies ahead of Perseverance to avoid ever having to fly past or near the rover, to minimize the risk of any close contact in a worst-case scenario.
Let’s review each of the factors above to see which option gives the best set of trade-offs to balance risk:
|Time of Sol
|Drift / Landing Site
|Keeping With Rover
|N/A. Too hot
|No landing in Séítah
|Have to wait
|10:00 (no change)
|Medium flight in Séítah;
EDL hardware risk
|A short flight in Séítah
Which option would you choose?
As is often the case in Ingenuity operations, there is no obvious solution that is the best for all factors: Trade-offs have to be made based on the available data and the judgment of team members. In this case, the helicopter team decided to go with option C.
Flight 24 Summary
With option C, flight 24 was a short hop and yaw at 09:30 LMST with 2,537 rpm, and set us up to exit Séítah on flight 25.
Flight #: 24
Goals: Test flight at 2,537 rpm, 09:30 LMST flight
Altitude: 10 meters
Time aloft: 69.5 seconds
Distance: 47 meters
With Flight 24 in our log book, it is now time to look forward to our upcoming effort that charts a course out of Séítah. Flight 25 – which was uplinked yesterday – will send Ingenuity 704 meters to the northwest (almost 80 meters longer than the current record – Flight 9). The helicopter’s ground speed will be about 5.5 meters per second (another record) and we expect to be in the rarefied Martian air for about 161.5 seconds.
See you at the delta!
Written by Ben Morrell, Ingenuity Operations Engineer at NASA’s Jet Propulsion Laboratory.