totm dec 2023 Artemis Discussion Thread

[tater]

2 hours ago, sevenperforce said:

It's the service module that cripples it. 

And the mass. The Orion capsule is 10.4 tons. Apollo CM was 5.56t.

The Orion CSM almost the same mass as the Apollo CSM—28.8t for Apollo, 26.5t for Orion.

An Apollo CSM could have a reduced prop load, and be capable of Apollo 8 style missions—the whole thing could be sent to TLI by SLS Block 1 in fact, to LLO and back. Remember, the Apollo SM was scaled to brake the CSM coupled to the LM for LOI.

I can't find the mass for CST-100 anywhere. The CSM is "~13,000kg" but that is all I can find.

[kerbiloid]

That's not Orion is small.
That's CrewDragon is overgrown.

A crew cabin cramped between the hypergolic tanks and engines.
What can be more safe and cozy?!

[sevenperforce]

10 hours ago, tater said:

And the mass. The Orion capsule is 10.4 tons. Apollo CM was 5.56t.

Oh yeah, you're right -- I forget that the Dragon 2's capsule mass includes its maneuvering propellant and engines.

Why the hell is it so heavy?

10 hours ago, tater said:

The Orion CSM almost the same mass as the Apollo CSM—28.8t for Apollo, 26.5t for Orion.

An Apollo CSM could have a reduced prop load, and be capable of Apollo 8 style missions—the whole thing could be sent to TLI by SLS Block 1 in fact, to LLO and back. Remember, the Apollo SM was scaled to brake the CSM coupled to the LM for LOI.

I can't find the mass for CST-100 anywhere. The CSM is "~13,000kg" but that is all I can find.

Yeah, I think it's proprietary. I wonder how it could be estimated.

In the pad abort test, Starliner's coast from abort motor cutoff to apogee lasted 11 seconds and its ascent angle was about 41°.  It is dense AF so I will ignore air resistance for a first-order estimate. This means the vertical component of its velocity was 11s*9.81m/s² or 107.9 m/s, making its total velocity 164.5 m/s. This accords fairly well with the observed plume expansion during cutoff (the first diffuse plume travels 336.6 pixels in 11 frames, and since Starliner is 31.4 pixels wide and the playback framerate is 31 fps, that's 48.9 meters in 0.355 seconds or 137.7 m/s which is to be expected since the plume needs time to slow down).

Boost duration was 5.355 seconds, split equally between 2.677 seconds nearly straight up and 2.677 seconds at 45 degrees, which is exactly what you want to get as far away from the fireball and from the exploding rocket as possible. It covers 70.3 meters in the 1.806 seconds it takes before the tower drops out of view, which tells us via kinematic equations that the acceleration during initial vertical boost is 43.1 m/s² (the felt-equivalent of about 5.4 gees for those with weak stomachs) and a speed of 77.8 m/s at an angle of 68.6°.

Treating the burn as two separate boosts (the first half at 68.6° and the second half at 45°) which are each split into vertical and horizontal components and then and solving like a good first-year physics student predicts a speed of 184.2 m/s at 47.8°, about 10% higher than estimated from the coast period. Of course this doesn't account for wind resistance so we would expect it to be slightly higher, so we know we're in the right ballpark.

Finally, Boeing has said that the four engines generate 40,000 pounds-force each or a total of 711.7 kN. The engines are just slightly canted out (my visual estimate is about 6.8°) so cosine losses will bring the effective thrust down to around 706.8 kN. If Starliner's pad weight is 13 tonnes, then they should supply a total acceleration of 54.4 m/s², which is just 3% more than my visual estimate once you subtract gravity (and it should be slightly over because, again, air resistance). All this gives us a good degree of confidence that the pad abort burn was full-thrust.

 

Specific impulse for hypergolic pump-fed SL engines range from the Viking's 248 s to the RD-273's 296 s. There aren't many numbers for pressure-fed hypergolic SL engines (mostly because they aren't often used) but of course we know the SuperDraco comes in at 235 s, so let's ballpark Starliner's abort motors (which are each roughly 2.5X the thrust of a SuperDraco) at the same number. At full thrust these would consume 308.7 kg/s which means a 5.355-second burn requires at least 1,653 kg of propellant. A pad abort would deplete most of the propellant...let's give it a 5% reserve and round it off to 1,750 kg.

It's tough to estimate the mass of these engines, but that won't stop me from guessing. According to SpaceFlightNow, each service module propulsion kit contains 24 additional OMS engines which each provide 6.7 kN as well as 28 RCS engines of unspecified thrust, but we can assume they are the modern version of the R-4D, since Aerojet Rocketdyne now manufactures those. They provide 490 N. All together the thrust capability of the service module is 886 kN, and since I'm lazy I will just borrow the 80:1 T/W ratio of the Viking first-stage engine, which would give them a combined mass of 1.1 tonnes.

The Agena A carried about about 1100 kg more propellant than Starliner, but at least we are in the right ballpark. Its dry mass (less its XLR81 engine) was 751 kg. Scaling with the square-cube law gives us a total tank and structural mass of about 536 kg, for a total service module mass of 3.4 tonnes. That sounds about right if you compare the gross mass and propellant load of the larger (and lacking-in-abort-engine) ESM. So that suggests a loaded capsule mass of around 9.5 tonnes. Not much less than Orion.

 

[Silavite]

4 hours ago, sevenperforce said:

Why the hell is it so heavy?

This thread from NSF gives a partial answer; Orion is just big.

Quote

The reason Orion is so much heaver is simple its huge size. For a pressure vessel, the mass is proportional to its volume. The Apollo CM mass was 5.84 t with a 3.91 m diameter. Orion is 5.0 m diameter, so directly scaling gives 5.84*(5/3.91)³ = 5.84*2.09 = 12.21 t. Orion CM mass is 19% less less at 9.89 t, probably due to using lighter materials, such as aluminium instead of steel for the pressure vessel, lithium-ion batteries and modern electronics. Orion was originally going to carry six crew so that it could be used for ISS and Mars missions. Had Orion been sized for four crew, its diameter would have been 3.91*(4/3)^{1/3} = 4.3 m. That would have reduced its mass to between 9.89*(4.3/5)³ = 6.29 t (Orion model) and 5.84*(4.3/3.91)³ = 7.77 t (Apollo model). Lets use the average of 7.03 t, which is a 29% reduction in mass. So instead of a 25.85 t Orion, it would now be a 18.37 t Orion, which would allow it to be launched by Atlas V and Falcon 9, instead of the only option being the very expensive Delta-IV Heavy.

 

[tater]

20 hours ago, Silavite said:

This thread from NSF gives a partial answer; Orion is just big.

The ESM (according to wiki) is 13.5t wet.

We know the TLI mass of the Orion CSM is supposed to be 26.5t, which suggests that Orion is 13t wet (including crew, presumably?) 8600kg pf props. 360kg of crew consumables.

Actually, ESA says the same: https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Orion/Artemis_1—though they only say  "Orion will weigh over 20 tonnes in total" Maybe the dry capsule is just under 10, then a few tons of consumables and crew?

With a total mass of 26.5, and 8.6t of props (and some of that is presumably RCS), Orion CSM has under 1300 m/s of dv.

No wonder it's useless.

 

[SpaceFace545]

On 4/16/2021 at 12:48 AM, kerbiloid said:

That's not Orion is small.
That's CrewDragon is overgrown.

A crew cabin cramped between the hypergolic tanks and engines.
What can be more safe and cozy?!

the igniting hypergols can keep you warm and give you horrible diseases!!!

[kerbiloid]

Is it now a time to expect the next replacement?

A Skylab Spaceshiplab instead of the modular LOP-G?

[tater]

 

[Mitchz95]

Another excellent deep dive from Apogee:

 

[tater]

I like to have this bloody thing handy.
 

 

 

 

[RCgothic]

Using BLTs to justify crewed missions to a nowhere orbit feel like cheating somehow. Not sure if that's a reasonable reaction or not.

[tater]

6 hours ago, RCgothic said:

Using BLTs to justify crewed missions to a nowhere orbit feel like cheating somehow. Not sure if that's a reasonable reaction or not.

The ballistic transfer is only for cargo (says so in the diagram). I think the combined crew+cargo segment are confusing, however.

 

A lot depends on things like LS mass (maybe Lunar Starship needs to be LS from now on?), but If we were to have LS stay at the Moon, vs coming back to Earth, NRHO might not be awful. LS can probably get to Gateway with about 100t of propellants (a little more would increase margin).

Alternate architectures could have it return to Earth for refilling and loading (including with astronauts), and the most efficient way would be aerobraking, but I think LS might not be ideal for 1-pass aerobraking. Staging another tanker can also obviously work.

Let's assume it stays at the Moon, however. How much could a tanker SS—optimized as a cislunar tug—take to Gateway in 1 go?

What if SS-Tug dumps the nose cone entirely. Gets solar, etc. Call the mass... 60t, and it has a "1 deck" crew cabin on top with a docking port (~190m³, 2.6X the Shuttle). If it gets tanked up to 1200t props, what could it deliver to Gateway in addition to the crew? It needs some 160t of props to propulsively return to LEO. It needs 600t to get to NRHO. So we've reserved 760t of props. It gets the crew to Gateway and back propulsively, AND it delivers 440t of props to the lander. That's actually just about 1 RT to the surface and back to Gateway with an additional 15t of cargo (or prop margin).

The SS-tug could of course also make the trip alone when crew is not there, refilling LS in advance, so that the crew tug then brings a little less propellant, and more cargo.

A regular SS tanker could of course aerobrake, saving some 3.2km/s of dv—which is propellant for LS. Empty 100t SS needs ~15t to do a TEI burn from NRHO—I'm assuming it will aerobrake to LEO, and remain as a space tanker, so it might take a few passes. Slop that to 20t, and a tanker can also deliver 440t to Gateway (higher dry mass hurts it here). I had this do the BLT. It could trade payload for propellant 1:1, so those flights in advance could pre-position elements to be loaded on LS.

[tater]

23 hours ago, Mitchz95 said:

Another excellent deep dive from Apogee:

Pretty good, and we know the numbers seem reasonable ( @sevenperforce nodded to in the vid).

I'd add that the GTO refilling is just one of many possible mission architectures with Starship, well, functioning. It completely changes the landscape. Cislunar transit is possible without direct entry from the Moon, for example via a number of possible mechanisms.

I posted about tug Starships, and with a docking port, and the low mass of CCVs compared to SS itself, it's possible to simply take the CCV to the Moon and back. Instead of a large SM, none is needed. The crew moves to the tug. The TPS could be improved if needed, but the goal can be that the tug propulsively enters an orbit that allows for safe EDL—this might be well above ISS alt, but well below direct entry velocities from cislunar space. If the TPS was improved for direct EDL, then the TEI burn can be shaped such that the CCVs can course correct to EDL themselves, or, the tug can burn to direct EDL, with the goal of separation and a course correction later. This gives abort capability to the CCV as a lifeboat. Tug engines don't restart, you leave the tug in the Dragon/Starliner, and simply offset slightly to avoid the tug burning up, and head home.

[cubinator]

Low lunar orbit would require more stationkeeping fuel than low Earth orbit, wouldn't it?

[tater]

Low lunar orbits for sure do, but there are some stable "frozen" lunar orbits that require much less station keeping.

 

[cubinator]

Interesting.

[Silavite]

For reference on frozen orbits.

The ~86° one would probably be most convenient for the polar activities of Artemis.

[kerbiloid]

41 minutes ago, tater said:

"frozen" lunar orbits

https://en.wikipedia.org/wiki/Frozen_orbit#Lunar_frozen_orbits

[sevenperforce]

On 4/17/2021 at 3:59 PM, tater said:

I want to see Iron Man suits for EVA, damn it.

Rigid mechanical counterpressure to provide constant-volume joints for a flexible pressure suit.

23 hours ago, tater said:

Pretty good, and we know the numbers seem reasonable ( @sevenperforce nodded to in the vid).

I'd add that the GTO refilling is just one of many possible mission architectures with Starship, well, functioning. It completely changes the landscape. Cislunar transit is possible without direct entry from the Moon, for example via a number of possible mechanisms.

I posted about tug Starships, and with a docking port, and the low mass of CCVs compared to SS itself, it's possible to simply take the CCV to the Moon and back. Instead of a large SM, none is needed. The crew moves to the tug. The TPS could be improved if needed, but the goal can be that the tug propulsively enters an orbit that allows for safe EDL—this might be well above ISS alt, but well below direct entry velocities from cislunar space. If the TPS was improved for direct EDL, then the TEI burn can be shaped such that the CCVs can course correct to EDL themselves, or, the tug can burn to direct EDL, with the goal of separation and a course correction later. This gives abort capability to the CCV as a lifeboat. Tug engines don't restart, you leave the tug in the Dragon/Starliner, and simply offset slightly to avoid the tug burning up, and head home.

Oh, wow! I didn't realize he gave me the shoutout. Lemme go thank him.

An intermediate option not noted in the video: you can man-rate Falcon Heavy but not otherwise update Dragon 2, which allows SpaceX to put an ordinary Dragon 2 into GTO. Dragon 2 already has enough propellant onboard to get back to Earth from GTO. So then you stage everything from GTO from start to finish.

The only issue is phasing -- you have a smaller window of possible return dates since you have to match Dragon 2's argument of periapsis.

Obviously doing a dedicated tug as you have proposed opens everything up even more.

[tater]

4 minutes ago, sevenperforce said:

Obviously doing a dedicated tug as you have proposed opens everything up even more.

Yeah, I started getting obsessed with it—there is a reason NASA, and all the "old space" people were pushing it as part of STS... it works. It's particularly useful to note that back in the "Shuttle, Tug, Nuclear Ferry" days of the Space Transportation System, it was not 1 vehicle (Shuttle), but a system. Shuttle was assumed to fly once a week or so in initial planning. Every mission it brings up something, being quickly reused—including props for the tug. Any depot/tug architecture really requires very inexpensive mass to LEO.

Depots and tugs (ACES, etc) all work, and can all leverage smaller LVs being able to build larger capabilities via distributed launch, but it doesn't really shine until the thing filling the LEO gas station is rapidly reusable. At that point, building "ships" in LEO makes far, far more sense than direct ascent from Earth (particularly with expensive, throw away rockets).

The mere existence of working Starship... changes everything.

TPS for direct entry from the Moon? OK, but why when you can simply use some propellant? If you're stuck with Saturn V or SLS, yeah, TPS is way less mass than the required props. If the props are cheap, and the price to orbit is cheap enough? Use props, and a less risky crew return vehicle.

SNC starts looking really good in this department, IMHO.

[cubinator]

Starship seems to have the potential to be the STS we really needed.

[tater]

14 minutes ago, cubinator said:

Starship seems to have the potential to be the STS we really needed.

We need the nuclear ferry!

Spoiler

 

 

[SunlitZelkova]

50 minutes ago, cubinator said:

Starship seems to have the potential to be the STS we really needed.

A look at some of the components, together with the Integrated Program Plan the STS was to serve-

1. Standard Starship (replaces Space Shuttle except way more capable obviously)

2. The Space Tug (based on Tater's design) (same role as the Space Tug in the IPP/STS)

3. Nuclear Starship (replaces Reusable Nuclear Shuttle, due to the radioactive environment, an uncrewed Space Tug docks with the it, Nuclear Starship brings the tug to the Moon and it offloads its cargo there. Standard Starship brings the Tug its cargo in LEO, or the Tug could return to Earth)

4. LEO space base (this would be built out of stripped down starships. It would be much closer to the space station NASA originally wanted in the 70s, which was modular but instead of Proton/Space Shuttle sized modules it had INT-21 sized modules (similar in dimensions to the S-II stage)

5. GEO space base (same as the other but in GEO)

6. Lunar space station (smaller than the space base, but also built out of Starships)

8. Lunar surface base (This could also be made out of Starships)

7. The Mars spacecraft could be just Standard Starship.

8. A space station in Mars orbit was envisioned (same as lunar space station)

9. Mars base (or as SpaceX likes to call it, city) Interestingly, based on IPP proposal concept art, the "using your ship as a permanent habitation module" idea seems to have been used by the IPP as well, so in that program the first Mars base was just MEMs that don't return, whereas it has been speculated that if SpaceX's Mars base plans succeed, the early stages will be probably just be Starships that don't go back

Thomas Paine's dream would come true at last. Might as well name SpaceX Boca Chica the Thomas Paine Space Center (the village of Boca Chica has been "renamed" Starbase if I recall correctly, not the SpaceX facility itself). Maybe reserve the former administrator's name for the Starship launch site on Mars?

Just typing this is exciting but nonetheless, I doubt something resembling this will ever happen.

42 minutes ago, tater said:

We need the nuclear ferry!

  Reveal hidden contents

 

 

You may have been joking but if SpaceX really wanted to do something resembling the above, would it be necessary? The 21st century Space Tug from the SpaceX thread can already get to the Moon on its own, and Starship doesn't need it to get to Mars either.

Nuclear Starship would be really cool though.

[tater]

9 minutes ago, SunlitZelkova said:

You may have been joking but if SpaceX really wanted to do something resembling the above, would it be necessary? The 21st century Space Tug from the SpaceX thread can already get to the Moon on its own, and Starship doesn't need it to get to Mars either.

Nuclear Starship would be really cool though.

I'm all-in for nuclear thermal propulsion.

If I could snap my fingers and take the SLS/Orion budget going forward and throw it at something already worked on at NASA, it'd stay in Alabama and go to nuclear thermal propulsion and I suppose to the Space Technology Mission Directorate in general to move interesting stuff forward with some real money.

FWIW, NTP (solid core) with CH₄ as the reaction mass has an I_sp ~640.

[cubinator]

Starship tank volume is approximately  a cylinder 9 m diameter and 30 m tall. That volume is 1908 m^3. LH2 density is 71 kg/m^3, so 135 tons of hydrogen would fit in the tanks. Let's now use the trusty rocket equation:

m₀ is 85 tons + fuel mass, m_f is just 85 tons. If you could just fill a Starship's fuel tanks with hydrogen and slap a NTR with I_sp of 1000 on the bottom, you'd get...

9348 m/s

Improved NTR designs might reach I_sp of 1500 (14022 m/s) or 2000 (18696 m/s).  Burning all that would get you to Mars in 2.5 months or less, though stopping once you get there would be a bigger challenge. 

Edited April 20, 2021 by cubinator