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