Author Topic: ITS for the Moon  (Read 15598 times)

Online DrRobin

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Re: ITS for the Moon
« Reply #20 on: 11/26/2016 04:55 PM »
(estimates I've seen before being ~30 tons to the lunar surface with only refueling in LEO, ~350 tons with refueling in lunar orbit after ascent, even the first of which is well beyond what previous studies indicated necessary to build a base)

Is there a reference for that 30 t value? My calculations show that a Direct Lunar mission (using refueling in LEO) gets a cargo mass of -36.7 t, which means the scheme won't work since you need a have a payload with negative mass!

https://www.reddit.com/r/spacex/comments/55k1f4/its_moon_landing_payloads_and_costs/ he finds 38 tons direct, 380 tons with lunar orbital refueling, I rounded down a bit to accommodate some safety margin and slight underperformance. Odd inconsistency here though. I'm trying to figure out where both of you got the delta v numbers from, neither matches up well with charts I've seen before. The guy on reddit has a higher delta v for "earth orbit to moon orbit" than you have for TLI+LOI, lower delta v for ascent and descent, lower for TEI, and higher for earth EDL, but the total delta v values only differ by about 500 m/s (9.7 km/s for the reddit post, 10.2 for yours), which isn't enough to explain such a huge discrepancy in payload capacity. And it looks like you're using the same mass and ISP values. He doesn't give his full calculations though, just spreadsheets

I was just about to post my own numbers when I saw this. Working through the numbers, I think the difference is whether you go to lunar orbit first or straight to the lunar surface (obviously a more risky approach but maybe worth it for lower-value payloads) and how much you budget for Earth EDL. Using the numbers from the Wikipedia page on delta-V budgets for a direct landing (5.93 km/sec from LLO to Lunar surface, 2.80 km/sec from Lunar surface to C3=0, 0.75km/sec EDL) gives a total dV of 9.48, for a mass fraction of 12.61, and a payload of 18t. (Adding Steven's 2% margin, gives 9t). Still, you can do a lot better with Steven's LLO propellant transfer scheme.

Online DrRobin

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Re: ITS for the Moon
« Reply #21 on: 11/26/2016 05:10 PM »
Space stations, tankers and propellant depots in lunar orbit will need station keeping. A set of ion thrusters should be able to supply this.

Too small for two thousand tonne tankers.  Fully loaded ITS tanker is 5-6x ISS mass, and lunar orbits are unstable for most part. Fine for Orbital Outpost (maybe 100 tonnes) in high orbit or EML-1/2.

I think "for the most part" is important. There are a small number of stable "frozen" lunar orbits that can be used for longer duration missions with very little station keeping. [See discussion of Frozen Orbits here: https://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit ]

Offline laszlo

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Re: ITS for the Moon
« Reply #22 on: 11/26/2016 05:51 PM »
Isn't leaving the return fuel in lunar orbit what the Apollo lander did? ;)

Online guckyfan

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Re: ITS for the Moon
« Reply #23 on: 11/26/2016 05:57 PM »

I think "for the most part" is important. There are a small number of stable "frozen" lunar orbits that can be used for longer duration missions with very little station keeping. [See discussion of Frozen Orbits here: https://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit ]

But are such stable orbits suitable for a fuel depot? I think it needs to be in low lunar orbit. Steven Pietoban may correct me if I am wrong.

But I don't think it would be a big problem. Make that depot only as large as needed. Just above 200t if we assume 2 landed ITS at the same time. 100 if we assume only one. ITS would not stay over night so be on the ground for maybe 10 days. During that time stability should not be the issue. For the empty depot some ion thrusters could do the station keeping.

Offline Steven Pietrobon

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Re: ITS for the Moon
« Reply #24 on: 11/27/2016 07:45 AM »
OMG, Pascal. I am old enough to have a few stacks of punchcards encoding batch FORTRAN code I wrote back when Pascal was a spiffy new programming language, all the rage for use as a teaching language, all clean and strict about good programming technique.

We must be of similar age as I also have stacks of punchcards written in Fortran. :-) Us students would write the code (80 characters to a line) and secretaries would punch out the cards for us! Any mistakes and we would punch the correction ourselves. Running the software involved submitting the punched cards, waiting a while while the technicians ran the program, and then collecting the printed output, hopefully without any errors, otherwise the whole process would have to be repeated. Just seems so archaic, but we accepted this method as pretty normal at the time.

Quote
Anyway, just to make sure to have something on-topic, I did want to highlight the general concept you employ: The exponential nature of the rocket equation delta-V budgets can be a killer for out-and-back missions, but you can sometimes soften the requirements by caching propellant at one or more stops partway out, in the mode of basecamps for mountaineering, where each expedition helps the ones that come after.

That sort is similar to what I'm doing, except instead of basecamps (or propellant depots in this case) we have back and forth travellers passing what they need to each other.
« Last Edit: 11/27/2016 07:48 AM by Steven Pietrobon »
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Offline Steven Pietrobon

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Re: ITS for the Moon
« Reply #25 on: 11/27/2016 07:56 AM »
https://www.reddit.com/r/spacex/comments/55k1f4/its_moon_landing_payloads_and_costs/ he finds 38 tons direct, 380 tons with lunar orbital refueling, I rounded down a bit to accommodate some safety margin and slight underperformance. Odd inconsistency here though. I'm trying to figure out where both of you got the delta v numbers from, neither matches up well with charts I've seen before. The guy on reddit has a higher delta v for "earth orbit to moon orbit" than you have for TLI+LOI, lower delta v for ascent and descent, lower for TEI, and higher for earth EDL, but the total delta v values only differ by about 500 m/s (9.7 km/s for the reddit post, 10.2 for yours), which isn't enough to explain such a huge discrepancy in payload capacity. And it looks like you're using the same mass and ISP values. He doesn't give his full calculations though, just spreadsheets

Looks like he gets his delta-V values from Wikipedia, not the most reliable of sources. I got most of my delta-V values from "Apollo: The Definitive Sourcebook" by Orloff and Harland.
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline Steven Pietrobon

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Re: ITS for the Moon
« Reply #26 on: 11/27/2016 08:02 AM »
I was just about to post my own numbers when I saw this. Working through the numbers, I think the difference is whether you go to lunar orbit first or straight to the lunar surface (obviously a more risky approach but maybe worth it for lower-value payloads) and how much you budget for Earth EDL. Using the numbers from the Wikipedia page on delta-V budgets for a direct landing (5.93 km/sec from LLO to Lunar surface, 2.80 km/sec from Lunar surface to C3=0, 0.75km/sec EDL) gives a total dV of 9.48, for a mass fraction of 12.61, and a payload of 18t. (Adding Steven's 2% margin, gives 9t). Still, you can do a lot better with Steven's LLO propellant transfer scheme.

Yes, that's what I was thinking for the difference. With a direct landing and ascent the delta-Vs are lower which would make a significant difference to the payload mass. My understanding is that only certain areas of the Moon can be reached using direct landing.
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline sdsds

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Re: ITS for the Moon
« Reply #27 on: 11/27/2016 08:55 AM »
It's an excellent concept!

I would appreciate thoughts on a modification of the idea. Suppose there were and additional something else at the rendezvous orbit. A deep space habitat in need of cargo resupply, for example. Could an ITS-like system earn revenue by delivering some of that cargo and then like Cygnus has just demonstrated proceed on to a secondary mission? For the ITS-like system that secondary mission would be a trip down to the lunar surface and back. Would this work with non-negative payload masses even if the habitat were in lunar DRO or NRO?
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Offline MikeAtkinson

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Re: ITS for the Moon
« Reply #28 on: 11/27/2016 01:27 PM »
Here is my spreadsheet.

https://docs.google.com/spreadsheets/d/15kgq-0x6BKnNXGO9WFKfjhtE42WJncbpZHq3VHoQ3OA/edit#gid=0

81 tonnes payload delivered to the moon.

Offline sdsds

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Re: ITS for the Moon
« Reply #29 on: 11/28/2016 01:43 AM »
Here is my spreadsheet.

https://docs.google.com/spreadsheets/d/15kgq-0x6BKnNXGO9WFKfjhtE42WJncbpZHq3VHoQ3OA/edit#gid=0

81 tonnes payload delivered to the moon.

Thanks for putting this together; it is very easy to understand!

But just to clarify the obvious: is the requirement that ITS must eject 81 tonnes of mass at the lunar surface for the mission to "close?"
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Offline MikeAtkinson

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Re: ITS for the Moon
« Reply #30 on: 11/28/2016 07:55 AM »
Here is my spreadsheet.

https://docs.google.com/spreadsheets/d/15kgq-0x6BKnNXGO9WFKfjhtE42WJncbpZHq3VHoQ3OA/edit#gid=0

81 tonnes payload delivered to the moon.

Thanks for putting this together; it is very easy to understand!

But just to clarify the obvious: is the requirement that ITS must eject 81 tonnes of mass at the lunar surface for the mission to "close?"

81 tonnes or less.

You could land an empty ITS and return with some payload, or land far less than 81 tonnes and return it. I perhaps should amend the spreadsheet to cover those cases.

Offline Steven Pietrobon

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Re: ITS for the Moon
« Reply #31 on: 11/28/2016 08:30 AM »
Here is my spreadsheet.

https://docs.google.com/spreadsheets/d/15kgq-0x6BKnNXGO9WFKfjhtE42WJncbpZHq3VHoQ3OA/edit#gid=0

81 tonnes payload delivered to the moon.

Lunar escape velocity is 2.38 km/s. Both your Lunar descent and ascent delta-V's are 2.4 km/s. You have only included 20 m/s for gravity losses, equivalent to having a burn time of 20/1.622 = 12 seconds! Lets say there are about 200 m/s of gravity losses (about 2 minutes at 1.622 m/s).

Also, escape velocity will leave you in orbit at Lunar distance. Additional delta-V is required to lower your perigee. This varies from 790 to 903 m/s depending if you are at Lunar perigee or apogee. Assuming 0.8 km/s required to lower the perigee, that gives a total delta-V of sqrt(2.38+0.8)+0.2 = 2.71 km/s. Thus, you need to add about 0.3 km/s to your delta-V values.
 
A TLI of 3.2 km/s is plenty to get to the Moon. You don't need 3.5 km/s.

Your landing delta-V of 700 m/s should be OK. I used 733 m/s (including margin).
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Offline sdsds

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Re: ITS for the Moon
« Reply #32 on: 11/29/2016 06:17 AM »
I used the spreadsheet provided by MikeAtkinson as the basis for a look into whether propellant transfer between ITS ships in an orbit used by a deep space habitat could enable delivery of cargo to both the hab and then subsequently to the lunar surface.

It /seems/ to show each ITS mission could deliver both 35 t of cargo to the hab and something like 20 t of cargo to the lunar surface. This is enabled by transferring 63 t of propellant between departing and returning ITS ships.

I assumed the ships for this would be 25 t more massive in dry weight (175 t total) to provide for robust cargo handling equipment, performance margin, etc. My delta-v numbers are from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019648.pdf, as best I understand it.
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Offline Steven Pietrobon

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Re: ITS for the Moon
« Reply #33 on: 11/30/2016 07:01 AM »
I used the spreadsheet provided by MikeAtkinson as the basis for a look into whether propellant transfer between ITS ships in an orbit used by a deep space habitat could enable delivery of cargo to both the hab and then subsequently to the lunar surface.

You have 2400 m/s for Lunar descent from LLO. That's a bit high. Apollo was 2069 m/s. A value of 2100 m/s would be plenty.
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Offline sdsds

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Re: ITS for the Moon
« Reply #34 on: 11/30/2016 07:19 AM »
Oh yikes, you're right! Looking closer it also seems I had an unreasonably low lunar ascent delta-v. Fixing that is going to hurt the payload performance, more than it is helped by fixing the descent delta-v error.
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Offline J-V

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Re: ITS for the Moon
« Reply #35 on: 11/30/2016 12:24 PM »
What would the numbers look like, if assuming ISRU LOX from lunar surface? Somewhat better I assume, but how much?

Online guckyfan

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Re: ITS for the Moon
« Reply #36 on: 11/30/2016 01:56 PM »
What would the numbers look like, if assuming ISRU LOX from lunar surface? Somewhat better I assume, but how much?

Without numbers, a lot better, given that LOX is most of propellant by mass. They could land empty of LOX. Somehow it feels like a waste though to process water to LOX and release the hydrogen for lack of CO2. Also water would imply the poles.

I have recently seen there is work to produce oxygen from SiO2. A process that would extract the oxygen using electrolysis at very high temperatures provided by concentrating sunlight. This could be done everywhere on the moon.

I like the concept of Steven Pietroban leaving return propellant in orbit more though. No massive ISRU needed and still more than 100t payload to the surface.

Offline Steven Pietrobon

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Re: ITS for the Moon
« Reply #37 on: 12/01/2016 03:48 AM »
Producing Lunar oxygen would be a future enhancement to increase payload mass. I think the best source is Lunar regolith instead of ice, since its so plentiful (although harder to extract). Here's a link various sources on the subject.

http://www.lunarpedia.org/index.php?title=Oxygen

The first 180 t payload could contain a Lunox factory.
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Online AncientU

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Re: ITS for the Moon
« Reply #38 on: 12/02/2016 12:03 AM »
What would the numbers look like, if assuming ISRU LOX from lunar surface? Somewhat better I assume, but how much?

Without numbers, a lot better, given that LOX is most of propellant by mass. They could land empty of LOX. Somehow it feels like a waste though to process water to LOX and release the hydrogen for lack of CO2. Also water would imply the poles.

I have recently seen there is work to produce oxygen from SiO2. A process that would extract the oxygen using electrolysis at very high temperatures provided by concentrating sunlight. This could be done everywhere on the moon.

I like the concept of Steven Pietroban leaving return propellant in orbit more though. No massive ISRU needed and still more than 100t payload to the surface.

Actually 180 tonnes per Steven's calculations. If you park a tanker in Lunar orbit, a series of ITS-payloads could land, return to orbit/refueling, and then back to Earth. The tanker would return to Earth on its last 100 tonnes or so of fuel.

ISRU is much harder than this strategy... good for the long run, but not a pre-req to get going.
« Last Edit: 12/02/2016 12:05 AM by AncientU »
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Offline sdsds

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Re: ITS for the Moon
« Reply #39 on: 12/02/2016 02:10 AM »
What would the numbers look like, if assuming ISRU LOX from lunar surface? Somewhat better I assume, but how much?

I think in this case the numbers are easy. Every kg of propellant you transfer aboard while on the lunar surface is a kg of propellant you didn't have to bring down to the lunar surface. So it frees up a kg of down-mass payload capability. Yes?
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