One Month to Mars -- Methods for Very Fast Settler Transit

[LMT]

...there are lots of numbers in that post. Please don't be so condescending...

And some of the VASIMR articles were talking about 39 days to Mars, so using electric propulsion for fast transits isn't just me being crazy.

That's another story omitting basic numbers.  The 39-day notion clustered 45 engines to push a tiny 19-ton capsule.  Check the OP and again performance fails.

[LMT]

re: shock absorbers, this text and imagery from a New Atlas article:

The team's solution was to equip Orion with the mother of all shock absorbers. In the design, directly behind the pusher plate was a huge accordion bag filled with an inert gas. Behind this was a ring of giant pistons that work like the steam catapult on an aircraft carrier, only in reverse, and it uses a system of cylinders, pulleys, springs, and magnetic clutches to absorb and spread out the impact of each shock wave.

By tuning the frequency of the shock absorber, the acceleration could be cut to an acceptable 4 g. However, the entire pusher plate and absorber system was a bit more sophisticated because it had to handle the problem of misfires. If a bomb failed to detonate, the absorber might shoot out and not contract, causing it to overextend and jam or rip the air bag, so the mechanism had to compensate with a two-stage detuned spring and piston shock absorber.

Worse, the bomb might detonate, but not reach critical mass. That would mean a conventional explosion, but no nuclear one – showering the pusher plate with shrapnel that could pit it. This, too, had to be accounted for in the design.

[Vultur]

...there are lots of numbers in that post. Please don't be so condescending...

And some of the VASIMR articles were talking about 39 days to Mars, so using electric propulsion for fast transits isn't just me being crazy.

That's another story omitting basic numbers.  The 39-day notion clustered 45 engines to push a tiny 19-ton capsule.  Check the OP and again performance fails.

Well, I'll just say that I would find getting significantly better kW/kg than their assumed reactor out of 2030s thin-film solar to be less implausible than getting a fusion drive working well enough to transport large numbers of people by 2039.

This is a discussion board, not a college course or a RFP. I don't see why you can just declare things fail when there's logic and work behind them. There are real issues with the more optimistic VASIMR claims, sure, but I can't see how you can consider a fusion drive or nuclear pulse concept to have less issues! It seems like you're not holding the concepts to an equal standard.

I'd love for fusion drives to work - you need some kind of nuclear propulsion to send humans to the outer solar system, barring something unusual.

But developing them within 15 years-- not just inventing them, but developing them to a point where they can be reliably/safely used for shiploads of settlers, and designing the actual spacecraft for them (you probably can't just jam it into a Starship... and you'll need the drive to be at a decent level of design maturity to design the ship) that strikes me as extremely implausible.

I think that very extreme solar-electric is the least unlikely to work, given the parameters you set in the OP.

Though nothing is actually likely to work. 15 years of development isn't all that much on this scale. Look at the actual timelines of 21st century human spaceflight projects...

- Orion will be over 20 years old when it first flies with crew, hopefully next year

-  SLS started with existing hardware and it will probably be 15 years from the 2010 NASA Authorization Act to SLS first flying humans.

- Starship will probably take 15+ years from start of development to first people on Mars, and it's less exotic than these concepts.

- SpaceShipOne 2004, VG's first commercial flight 2023 = 19 years.

- New Shepard: development started in the 2000s, first crewed flight 2021. Maybe 15 years?

- Starliner... still waiting...

- (I'm not really sure what dates to quote for Crew Dragon since Cargo was developed first. It's also about 15 years if you count from the start of development of Dragon...

- (also not sure what start date to use for Shenzhou...)

I think you're looking just at the hypothetical performance of the technology, not the necessary development timelines.

[LMT]

...very extreme solar-electric is the least unlikely to work...

...you're looking just at the hypothetical performance of the technology, not the necessary development timelines.

You're still ignoring, or don't understand, physical basics, such as the fundamental performance curve of electric thrusters.  False repetition is clutter.

Useful tech proved out previously, such as 3F microexplosion tech, is fine for 2039 discussion.  Re: "development timelines", Boca Chica steelworks could try out a 3F steel reflector / magnetic mirror at min scale, with 3 tons of MOX, at any time.  If desired, superconducting loops could augment the conventional explosion's weak magnetic mirror field to illustrate the protective 10 Tesla field of the equivalent 3F detonation.  Winterberg 2015.

[Vultur]

...very extreme solar-electric is the least unlikely to work...

...you're looking just at the hypothetical performance of the technology, not the necessary development timelines.

You're still ignoring, or don't understand, physical basics, such as the fundamental performance curve of electric thrusters.  False repetition is clutter.

Useful tech proved out previously, such as 3F microexplosion tech, is fine for 2039 discussion.  Re: "development timelines", Boca Chica steelworks could try out a 3F steel reflector / magnetic mirror at min scale, with 3 tons of MOX, at any time.  If desired, superconducting loops could augment the conventional explosion's weak magnetic mirror field to illustrate the protective 10 Tesla field of the equivalent 3F detonation.  Winterberg 2015.

1) I don't think your link on electric thrusters goes to where you want it to go; that post doesn't say anything about a fundamental curve. (You're also linking to a thread page I participated in, so your assumption I don't know about it is a bit odd.)

If you're talking about the "20 days per km/s" number which that post is quoting from an earlier post, that isn't fundamental at all. It assumes 10 tons per MW ... i.e. 0.1 kW/kg. I was assuming 6 kW/kg. That post also says "rigid solar" (i.e. not thin-film) and "conservative". It's nothing near the limit of the possible.  If you have a 60x better power source for your electric thrusters, you will get way more out of them!

60x power, same Isp = 60x thrust. 20 days per km/s goes to 3 km/s per day, easily enough to complete a 55 km/s maneuver in 30 days.

I don't think I am ignoring or not understanding anything; I think you are really underestimating the specific power modern or near-term/in-development thin-film solar can give. It's way better than the nuclear power sources that are generally assumed in older (even 2000s) work, like HiPEP or the early VASIMR stuff.

2) If you think prototyping nuclear pulse 'Starship at Boca Chica style' is at all feasible, our assumptions about technology development are probably too different to make discussion meaningful. I cannot see how development of nuclear pulse could be considered easier than using solar cells which already exist at least at lab scale to power well-demonstrated electric propulsion.

[LMT]

...very extreme solar-electric is the least unlikely to work...

...you're looking just at the hypothetical performance of the technology, not the necessary development timelines.

You're still ignoring, or don't understand, physical basics, such as the fundamental performance curve of electric thrusters.  False repetition is clutter.

Useful tech proved out previously, such as 3F microexplosion tech, is fine for 2039 discussion.  Re: "development timelines", Boca Chica steelworks could try out a 3F steel reflector / magnetic mirror at min scale, with 3 tons of MOX, at any time.  If desired, superconducting loops could augment the conventional explosion's weak magnetic mirror field to illustrate the protective 10 Tesla field of the equivalent 3F detonation.  Winterberg 2015.

...that post doesn't say anything about a fundamental curve. (You're also linking to a thread page I participated in, so your assumption I don't know about it is a bit odd.)

You didn't understand the electric performance curve then, either, and you didn't ask about it.  So count the engines now, and the total mass.  Cost with propellant?  Then multiply for the settlement fleet.  Oho.

But again, that's OT, like other notions that fail in context, such as VASIMR handwaving.

[LMT]

Tarantini 2015 simulates a regenerative electromagnetic shock absorber system, featuring a motion rectifier for improved mechanical efficiency.  Essential equations are presented. 

On a cryogenic platform, such a system could incorporate superconducting circuitry for even better performance.

Refs.

Tarantini, F., 2015. Simulation of a regenerative electromagnetic vehicle suspension.

[Vultur]

...very extreme solar-electric is the least unlikely to work...

...you're looking just at the hypothetical performance of the technology, not the necessary development timelines.

You're still ignoring, or don't understand, physical basics, such as the fundamental performance curve of electric thrusters.  False repetition is clutter.

Useful tech proved out previously, such as 3F microexplosion tech, is fine for 2039 discussion.  Re: "development timelines", Boca Chica steelworks could try out a 3F steel reflector / magnetic mirror at min scale, with 3 tons of MOX, at any time.  If desired, superconducting loops could augment the conventional explosion's weak magnetic mirror field to illustrate the protective 10 Tesla field of the equivalent 3F detonation.  Winterberg 2015.

...that post doesn't say anything about a fundamental curve. (You're also linking to a thread page I participated in, so your assumption I don't know about it is a bit odd.)

You didn't understand the electric performance curve then, either, and you didn't ask about it.  So count the engines now, and the total mass.  Cost with propellant?  Then multiply for the settlement fleet.  Oho.

But again, that's OT, like other notions that fail in context, such as VASIMR handwaving.

I still disagree, and I don't see how you can just say I don't understand without actually engaging with my numbers.

Here's what I'm missing: do you disagree that modern or near-term thin film solar can give kW/kg significantly better than what was assumed in the "VASIMR to Mars" concepts?

Or are you arguing that electric propulsion for some fundamental reason just can't hit thrust levels of thousands of Newtons at say 3000 Isp, even if you had as much power as you could ever want?

---

As for your specific comments:

Yes, it would involve a lot of engines. So what? Mass production of electric propulsion can make it cheaper - this has been demonstrated at least for small satellites (Starlink uses electric propulsion); if we were building thousands of VASIMR engines for a settlement fleet the cost-per-unit would go down a lot.

Propellant cost will not be that high since you won't be using xenon. (There isn't enough xenon anyway.) Even Starlink has had to move away from xenon.

If you are saying the engines themselves would mass too much, I'm not seeing it, at least not for a VASIMR type design. Electric propulsion historically can't do stuff like this because of the specific power (kW/kg) of the power source. If that constraint is removed...

If you can deploy very large areas of very thin film solar in space, you just don't *need* nuclear propulsion for the Inner Solar System. If you want a problem set where these kinds of things *do* make sense, humans to Titan / Enceladus or something would fit better.

[LMT]

...if we were building thousands of VASIMR engines for a settlement fleet...

Ignoring the OP and the rocket equation, presenting yet another convenient false number.  Enough with all that.

[TheRadicalModerate]

If you are saying the engines themselves would mass too much, I'm not seeing it, at least not for a VASIMR type design. Electric propulsion historically can't do stuff like this because of the specific power (kW/kg) of the power source. If that constraint is removed...

If you can deploy very large areas of very thin film solar in space, you just don't *need* nuclear propulsion for the Inner Solar System. If you want a problem set where these kinds of things *do* make sense, humans to Titan / Enceladus or something would fit better.

Specific power of solar arrays is nearly constant as you scale up their total power.  That's not true for nukes:  specific power rises as total power increases.  At some point, the nuke will outperform the solar array, even when burdened with the required heat rejection.

I crunched some numbers from the VASIMR paper here.  From what they have, it's hard to separate the pure power supply from the thrusters, but here's a quick roll-up:

90-day transit time: 12MW system, power+propulsion+thermal = 58t, so 207W/kg.
39-day transit time: 200MW system, power+propulsion+thermal = 154t, so 1299W/kg.

Last I checked, space solar arrays are getting close to a specific power of 350W/kg (at 1AU).  I'm willing to believe that specific power will triple over the next 20 years.  So let's say 1050W/kg.  But the arrays will be operating at an average distance of 1.26AU, which takes them down to 660W/kg.

I'm willing to believe that the thrusters and thruster-specific heat rejection scale substantially sub-linearly.  (The 200MW description didn't break them out.)  Let's say that they only increase from 21t in the 12MW case to 40t in the 200MW case.  That makes the whole system 583W/kg, which is less than half the specific power of the nuke.

Continuous low thrust systems are incredibly sensitive to specific power.  I conclude from this that SEP is probably a loser for an architecture like this.  You need the big nuke.

PS:  Don't get me wrong:  the big nuke and scaling up VASIMR is probably 0.1% of the engineering cost of getting an exploding inside-out boosted fission propulsion system to work.  If you really need to get people to Mars in one month, this is probably the best option.  But getting spin gravity and decent shielding to work for a 4-month trip is probably 1% of the engineering cost of getting the big nuke to work.  And it's about 0.005% of the political cost.

[Vultur]

If you are saying the engines themselves would mass too much, I'm not seeing it, at least not for a VASIMR type design. Electric propulsion historically can't do stuff like this because of the specific power (kW/kg) of the power source. If that constraint is removed...

If you can deploy very large areas of very thin film solar in space, you just don't *need* nuclear propulsion for the Inner Solar System. If you want a problem set where these kinds of things *do* make sense, humans to Titan / Enceladus or something would fit better.

Specific power of solar arrays is nearly constant as you scale up their total power.  That's not true for nukes:  specific power rises as total power increases.  At some point, the nuke will outperform the solar array, even when burdened with the required heat rejection.

I crunched some numbers from the VASIMR paper here.  From what they have, it's hard to separate the pure power supply from the thrusters, but here's a quick roll-up:

90-day transit time: 12MW system, power+propulsion+thermal = 58t, so 207W/kg.
39-day transit time: 200MW system, power+propulsion+thermal = 154t, so 1299W/kg.

Last I checked, space solar arrays are getting close to a specific power of 350W/kg (at 1AU).  I'm willing to believe that specific power will triple over the next 20 years.  So let's say 1050W/kg.  But the arrays will be operating at an average distance of 1.26AU, which takes them down to 660W/kg.

I'm willing to believe that the thrusters and thruster-specific heat rejection scale substantially sub-linearly.  (The 200MW description didn't break them out.)  Let's say that they only increase from 21t in the 12MW case to 40t in the 200MW case.  That makes the whole system 583W/kg, which is less than half the specific power of the nuke.

Continuous low thrust systems are incredibly sensitive to specific power.  I conclude from this that SEP is probably a loser for an architecture like this.  You need the big nuke.

PS:  Don't get me wrong:  the big nuke and scaling up VASIMR is probably 0.1% of the engineering cost of getting an exploding inside-out boosted fission propulsion system to work.  If you really need to get people to Mars in one month, this is probably the best option.  But getting spin gravity and decent shielding to work for a 4-month trip is probably 1% of the engineering cost of getting the big nuke to work.  And it's about 0.005% of the political cost.

Oh I'm not necessarily disagreeing that at some scale nuclear power would win out. I'm saying that space nuclear power at that scale is not happening in 15 years.

But I do think that given an Apollo-scale push to the limits of the possible*, you could probably get way better W/kg than you suggest out of solar. I'm not describing improved versions of current space solar arrays: I'm talking about something made out of thin films and deployed like a solar sail, not like a normal solar array. This would be something like the (unfortunately not being built) proposed JAXA follow-on to IKAROS, a massive sail with thin-film solar cells and ion engines. Just much larger, with a much larger payload.

*which is the only meaningful context to talk about super-fast Mars transits on this timescale; NASA or CNSA without Starship would be very lucky to get people to Mars at all by 2039, and even with optimistic Starship development timelines they'd only be a few synods into humans-to-Mars by then.

[LMT]

...getting spin gravity and decent shielding to work for a 4-month trip is probably 1% of the engineering cost of getting the big nuke to work.

~ 6 months, unshielded.

"Decent shielding", that truly protects on slow transits, would easily add tens of thousands of tons to fleet transit mass, at a launch cost likely beyond $1 billion.  The extra mass would stretch transit toward 7 months, too.

Boca Chica steelworks could do a lot with a fraction of the $ billions saved annually by minimizing shielding requirement and transiting Starships on empty tanks -- also, the many $ billions saved up front through fleet reduction:  i.e., infrastructure savings accrued both on Earth and on Mars.  Don't lose sight of the value of very fast transit.

[LMT]

Funding for Uranium Enrichment Is on the House
By Rachael Zisk
February 22, 2024

The bill states that the $2.7B will be reallocated from unused 2022, 2023, and 2024 funding to carry out the Nuclear Fuel Security Act of 2023 by establishing new programs. The bill would also continue existing programs to expand the production of low-enriched uranium (LEU) and high-assay low-enriched uranium (HALEU), essential reactor fuels...

For what it’s worth, the funding for uranium enrichment wasn’t a sticking point in the prolonged discussion of the bill...

The bill... passed the Senate on Feb. 13, and now it’s up to the House to decide whether it moves forward.

[LMT]

Fusion Neutron Shielding / Thrust Augmentation

DT fusion releases 80% of its energy as neutrons.  In the 3F sequence of Winterberg 2004, these neutrons feed the autocatalytic implosion.  However, not all neutrons are absorbed by combustion gases.  Only "a good fraction of the neutrons" are absorbed, leaving some neutron hazard and wasted energy. 

Hydrogen, having nucleus mass matching the neutron, scatters and absorbs the energy of fast neutrons well, to serve as a neutron shield.  Therefore, methane, at 25% hydrogen, can also shield.

Slow neutrons, with energies thousands of times lower, escape methane.  Slow neutrons can be shielded with boron, incorporated perhaps as a hardening additive in a tug's steel reflector and elsewhere in the structure.  If the reflector is supported by the dispersive AlO3 backing of Winterberg 2015, boron might be incorporated there as well.

Conceivably, when a spherical 3F detonator is loaded with MOX, a roughly hemispherical cap can be loaded with extra LCH4, on just the reflector-facing side.  At detonation, the LCH4 clears the principal, fast neutron hazard -- outside the tug structure -- and converts that neutron "waste" energy into significant augmented thrust.  Here a standard Starship / tug propellant would be serving two additional roles.

What are some other potentially useful shielding / thrust augmentation methods for such a fusion tug?

[TheRadicalModerate]

~ 6 months, unshielded.

Less than that, if you assume a stretched Starship, with 1500t of prop and 9 engines.  Since that's already been discussed as Starship 3, it's probably a good assumption.  My numbers attached.  These take into account enough delta-v for braking to meet the specified entry speed.  The landing delta-v of 675m/s is good for 200t with 9 engines and a peak acceleration of 5G, if my sim is correct.

Refueling from a high-energy HEEO obviously takes a lot of methalox.  If propellant is the long pole in the tent, then this is a problem.  LUNOX would help somewhat, although they'd still have to get the LCH4 into the HEEO.  But it's not clear if shaving off a bit more than 5 weeks from the trip is worth it over the 200x200 LEO departure.

It's a reasonable trade if you really have an unacceptable radiation problem.

"Decent shielding", that truly protects on slow transits...

I don't know what "true protection" means.  If you're talking about one-shot settlers who are going to live the rest of their lives in shielded habitats, 200-300mSv probably isn't terrible, and doesn't require anything special, other than shaving a month or two off the ToF, which, as you can see, isn't that hard. 

If you're talking about people who commute multiple times over the course of their lives, then that'll be a problem.  Best solution for those people (who are either obscenely rich or sufficiently high-value that somebody is paying for them), would be to trade away passenger capacity for more shielding, with the fares reflecting the premium.  Think of it as business class to Mars.

Don't lose sight of the value of very fast transit.

I'm not losing sight of it.  But what you're proposing is science fiction.  If you want something that's not quite as science fictiony, VASIMR with a big nuke is a better bet--not a great one, but better.  It's definitely not as sexy, but that's not really a requirement, is it?

[Coastal Ron]

Fusion Neutron Shielding / Thrust Augmentation

DT fusion releases 80% of its energy as neutrons.  In the 3F sequence of Winterberg 2004...

Developing anything nuclear takes $Billions of dollars, if not hundreds of $Billions. And unless it has some sort of big payoff here on Earth, no one is going to pay for it.

Know why? Because no one cares about getting to Mars a few months quicker. No politicians, no investors, and most of the science community.

So unless what you are referencing is dual use, in that it can be used here on Earth for power generation too, and it can be developed in less than a decade, you can forget about it being used for the initial wave of colonists going to Mars.

Maybe you should focus on technologies that are already close to be matured, and can provide something quicker than 6 month transits. But otherwise it seems clear that one month is not achievable with proven technologies today, AND could be affordable for the vast number of transits needed to start colonizing Mars.

[LMT]

...if you assume a stretched Starship, with 1500t of prop...

They wouldn't cut "stretched" payload to 200 t, or less, just to shave a few days off transit -- while increasing the gigantic fleet count and cost to compensate.  Your story is irrational.

But what you're proposing is science fiction.  If you want something that's not quite as science fictiony, VASIMR with a big nuke is a better bet...

No, VASIMR has middling electric performance and 6 N max thrust.  If you ever calculate the number, mass, and cost for a settlement fleet... oho.  You and Vultur might compare notes, but don't post breathless fiction.

[LMT]

Fusion Neutron Shielding / Thrust Augmentation

DT fusion releases 80% of its energy as neutrons.  In the 3F sequence of Winterberg 2004...

Maybe you should focus on technologies that are already close to be matured, and can provide something quicker than 6 month transits. But otherwise it seems clear that one month is not achievable with proven technologies today, AND could be affordable for the vast number of transits needed to start colonizing Mars.

He'd have you believe 3F DT fusion isn't "close to be matured".  Who wants to tell him?   

[john smith 19]

I haven't seen this thread before.

From running my settlement game the real problem in settling mars (as opposed to America) was the roughly 26 month delay between launches, necessitating a large (and growing) fleet of ships.

At 1 starship/week you could send as many settlers as a single fleet of 104 ships per window with as little as 3 ships, assuming it took a week to return as well. Obviously any increase in size would immediately speed up settlement (assuming a ready supply of settlers).

High thrust/long duration engines are still SF. One option that's never really been explored is the idea of short burst of high(ish) g.

Work in the 50's and 60's showed people could take 45g in water tanks and -60g on rocket sleds without ill effects (once their retinas had been reattached  ). But on a more mundane level 3g is nothing unusual on a modern roller coaster.

The fission fragment rocket is usually viewed as a pulse design generating intense burst of fission fragments at about 3-5% of c.
For a human ship what would be needed would be massive thrust in a pulse design.

In fact pulse reactors do  exist as a test tool capable of massive energy outputs for very brief period. They've been employed by all major nuclear nations since the 50's to do accelerated life testing of parts for conventional reactors and other nuclear hardware.

Building a flight weight design would be challenging. An interesting feature would be wheather you incorporate the propellant as part of the moderator. The reactor is near cold until  the propellant starts flowing (or some kind of discrete tank package?) is in the core and shuts down once it's run out. For a really clever design you don't rely on the operating temperature of the reactor to heat the propellant by contact but by neutrons and gamma rays passing through (or being stopped by) it.

While not trivial it offers the possibility of a)Much shorter travel times b)Not needing an asteroid (which is what you're going to need if you really want to cut down radiation exposure with conventional options, including ion thrusters) c)Not needing fighter pilot levels of fitness to cope with the acceleration d)A solid existing technology base requiring no breakthrough physics to make it work. There is no existing engine design that can deliver sustained high levels of acceleration (and in this context 0.1g is "high" g  )

[LMT]

A solid existing technology base requiring no breakthrough physics to make it work.

Which I'm presenting, topically.  Posters might read the papers and explore options.