CLPS Mission Design Trade-Offs

[sdsds]

Starting a thread here to divert discussion/speculation about individual CLPS mission lander and trajectory design choices from the mission update threads.

There are two flown missions to compare, plus more unflown missions.

IM-1: https://forum.nasaspaceflight.com/index.php?topic=59696

Peregrine: https://forum.nasaspaceflight.com/index.php?topic=60169

[sdsds]

I'm pretty sure they did not use all available space.   Here's a picture of the lander in the spacecraft adapter.  Assuming it's a standard spacecraft adapter (1.575m) (330 pixels), then the landing gear spans 903 pixels, or about 4.3 meters.  I get similar results measuring other pictures.   So they could have widened the stance at least some.

Possible error in your calculus: You are measuring the length of a side of the square, not the diagonal across the square, which is actually the controlling dimension.  From what I can find, the payload envelope size in the largest Falcon 9 fairing allows for a maximum diameter of 4.572m (180").  Intuitive Machines describe their lander as having legs that are 4.6m wide.  I think it's safe to say that they used up all available space for a fixed leg design.

You are correct.  IM says 4.6 meters in many places.  And the inside of the fairing (as shown in diagrams above) allows 4.604 meters.  So they did use the available room.

That's excellent -- thanks!

Given the choice to stack the propellant tanks and the use of all available (non-deployable) leg stance width, the solution to prevent tipping over on future missions must be ... software!

[sdsds]

Both Astrobotic and Intuitive chose pressure-fed engine designs. Is it a 'well known result' that more complex engine cycles don't scale down well to CLPS-sized spacecraft?

[sdsds]

Several aspects of  the IM-1 mission give the appearance that they were working with a 'ticking clock.'

a) They chose a direct TLI rather than the Peregrine phasing loop trajectory.
b) They worked like the dickens to start lunar descent within their pre-announced 24 hour time budget, despite facing a hardware issue that required a massive effort software/hardware work-around.

Was this because:
a) their supply of gaseous helium, used for both RCS and tank pressurization, was running low?
b) their choice to contain the propellants rather than allow boil-off meant temperature/pressure were rising and they risked loss of tank structural integrity?

[tolis]

For a safe landing irrespective of attitude, a caltrop might work best: https://en.wikipedia.org/wiki/Caltrop
However, most payloads come with bespoke pointing/operability constraints, requiring some mechanism to change the lander attitude post-landing. Something along those lines was used in Mars Pathfinder I believe. For the Moon, you might have legs that can be extended or shortened (as opposed to deployed from a stowed position), same as with the legs that come out on a photo tripod.

[Jim]

Both Astrobotic and Intuitive chose pressure-fed engine designs. Is it a 'well known result' that more complex engine cycles don't scale down well to CLPS-sized spacecraft?

CLSP size?  Show me any spacecraft that doesn't use pressure fed.  Hint it isn't determined by size.

[Jim]

Several aspects of  the IM-1 mission give the appearance that they were working with a 'ticking clock.'

a) They chose a direct TLI rather than the Peregrine phasing loop trajectory.
b) They worked like the dickens to start lunar descent within their pre-announced 24 hour time budget, despite facing a hardware issue that required a massive effort software/hardware work-around.

Was this because:
a) their supply of gaseous helium, used for both RCS and tank pressurization, was running low?
b) their choice to contain the propellants rather than allow boil-off meant temperature/pressure were rising and they risked loss of tank structural integrity?

A.  Just direct (no "TLI").  No.  The Falcon 9 second stage second burn provided the transfer orbit.
b.  They likely had vents

[Asteroza]

Both Astrobotic and Intuitive chose pressure-fed engine designs. Is it a 'well known result' that more complex engine cycles don't scale down well to CLPS-sized spacecraft?

CLSP size?  Show me any spacecraft that doesn't use pressure fed.  Hint it isn't determined by size.

Is that related to the pressure-fed/turbine sizing line, which is about 5000 lbsf thrust?

[Jim]

Both Astrobotic and Intuitive chose pressure-fed engine designs. Is it a 'well known result' that more complex engine cycles don't scale down well to CLPS-sized spacecraft?

CLSP size?  Show me any spacecraft that doesn't use pressure fed.  Hint it isn't determined by size.

Is that related to the pressure-fed/turbine sizing line, which is about 5000 lbsf thrust?

I don't know.  Look at the Apollo spacecraft "engines"

[sdsds]

[...]
b) their choice to contain the propellants rather than allow boil-off meant temperature/pressure were rising and they risked loss of tank structural integrity?

[...]
b.  They likely had vents

That was my assumption, until the media telecon where Altemus or Crain seemed to say they had zero boil-off. Any use of a pressure-relief vent counts as boil-off, yes?

[trimeta]

Both Astrobotic and Intuitive chose pressure-fed engine designs. Is it a 'well known result' that more complex engine cycles don't scale down well to CLPS-sized spacecraft?

CLSP size?  Show me any spacecraft that doesn't use pressure fed.  Hint it isn't determined by size.

Doesn't interplanetary Photon use a special electric-pump-fed variant of Curie? Plus, I don't know whether electric thrusters (Hall effect, etc.) count as pressure-fed.

That said, those have very particular/unique applications, pressure-fed is certainly the standard for in-space usage, especially if you want a decent amount of thrust quickly (which would of course be necessary for a Moon landing).

[Jim]

Doesn't interplanetary Photon use a special electric-pump-fed variant of Curie?

Has it flown?

 Plus, I don't know whether electric thrusters (Hall effect, etc.) count as pressure-fed.

no pumps.

[trimeta]

Doesn't interplanetary Photon use a special electric-pump-fed variant of Curie?

Has it flown?

Indeed it has, on the CAPSTONE mission. Per the Lunar Photon brochure, "ΔV greater than 3 km/sec is provided by a storable, re-startable bi-propellant propulsion system called HyperCurie, evolved from the heritage Curie engine, using electric pumps to supply pressurized propellant to a thrust vector-controlled engine."

 Plus, I don't know whether electric thrusters (Hall effect, etc.) count as pressure-fed.

no pumps.

I believe that electric thrusters are different enough from a "pressure-fed engine" as to count as a wholly separate engine cycle. But sure, to my knowledge they do not use pumps, unless we count the electrical fields which pull ions along as "pumps" (which I wouldn't). And they're certainly not the "more complex engine cycles" which the original question asked about.

I also didn't mention solid rocket motors (as seen on boost stages like the Star 48), which also don't use the "pressure-fed engine" cycle but don't use pumps either. These likewise aren't "more complex" engines (or engines at all, I suppose). And I guess there's an argument that if they're single-use, they're a rocket stage, not a spacecraft.

[sdsds]

Both Astrobotic and Intuitive chose pressure-fed engine designs. Is it a 'well known result' that more complex engine cycles don't scale down well to CLPS-sized spacecraft?

[...] Show me any spacecraft that doesn't use pressure fed.  Hint it isn't determined by size.

I assert size does matter, along with complexity/reliability and parasitic mass. (I'll further assert a long-duration Centaur stage could count as a spacecraft, enabled by the near-magic of an RL10 engine. And of course there was the ACES design. And this thing called Starship.)

What I can't show is any small spacecraft prior to Nova-C that used cryo-propellants, which enable use of an expander engine cycle. If expander cycle turbo-machinery were small enough it could conceivably mass less than a tank of helium. So ... that's what motivates the size question. Can turbo-pumps be made small enough for this, or does the physics not scale down?

[Jim]

I believe that electric thrusters are different enough from a "pressure-fed engine" as to count as a wholly separate engine cycle. But sure, to my knowledge they do not use pumps, unless we count the electrical fields which pull ions along as "pumps" (which I wouldn't). And they're certainly not the "more complex engine cycles" which the original question asked about.

Their propellant is compressed gases like Xenon or Argon.

[Jim]

I assert size does matter, along with complexity/reliability and parasitic mass. (I'll further assert a long-duration Centaur stage could count as a spacecraft, enabled by the near-magic of an RL10 engine. And of course there was the ACES design. And this thing called Starship.)

Those are launch vehicle stages before they are spacecraft buses.  That is what drives the propulsion design.

Agena, when used as a spacecraft bus, seldom re-ignited the Hustler engine and even added a secondary propulsion system or solid motors for delta V later in the mission.  Restarts were mostly common when it was used as an upperstage or tug.

[sdsds]

Did the Apollo LM contact probes actually extend as far as shown in this diagram? And ... are contact probes still a thing?

https://ocw.mit.edu/courses/sts-471j-engineering-apollo-the-moon-project-as-a-complex-system-spring-2007/0028108a6ba8ed1910de0b0940194f7f_4_2_lunr_landing.pdf

[Jim]

Did the Apollo LM contact probes actually extend as far as shown in this diagram? And ... are contact probes still a thing?

5 feet I believe.  Haven't seen them

[sdsds]

So how silly would it be to put an Estes-E sized solid (Burn Time: 2.4 sec; Total Impulse: 27.2 N-sec; Mass: 59.9 g) on each leg and auto-fire it just before contact? Soyuz capsules (and New Shepard?) sort-of kind-of do this.... It softens the blow, so to speak, reducing 'jerk.' They would also help pitch the lander to match the slope. Once they've all fired the total imposed torque would be zero. Retail cost is less than $12 per motor, and if you agree to slap an Estes decal on your lander I bet you get them for free....

What could possibly go wrong?

[Jim]

So how silly would it be to put an Estes-E sized solid (Burn Time: 2.4 sec; Total Impulse: 27.2 N-sec; Mass: 59.9 g) on each leg and auto-fire it just before contact? Soyuz capsules (and New Shepard?) sort-of kind-of do this.... It softens the blow, so to speak, reducing 'jerk.' They would also help pitch the lander to match the slope. Once they've all fired the total imposed torque would be zero. Retail cost is less than $12 per motor, and if you agree to slap an Estes decal on your lander I bet you get them for free....

What could possibly go wrong?

they don't fire at the same time

[Zed_Noir]

So how silly would it be to put an Estes-E sized solid (Burn Time: 2.4 sec; Total Impulse: 27.2 N-sec; Mass: 59.9 g) on each leg and auto-fire it just before contact? Soyuz capsules (and New Shepard?) sort-of kind-of do this.... It softens the blow, so to speak, reducing 'jerk.' They would also help pitch the lander to match the slope. Once they've all fired the total imposed torque would be zero. Retail cost is less than $12 per motor, and if you agree to slap an Estes decal on your lander I bet you get them for free....

What could possibly go wrong?

they don't fire at the same time

Plus what to use as proximity sensor to initiated ignition of each motor along with power and control issues.

Also how would the Estes motors fared in vacuum and zero-G conditions along with possible thermal issues from being in sunlight and in the shade.

Until the Estes motors have demonstrated to work nominally in near Lunar surface environment after transit from LEO. They shouldn't be considered for use on a Moon lander. Someone will have to do kamikaze lithobraking with maybe a cubesat Estes motor test rig over the Lunar regolith first.

[edzieba]

So how silly would it be to put an Estes-E sized solid (Burn Time: 2.4 sec; Total Impulse: 27.2 N-sec; Mass: 59.9 g) on each leg and auto-fire it just before contact? Soyuz capsules (and New Shepard?) sort-of kind-of do this.... It softens the blow, so to speak, reducing 'jerk.' They would also help pitch the lander to match the slope. Once they've all fired the total imposed torque would be zero. Retail cost is less than $12 per motor, and if you agree to slap an Estes decal on your lander I bet you get them for free....

What could possibly go wrong?

I assume you mean firing up, unlike the Soyuz and Shenzhou landing motors?
Rather than on the legs, a single upward-firing thruster triggered by any leg contact may work to settle the lander and null horizontal and rotation residuals through constraint (friction against the ground) as long as the legs are beefy enough that the maximum impact force from landing plus hold-down thrust does not snap one - at which point your settling thruster has just made things worse.

However, any extra thrusters/giant wide legs/harpoons/sticky goop/other-ACME-catalogue-sourced-mechanisms all need to trade against 'land without the residuals in the first place'. We have existence proof that tall spindly objects can reliably land vertically after descent from high velocities, even with the added problems of wind and the ground bouncing up and down and rolling about. This is not an intractable problem that needs more hardware thrown at it, it just takes a few tries to debug.

[sdsds]

I assume you mean firing up, unlike the Soyuz and Shenzhou landing motors?
[...]

Hadn't thought of that option! Currently knowing nothing about the failure mode a certain six-legged lander experienced, it's tough to guess if hold-down thrust would have helped.

Plus what to use as proximity sensor to initiated ignition of each motor along with power and control issues. [...]

Yes, in truth it's hard to envision any actively controlled solution that wins over purely passive, mechanical, one-time-use 'crush zone' shock absorbers.

[Jim]

Plus what to use as proximity sensor to initiated ignition of each motor along with power and control issues.

Also how would the Estes motors fared in vacuum and zero-G conditions along with possible thermal issues from being in sunlight and in the shade.

Until the Estes motors have demonstrated to work nominally in near Lunar surface environment after transit from LEO. They shouldn't be considered for use on a Moon lander. Someone will have to do kamikaze lithobraking with maybe a cubesat Estes motor test rig over the Lunar regolith first.

Use Aerotech motors

[Zed_Noir]

Plus what to use as proximity sensor to initiated ignition of each motor along with power and control issues.

Also how would the Estes motors fared in vacuum and zero-G conditions along with possible thermal issues from being in sunlight and in the shade.

Until the Estes motors have demonstrated to work nominally in near Lunar surface environment after transit from LEO. They shouldn't be considered for use on a Moon lander. Someone will have to do kamikaze lithobraking with maybe a cubesat Estes motor test rig over the Lunar regolith first.

Use Aerotech motors

@Jim, you are no fun.
Would really want to see someone try to test fire an Estes motor over the Lunar regolith.

Are there solid Aerotech motors as puny as an Estees motor?

[sdsds]

[...]
b) their choice to contain the propellants rather than allow boil-off meant temperature/pressure were rising and they risked loss of tank structural integrity?

[...]
b.  They likely had vents

That was my assumption, until the media telecon where Altemus or Crain seemed to say they had zero boil-off. Any use of a pressure-relief vent counts as boil-off, yes?

So yes, Crain confirmed they used vents, making no statement about boil-off rates though.

[sdsds]

a) their supply of gaseous helium, used for both RCS and tank pressurization, was running low?

Crain or Altemus confirmed the lander ended up on the surface with no remaining consumables. Yet they showed the engine firing at the surface, so apparently ran dry immediately after propulsion was no longer necessary, i.e. near-zero margin.

[Jim]

Plus what to use as proximity sensor to initiated ignition of each motor along with power and control issues.

Also how would the Estes motors fared in vacuum and zero-G conditions along with possible thermal issues from being in sunlight and in the shade.

Until the Estes motors have demonstrated to work nominally in near Lunar surface environment after transit from LEO. They shouldn't be considered for use on a Moon lander. Someone will have to do kamikaze lithobraking with maybe a cubesat Estes motor test rig over the Lunar regolith first.

Use Aerotech motors

@Jim, you are no fun.
Would really want to see someone try to test fire an Estes motor over the Lunar regolith.

Are there solid Aerotech motors as puny as an Estees motor?

Don't think black powder would work

[Comga]

So how silly would it be to put an Estes-E sized solid (Burn Time: 2.4 sec; Total Impulse: 27.2 N-sec; Mass: 59.9 g) on each leg and auto-fire it just before contact? Soyuz capsules (and New Shepard?) sort-of kind-of do this.... It softens the blow, so to speak, reducing 'jerk.' They would also help pitch the lander to match the slope. Once they've all fired the total imposed torque would be zero. Retail cost is less than $12 per motor, and if you agree to slap an Estes decal on your lander I bet you get them for free....

What could possibly go wrong?

they don't fire at the same time

They are not supposed to fire at the same time in this (unlikely) scheme
Each pad has a contact probe that triggers the little motor
If one leg makes contact earlier than the others it fires earlier and the torque helps align the lander with the surface.

Of course, those of us who flew Estes rockets (Is there a weirdo here who didn’t?) know how long it takes for the electrical igniters to work. (I once built a giant model rocket with three engines side by side.  The third lit about 100 feet above the launch pad. Still flew straight!)


If all worked as conceived it would work almost as well as big balls of compressible aluminum sponge.

But that brings to mind another option for a future CLPS lander: a lunar version of the 1970’s Estes rocketcam, an Eaglecam from overhead.  Carry an Estes rocket with a rearward facing camera and a tiny transmitter.  Take context images of the lander. Get Estes to sponsor it.

[HMXHMX]

Or perhaps just build adequate gear?

[sdsds]

[...]
But that brings to mind another option for a future CLPS lander: a lunar version of the 1970’s Estes rocketcam, an Eaglecam from overhead.  Carry an Estes rocket with a rearward facing camera and a tiny transmitter.  Take context images of the lander. Get Estes to sponsor it.

This imaging from above scheme seems grand! I suppose as with any separation event someone would need to perform a re-contact analysis. Presumably that motivated sideways ejection of EagleCam.

[sdsds]

NASA (and previously the NACA) have a mandate to foster the aerospace sector of the US economy. Terms like commercial or private largely mean spaceflight conducted by 'for-profit' enterprises. CLPS markedly advances that approach.

For-profit enterprises bring to spaceflight a set of priorities different from those of academic research entities. CLPS providers care about lunar science only because customers might pay them to facilitate collection of science data. CLPS providers care about technology development either because customers will pay to facilitate it, or because their business plan calls for use of the technology to provide service to customers. CLPS providers care about prestige not out of nationalistic fervor but out of a desire to attract new customers to their brand.

Prestige has marketing value. Coca-cola is just flavored sugar water, and yet the brand is a major asset of the for-profit company that owns it. To succeed, CLPS providers need to establish awareness of their brands, and positive brand impressions. If doing that isn't part of a business plan the business won't thrive in a competitive marketplace.

The early CLPS missions have huge marketing potential. The EagleCam photo that wasn't obtained would have dramatically increased the value of the Nova-C and Intuitive Machines brands. It could have made front-page above-the-fold news in major media outlets. Those are  print-media phrases from days long past. Today still image photos are archaic, replaced by videos. So thus the marketing organization in a forward-thinking CLPS company should be strongly motivated to capture third-person perspective video of a lunar landing.

Is that actually difficult? I suggest a micro-sat payload released after descent initiation could use a telescopic camera to capture a video of the landing from a location in orbit just past perilune. Pointing could perhaps be done solely by tracking the telemetry transmissions of the lander. On the first perilune pass the micro-sat would be radio-silent; on a subsequent pass it would transmit the video to the lander for relay to Earth. Sure, lander telemetry gives more technology development data, but the video would have much more marketing value.

[Steven Pietrobon]

Did the Apollo LM contact probes actually extend as far as shown in this diagram?

Yes they did. For Apollo 11, they removed the front probe to prevent it interfering with the astronauts exiting the lander. Each probe was 1.7 m long.

https://space1.com/Artifacts/Lunar_Module_Artifacts/Surface_Sensing_Probe/surface_sensing_probe.html
https://www.nasa.gov/history/alsj/a11/a11.landing.html

And ... are contact probes still a thing?

I'm not aware of them being used on recent landers.

[Steven Pietrobon]

So how silly would it be to put an Estes-E sized solid (Burn Time: 2.4 sec; Total Impulse: 27.2 N-sec; Mass: 59.9 g) on each leg and auto-fire it just before contact?

Well, perhaps using Estes motors might be bit a silly, but the Russian LK crewed Lunar lander was going to use four upward firing motors to settle the vehicle on landing!

https://nick-stevens.com/2016/02/20/soviet-lk-moon-lander/

[Steven Pietrobon]

So thus the marketing organization in a forward-thinking CLPS company should be strongly motivated to capture third-person perspective video of a lunar landing. Is that actually difficult?

Yes! Transmitting live video from 400,000 km away is not trivial. You a need a steerable high gain antenna with a few watts behind it to get the required link margin.

[sdsds]

So thus the marketing organization in a forward-thinking CLPS company should be strongly motivated to capture third-person perspective video of a lunar landing. Is that actually difficult?

Yes! Transmitting live video from 400,000 km away is not trivial. You a need a steerable high gain antenna with a few watts behind it to get the required link margin.

Live video would be really challenging, but once landed the lander could relay to Earth cached video received from the still-orbiting camera?

[chopsticks]

So thus the marketing organization in a forward-thinking CLPS company should be strongly motivated to capture third-person perspective video of a lunar landing. Is that actually difficult?

Yes! Transmitting live video from 400,000 km away is not trivial. You a need a steerable high gain antenna with a few watts behind it to get the required link margin.

Yet they did it on Apollo (TV camera).

[Bunsen]

So thus the marketing organization in a forward-thinking CLPS company should be strongly motivated to capture third-person perspective video of a lunar landing. Is that actually difficult?

Yes! Transmitting live video from 400,000 km away is not trivial. You a need a steerable high gain antenna with a few watts behind it to get the required link margin.

Yet they did it on Apollo (TV camera).

Apollo needed 20 watts of RF into a 20.5 dB gain¹ steerable antenna on the LM and the 70 m DSN antennas receiving.  Technology has improved since then, of course, so you might be able to do it with around 20 dB less in the link budget depending on desired video quality.  That would let you ditch either the high power amp and need for the 70 m dishes or the steerable antenna on the lander.  That's a significant burden -- 20 W of radiated RF probably means 100-200 W of power consumption, a steerable antenna is heavy and bulky (and relies on correct attitude knowledge and body motion staying in bounds and below its maximum slew rate), time on the big DSN dishes is scarce and expensive, and you need at least one of those or some combination of all three.  It's entirely doable, but plenty costly.

[1] I think this is dBi, but the technical note I'm reading isn't explicit.

[edzieba]

Apollo also sent slow-scan TV (200 TV lines, 10 FPS, colour frame sequential). By today's standards, that would be 'not even potato'. It was also not sent live during descent. Apollo also had the advantage of a much more extensive network of fixed and mobile ground stations than the DSN has available today (as well as having access to the DSN at the time, too).

[sdsds]

On the topic of physical contact sensors and other 'old school' ways of doing things, what's the status of radar for range detection during terminal descent? Given the ... technical difficulties ... with lidar lasers, not to mention their cost, is lidar really a design solution superior to radar?

[edzieba]

On the topic of physical contact sensors and other 'old school' ways of doing things, what's the status of radar for range detection during terminal descent? Given the ... technical difficulties ... with lidar lasers, not to mention their cost, is lidar really a design solution superior to radar?

LIDAR is nice as it gives you pretty precise range at a specific angle for relatively low power - and, depending on the LIDAR module, can do so in a wide array to start generating a point-cloud - with low latency. RADAR generally has higher power requirements, antenna design constrains your choice of wide-coverage-low-resolution or high-resolution-low-coverage (beam angle) and slewing an antenna or dish is much slower than slewing a mirror. RADAR has its advantages is being less dependant on optical surface properties (though surfaces have varying RF properties too). Tricks like SAR can up resolution, but at a hit to latency.
Having both is nice. Having both is not always within your mass budget or power budget (or budget budget).