Author Topic: One Month to Mars -- Methods for Very Fast Settler Transit  (Read 32778 times)

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #60 on: 02/13/2024 09:15 pm »
What are the most promising exotic propulsion concepts, for the given purpose and timeframe?  Why?

NTR or pulsed fission (maybe fusion), if we are constrained to reasonable TRL. So much for propulsion; that was easy.

Pulsed nuclear thermal in Arias 2016 is TRL 1-2, with ballpark equations, yes?  Isn't that min TRL, for any published propulsion concept?

The problem here is that it's all dependent on the quench rate of the heat rejection medium, which is in turn dependent on... you, know, heat rejection.  It's not a system until you figure out the thermal side.

This gets back to my "small mass flows suck, but large mass flows are impossible" issue.  The only way to make the system efficient is to scale it up, but that requires unobtanium.

Find a way to make Orion-style detonation systems clean enough to be viable and I'm all ears.  Until then, NTR is as good as you're gonna get, and it's not good enough for 1 month trips, even at scale.

After thinking about it, I object to TRL=2 being worth discussion.  I'd put the cutoff at at least 3, maybe 4.  Otherwise, you're just having a science fiction fan discussion.

Improvement on Project Orion is noted already in thread.

Also, you made several noted mistakes, recently; will you address them?  You don't want casual accusations tossed your way, right?
« Last Edit: 02/14/2024 02:48 am by LMT »

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #61 on: 02/13/2024 09:35 pm »
...VASIMR engines could do this.

After 40 years, VASIMR thrust is still... rather low.  Is there a roadmap to better?

Low thrust is perfectly fine when applied over a long period of time. It adds up, that's the point of continuous acceleration.

And yes, it's scalable. The 40 days to Mars concept "just" needs a low-mass space-rated 200 mW nuclear reactor to power it.

But don't assume; calculate, to check basics. 

Currently, 200 MW feeding 1000 VASIMRs (!) would give a ship just 6 kN thrust.  How does the delta-v compare to the impulsive one-month min-delta-v requirement?  How much power, and how many VASIMRs, to meet that requirement with finite burns?  For that calc, take ship transit mass as 400 t, apart from VASIMR, and don't bother with fast return.

Total dV is always going to be about the same. Low thrust isn't a problem when you're continuously burning the engine.

That wasn't the calc.  Do you know what the finite-burn delta-v is?

Offline Todd Martin

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #62 on: 02/13/2024 10:09 pm »
I have one note to help on this DV budget for the OP.  Nowhere did it say that the journey to Mars has the clock started in LEO.  If the crew were to transfer onto a spacecraft in L1 before heading to Mars, the DV budget would be significantly less (see attachment).  Breaking up the journey into more manageable chunks is kinder to the rocket equation and makes a lower thrust higher ISP option easier.
 

Offline Vultur

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #63 on: 02/13/2024 11:22 pm »
I think that 1 month is unnecessarily constraining: you don't need artificial gravity to survive ordinary Mars trip times. (Even less so if we are speaking of *settlers*: ISS astronauts have to go back to 1 g, Mars settlers will be living only in 0.38 g.)

But accepting the constraints of the thread ... and assuming that the 2039 mention means we're ruling out options that require major in-space infrastructure already to exist (like light sails pushed by massive off-Earth laser arrays) ...

...well, you need high Isp and decent thrust. Absent said giant laser arrays, sail propulsion types (solar, electric, whatever) are way too low thrust.

 Chemical and NERVA or DRACO style solid-core nuclear thermal are too low Isp.

That leaves a number of concepts:
- Orion style nuclear pulse. Sufficient TRL, but utterly prohibitive political problems and cost

- More advanced nuclear pulse. Similar problems; lower TRL, some versions maybe somewhat less politically impossible, but still way too much so.

- Gas-core nuclear thermal, possibly. But how would you test it without massive irradiation? Similar political problems.

- The Zubrin "nuclear-salt-water rocket". Easily has the performance, probably way more than needed. But testing would be even worse than the gas-core probably...

-Electric with an *incredibly* high power reactor, or truly colossal, massively thin "solar wings" to get the horrendous power needed for decent thrust.  Space nuclear reactors are probably politically viable, but I don't think you could get one with the huge needed power and W/kg ratio by 2039, or even 2059. Solar has no political problems but making and deploying the colossal super-light solar thin-film wings needed... ouch.

No way this is happening in the next few decades.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #64 on: 02/14/2024 12:43 am »
...you don't need artificial gravity to survive ordinary Mars trip times.

Region 3, ISS 1-8, 150 days.

Region 4, Soyuz 31, 140 days.

With large crews, you'd probably worry about outliers.

Offline Vultur

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #65 on: 02/14/2024 12:47 am »
...you don't need artificial gravity to survive ordinary Mars trip times.

Region 3, ISS 1-8, 150 days.

Region 4, Soyuz 31, 140 days.

With large crews, you'd probably worry about outliers.

Are we discussing crews, or settlers?

Mars settlers do not need 100% bone density. 

Assuming we're discussing very large crews (large enough to care about outliers) but not permanent settlers, there's a big gap between 140 days and one month, and getting the trip time down more and more gets increasingly expensive. What about 120 days? 100? 80? 60?
« Last Edit: 02/14/2024 12:52 am by Vultur »

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #66 on: 02/14/2024 12:53 am »
...you don't need artificial gravity to survive ordinary Mars trip times.

Region 3, ISS 1-8, 150 days.

Region 4, Soyuz 31, 140 days.

With large crews, you'd probably worry about outliers.

Are we discussing crews, or settlers?

Mars settlers do not need 100% bone density.

Settlers, obviously.  It's strange that you care less about their health.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #67 on: 02/14/2024 12:57 am »
political problems

Future "K Street" hypotheticals are probably OT.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #68 on: 02/14/2024 06:29 am »
...VASIMR engines could do this.

After 40 years, VASIMR thrust is still... rather low.  Is there a roadmap to better?

Low thrust is perfectly fine when applied over a long period of time. It adds up, that's the point of continuous acceleration.

And yes, it's scalable. The 40 days to Mars concept "just" needs a low-mass space-rated 200 mW nuclear reactor to power it.

Best I can tell, in your cited 2011 paper,¹﹐² Ad Astra is assuming about 75mN/kW as specific thrust at an average Isp of about 20,000s, in an Isp range of 4000s - 30,000s.  In a 2012 paper, they measured about 55mN/kW at Isp=1800s (Fig 4), and an Isp no higher than about 5000s (Fig. 7) with lower specific thrusts.

In a 2022 paper that focused on long-duration testing, they measured specific thrust no higher than 28mN/kW.  The trend is not reassuring.

In a TRL 2 kinda way, VASIMR is fairly promising, in that all it requires is an arbitrarily large nuclear electric system and the heat rejection to go with it.  If they can really do Isp>10,000s with ~100mN/kW, then it might do the job.

But always, always, always:  mdot = thrust / ( Isp*9.8 ).  If the prop load required to do the mission in the allotted time is so large that prop/mdot > missionTime, then it doesn't work.  The paper only passes that test if you believe the implied specific thrusts and Isps.  However, compared to some of the other proposals floating around on this thread, it's close to a slam-dunk.

__________
¹I really wanted to dig out some delta-v numbers from this mess of a paper.  (Would it have killed them to include a friggin' table??)  If I gathered stuff correctly from hither and yon, and lawyered their definitely-not-apples-to-apples numbers properly, then, assuming an average Isp of 20,000s (an eyeball of the graphs), I got the attached.

Note that, in both cases, the lander separates for an entry speed of 6.8km/s, but the propulsion section travels on, braking until it re-encounters Mars to be captured into LMO.  I can't explain why the delta-v for the heavier ship, which re-encounters Mars in 36 days, is less than the 12MW version, which requires 110 days.  It's possible that the mass of the drop tanks changes the T/W enough to make a difference.  Or I could've messed something up.  I think I've lost the will to live on running this down any further.

²Different link, same paper.
« Last Edit: 02/14/2024 06:43 am by TheRadicalModerate »

Offline TheRadicalModerate

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #69 on: 02/14/2024 06:38 am »
Also, you made several noted mistakes, recently; will you address them?  You don't want casual accusations tossed your way, right?

Nope, I'm not going to correct, and I'm not willing argue about exoatmospheric sandbraking.  The idea is silly on its face.  I do this for fun, but there's a limit.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #70 on: 02/14/2024 07:50 am »
What do you think is a reasonable payload mass for a crew module + passengers + consumables for a Starship?  I used to be using 50t, but my guess is that 100t is probably closer.  That'd be 1t/person for a 100-passenger ship.

Also, per what I read in the VASIMR paper, which I assume is cribbed from DRA 5, I reduced entry speed to 6800m/s.

At that loading, with a Starship v3:

133t dry (Starship v2 120t baseline, plus 3 ring segments, plus 3 RVacs)
100t payload
1500t prop (my guess)
Avg Isp 369s

Departure delta-v from 200x200 LEO to v∞=4510m/s (C3=20.3):  4110m/s
Flight path angle from heliocentric Earth orbit:  17º
Braking delta-v just before 100km Mars entry interface:  2240m/s
Entry interface speed:  6800m/s
Landing delta-v:  800m/s

Time of flight from LEO:  4.5 months

Remember, this is all stuff that's TRL 8 or 9.  It's likely to be real in a couple of years.  Cutting that ToF by a factor of 4 would be lovely, but it's pretty good. It's already two-and-a-half months shorter than what DRA 5 envisioned.

I guess this is technically OT for our 1 month requirement, but it's worth remembering that we're only talking about an extra 3.5 months in space on the outbound trip, using  technology that's almost on the pad.  No nukes involved.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #71 on: 02/14/2024 12:21 pm »
Also, you made several noted mistakes, recently; will you address them?  You don't want casual accusations tossed your way, right?

Nope, I'm not going to correct, and I'm not willing argue about exoatmospheric sandbraking.  The idea is silly on its face.  I do this for fun, but there's a limit.

Your very critical post was a word salad, from the first sentence.  That's why you couldn't quote or calculate anything to back it up.  But if that's your limit, ok.
« Last Edit: 02/14/2024 12:30 pm by LMT »

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #72 on: 02/14/2024 12:35 pm »
What do you think is a reasonable payload mass for a crew module + passengers + consumables for a Starship?  I used to be using 50t, but my guess is that 100t is probably closer.

Where did you get those numbers? 

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #73 on: 02/14/2024 01:53 pm »
If you want to minimize TOF, you need to minimize payload mass. People don’t weigh very much, and they need a certain volume to keep from going crazy. 100 people @ 1t/person and ~10m^3/person sounded about right. But that’s why I’m asking the question.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #74 on: 02/14/2024 02:45 pm »
If you want to minimize TOF, you need to minimize payload mass.  People don’t weigh very much...

No, you'd maximize crewed payload for efficiency, obviously.  No one recommends, say, cutting crewed payload in half to save a few days -- not months -- and at roughly twice the fleet price.
« Last Edit: 02/14/2024 02:55 pm by LMT »

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #75 on: 02/14/2024 03:59 pm »
Time of flight from LEO:  4.5 months

Remember, this is all stuff that's TRL 8 or 9.  It's likely to be real in a couple of years.  Cutting that ToF by a factor of 4 would be lovely, but it's pretty good. It's already two-and-a-half months shorter than what DRA 5 envisioned.

I guess this is technically OT for our 1 month requirement, but it's worth remembering that we're only talking about an extra 3.5 months in space on the outbound trip, using  technology that's almost on the pad.  No nukes involved.

I agree, moderate reductions in travel time are quite doable with chemical propulsion and a better idea than one month travel. I think the sweet spot for crews is modestly accelerated all-chemical transits with roughly 4-6 months travel time (min delta vee takes ~7 months), which doesn't cost much (see e.g. https://mars.nasa.gov/spotlight/images/porkchop_lg.gif). ISS experience shows that people can handle months in space OK. If people are scared of bone loss or radiation one can probably avoid that using artificial gravity or heavy radiation shielding respectively for a lot less money than nuclear propulsion. Just throw some mass at the problem, and with reusable launch vehicles mass is a lot less expensive than it used to be.

Nuclear propulsion of some sort (thermal, electric, or explosive) is required for one month transits without ridiculous mass. Nuclear propulsion is a bad idea currently for a few reasons. Firstly it's going to cost at least as much as SLS and leave NASA without any money for actually doing anything significant on Mars. Secondly there are far more people who don't like radioactive debris falling on their heads after rapid unscheduled disassemblies than care about space so Congress may not allow NASA to operate nuclear propulsion anywhere near Earth, which makes it a lot less useful. Thirdly the lower risk versions of nuclear thermal have only a relatively modest advantage over chemical because the advantage of higher specific impulse is partially canceled by the larger dry mass.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #76 on: 02/14/2024 04:51 pm »
I think the sweet spot for crews is modestly accelerated all-chemical transits with roughly 4-6 months travel time...

...people can handle months in space OK.

Quote
Rehabilitation After International Space Station Flights

Rehabilitating U.S. crew members to preflight status following flights on the Russian Mir Space Station required longer than six months for full functional recovery of some of the seven crew members...

The [newer ISS] rehabilitation program lasts for 45 days...  Some crew members subjectively indicated the need for a longer rehabilitation period..."

Mir flight durations:  115-188 days

Quote
Astronauts’ brains take a hit during long spaceflights
CNN, 6/8/23

"We found that the more time people spent in space, the larger their ventricles became," said lead study author Rachael Seidler...  "The biggest jump comes when you go from two weeks to six months in space..."

"...the team concluded that astronauts need at least three years between missions to allow their ventricles to recover fully."

-

Use delta-v plots for clarity.  Using a published plot (Moraguez et al. 2018) for your 2005 window:

1.  We know that a 200-t payload sets max Starship delta-v at ~ 5.5 km/s. 

2.  Min transit time is ~ 170 days, so 6 months may be typical for fleet transits.

Refs.

Moraguez, M., Miller, D. and Vanatta, M., 2018, July. Mass-Optimal Transit Time for Acceptable Effective Radiation Dose on Manned Deep Space Exploration Missions. 48th International Conference on Environmental Systems.
« Last Edit: 02/14/2024 08:10 pm by LMT »

Offline sdsds

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #77 on: 02/14/2024 05:52 pm »
guess this is technically OT for our 1 month requirement [...]

Not really. Given the constraints presented initially the solution set is empty. Those constraints were:

  * no magic
  * 1 month transit
  * transport settlers to Mars

Relaxing any one of those constraints might yield a non-empty solution set.

I think fast transport of wealthy settlers or critical replacement parts might be accomplished via alternate means, but for settlement to succeed there needs to be bulk transport of cargo and mass transport of settlers. So this discussion becomes about which constraint to relax: the 1 month constraint, the payload mass constraint, or the reality constraint.
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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #78 on: 02/14/2024 06:07 pm »
the solution set is empty.

But you haven't evaluated or even listed potential solutions yet.  For 2039.
« Last Edit: 02/14/2024 06:20 pm by LMT »

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #79 on: 02/14/2024 06:37 pm »
the solution set is empty.

But you haven't evaluated or even listed potential solutions yet.  For 2039.

My personal crystal ball is definitely a magical device.
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