Author Topic: One Month to Mars -- Methods for Very Fast Settler Transit  (Read 19249 times)

Online armchairfan

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #40 on: 02/12/2024 03:07 am »
But you need to check the basic magsail parameters first.  As Tolley noted, "a 100 km radius coil... generates a thrust of 225 N."

What delta-v does it give a 400-ton Starship over two weeks?

Not much. 225 / 400e3 * 2 * 7 * 24 * 3600 = 680 m/s.

Offline TheRadicalModerate

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #41 on: 02/12/2024 05:13 am »
I was looking at ~30-40 day transfers that use aerocapture/entry at the end, and you start to exceed the usual unassisted acceleration limits. Hence fluid immersion (which itself has limits due to air cavities... although even with liquid breathing, there are still limits).

I got a 33km/s entry speed for a one-month transit time.  Let's assume that our aerocapture/EDL corridor is 80km from the surface.  To keep the trajectory from just boring an ionized hole through the upper atmosphere and the vehicle skipping back into a hyperbolic orbit, you need to apply downward lift.  Mars has an average radius of 3390km. 

So:  liftAcceleration = (33km/s)²/3470km - 3.72m/s² = 310m/s² = 31.6G.
Figure L/D = 1.5, and you have 21.1G of drag. 
Our buddy Pythagoras says the vehicle will be experiencing 38G of acceleration.

And that of course leaves out the thermal issues.  Forget turning the passengers into fish to survive.  You have to make your ship out of unobtanium.
The Galileo probe was even more insane.

A fair point.  But the Galileo atmospheric probe went from entry interface to going slow enough to pop its drogue chute in less than two minutes (00:01:52)--and then it ejected its heat shield, along with a bunch of residual heat.  It was basically a fancy ICBM entry vehicle.  That's not going to be the case for a human Mars mission.

It's not the mechanical forces that get you; it's the sustained heat pulse.

BTW, I've read science fiction stories that use liquid in and around all human body cavities to mitigate acceleration, but I don't know any of the actual scientific literature surrounding this.  How do you protect a brain (which is already in a liquid filled cavity) from inertial acceleration of multiple tens of G?  Brain density and CSF density aren't the same.  In short order, the brain will be squished up against the skull, and then the person will be dead.

What am I missing?

No, reasonable aerocapture numbers are right above.

I was responding to RB's conops, which didn't use propulsive braking.  That's a very different aerocapture case.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #42 on: 02/12/2024 06:14 am »
DusTug

re: Minimizing Exotic Delta-V

Sandbraking 1 2 3 4 could conceivably offer the required deceleration near Mars.

Here a "Deimostation" ISRU depot 1 2 3 4 5 6 acts as the staging area.  In the Deimostation, processed regolith waste is sifted for fine particles (dust, < 50 microns).  These are loaded into ASCENT service vehicles or cargo Starships, acting as "Interceptors".  Dust is deployed gently into a narrow sandbraking corridor extending between Deimos and Phobos orbits.

An arriving Starship crosses Deimos' diameter in 0.4 seconds.  Thereafter, GNC puts it into the corridor, actively, for 3 g sandbrake deceleration over ~ 15 minutes.  The Starship diverts out of the corridor when speed drops below 5 km/s, near Phobos orbit.  Then it corrects course on Raptors and RCS for conventional Mars EDL.  Alternately, the corridor could be deployed further out.

< 500 t of dust would decelerate a 400 t Starship to 5 km/s.  Thus, if 5000 t were deployed, < 10% would actually be used in momentum exchange targeting.  Corridor control is challenging in space, where both positioning and density must be managed over a great distance; perfection of control would impact deceleration smoothness and corridor mass-efficiency.

The corridor is out-of-plane, on a deceleration trajectory with periapsis in the Martian thermosphere.  Most of the dust would plunge into the thermosphere shortly after sandbraking.  Braking would eject a small fraction out of Mars orbit, and the tiny residual would be cleared out by solar radiation pressure.  Notably, the corridor would clear much faster than the dust rings created already by impacts on Phobos and Deimos

Benefits of this "DusTug" scheme:

1.  Propellant cut:
Halving exotic delta-v slashes exotic propellant mass and cost.

2.  Engine cut:
Exotic engines are no longer carried to Mars for deceleration.  Instead, they act as short-range tugs in high Earth orbit, cycling frequently, perhaps daily.  If daily departures filled an expanded 60-day synodic window, the notional 667 departures would use a dozen exotic tugs in HEO, plus a matching dozen in Mars orbit reserved as emergency backup decelerators -- altogether a 96% cut in exotic engine production requirement.

Or if corridor control proved too difficult in space, the dozen reserve tugs in Mars orbit would be pressed into daily service.  They would be supplied with exotic propellant, perhaps from Earth, for deceleration of all ships.  The 96% cut in engine production would be retained.
« Last Edit: 02/12/2024 03:45 pm by LMT »

Offline TheRadicalModerate

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #43 on: 02/12/2024 07:43 pm »
In the Deimostation, processed regolith waste is sifted for fine particles (dust, < 50 microns).  These are loaded into ASCENT service vehicles or cargo Starships, acting as "Interceptors".  Dust is deployed gently into a narrow sandbraking corridor extending between Deimos and Phobos orbits.

An arriving Starship crosses Deimos' diameter in 0.4 seconds.  Thereafter, GNC puts it into the corridor, actively, for 3 g sandbrake deceleration over ~ 15 minutes.  The Starship diverts out of the corridor when speed drops below 5 km/s, near Phobos orbit.  Then it corrects course on Raptors and RCS for conventional Mars EDL.  Alternately, the corridor could be deployed further out.

You can't sandbrake unless you're in an atmosphere.  The whole scheme depends on the vehicle, which has to carry its own sand, popping the stuff out in front of it, and then taking advantage of the large ratio of the ballistic coefficient of the vehicle to the BC of the sand.  In vacuum, the ratio is either meaningless (∞/∞) or, in a soft vacuum, not very large.

It's that large ratio (but not too large), that causes the impact of the sand on the vehicle to be at 1km/s, instead of at 33km/s.  That makes it survivable with only a giant pusher plate.

So this "corridor" of which you speak has to be a stream of sand, on exactly the right
(hyperbolic) trajectory, which varies through the transfer window, moving at a speed of 25-30km/s.  Otherwise, it's not sandbraking; it's a collision with a Kessler-syndrome-sized field of MMOD.  Which, after scattering (both the sand and the debris from the hapless vehicle), is exactly what you'll have in large portions of cismartian space.

No.  Just no.

This might be an interesting technique for vehicles that are already in some kind of cismartian capture orbit, because a stop at the Deimos sand station is... possible?  But unless you're going to load the Mars transfer vehicle up with a bunch of lunar dust before departure, this is nuts.

(Note:  Since we're already talking about ludicrous amounts of delta-v, it's possible that you can load hundreds of tonnes of lunar dust onto your vehicle and not change the amount of propellant very much.  But you're still going to require massive lift to keep the vehicle in the atmosphere long enough for the sandbraking to be effective without the deceleration killing the crew.)

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #44 on: 02/12/2024 08:05 pm »
In the Deimostation, processed regolith waste is sifted for fine particles (dust, < 50 microns).  These are loaded into ASCENT service vehicles or cargo Starships, acting as "Interceptors".  Dust is deployed gently into a narrow sandbraking corridor extending between Deimos and Phobos orbits.

An arriving Starship crosses Deimos' diameter in 0.4 seconds.  Thereafter, GNC puts it into the corridor, actively, for 3 g sandbrake deceleration over ~ 15 minutes.  The Starship diverts out of the corridor when speed drops below 5 km/s, near Phobos orbit.  Then it corrects course on Raptors and RCS for conventional Mars EDL.  Alternately, the corridor could be deployed further out.

You can't sandbrake unless you're in an atmosphere.  The whole scheme depends on the vehicle, which has to carry its own sand, popping the stuff out in front of it, and then taking advantage of the large ratio of the ballistic coefficient of the vehicle to the BC of the sand.  In vacuum, the ratio is either meaningless (∞/∞) or, in a soft vacuum, not very large.

It's that large ratio (but not too large), that causes the impact of the sand on the vehicle to be at 1km/s, instead of at 33km/s.  That makes it survivable with only a giant pusher plate.

So this "corridor" of which you speak has to be a stream of sand, on exactly the right
(hyperbolic) trajectory, which varies through the transfer window, moving at a speed of 25-30km/s.  Otherwise, it's not sandbraking; it's a collision with a Kessler-syndrome-sized field of MMOD.  Which, after scattering (both the sand and the debris from the hapless vehicle), is exactly what you'll have in large portions of cismartian space.

No.  Just no.

This might be an interesting technique for vehicles that are already in some kind of cismartian capture orbit, because a stop at the Deimos sand station is... possible?  But unless you're going to load the Mars transfer vehicle up with a bunch of lunar dust before departure, this is nuts.

(Note:  Since we're already talking about ludicrous amounts of delta-v, it's possible that you can load hundreds of tonnes of lunar dust onto your vehicle and not change the amount of propellant very much.  But you're still going to require massive lift to keep the vehicle in the atmosphere long enough for the sandbraking to be effective without the deceleration killing the crew.)

Here it's just momentum exchange in vacuum.  Relative speed of corridor dust is ~ 31 km/s near Deimos.  Thus, momentum exchange is unavoidable at corridor entry, right?

You avoid a Kessler cascade by using fine-particle dust, e.g., < 50 microns.  Nearly all of it is removed from Mars orbit right away; the tiny residual is removed by radiation pressure in the manner of Phobos/Deimos ring particles -- and you haven't predicted a Kessler cascade in those dusty rings, right?

btw, how big are the ring particles?

Also, a capture orbit isn't required in this scheme.  Decelerate in the corridor, and you can go straight to EDL.  Do you understand the Deimostation concept and, e.g., its service vehicles?
« Last Edit: 02/13/2024 03:04 am by LMT »

Offline Coastal Ron

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #45 on: 02/12/2024 08:12 pm »
In the Deimostation, processed regolith waste is sifted for fine particles (dust, < 50 microns).  These are loaded into ASCENT service vehicles or cargo Starships, acting as "Interceptors".  Dust is deployed gently into a narrow sandbraking corridor extending between Deimos and Phobos orbits.

An arriving Starship crosses Deimos' diameter in 0.4 seconds.  Thereafter, GNC puts it into the corridor, actively, for 3 g sandbrake deceleration over ~ 15 minutes.  The Starship diverts out of the corridor when speed drops below 5 km/s, near Phobos orbit.  Then it corrects course on Raptors and RCS for conventional Mars EDL.  Alternately, the corridor could be deployed further out.
...
No.  Just no.

LMT had a separate thread for this concept, and it never gained any traction for all the reasons brought up here, plus cost, complexity, and availability of other alternatives that are less costly and complex.

I think LMT just wanted to resurrect the topic, so he started a new thread with a new name... ::)
« Last Edit: 02/12/2024 10:34 pm by Coastal Ron »
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

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For ~1 month to Mars, you really only have one practical option:

Burn your fuel relatively slowly, over the entire transit, flip around halfway to slow down. Transit ships with clusters of ion thrusters or VASIMR engines could do this. Transit vehicles like this would either have a relatively slow flyby of Mars or it would enter Mars orbit, and drop off crew / cargo in separate landers.

This could result in transit times well under 100 days, heavily depending on system architecture and transfer window. VASIMR had a design concept for 40 days to Mars. If I've done the math right, a 40 day flight to Mars (near its closest approach to Earth) would require a constant acceleration of about 0.0041 g.

You could send many Cargo Starships on slow transits that take several months and only put crew on the faster ship.
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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #47 on: 02/12/2024 08:52 pm »
LMT had a separate thread for this concept, and it never gained any traction for all the reasons brought here, plus cost, complexity, and availability of other alternatives that are less costly and complex.

I think LMT just wanted to resurrect the topic, so he started a new thread with a new name... ::)

Several posters struggled with concepts like momentum exchange and point-to-point flight there.  Listen to the Martian wind at night, and you can still hear their screams.

Next:  SOTA nuclear rockets
« Last Edit: 02/12/2024 08:53 pm by LMT »

Online Bob Shaw

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #48 on: 02/12/2024 09:35 pm »
Rather than speeding up crewed Starship transits, I think that continuous low-thrust velocity changes would be better employed to actively shape trajectories in order to enlarge the set of possible good transfer orbits, delivering more cargo (and passengers) for fewer flights. This approach also has the big advantage that a period of deceleration isn't necessarily needed, thus saving fuel mass etc. As these hybrid trajectories would entail considerably more risk than simple ballistic paths I would imagine that passengers would not be carried until reliability was well demonstrated!

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #49 on: 02/12/2024 09:45 pm »
...VASIMR engines could do this.

After 40 years, VASIMR thrust is still... rather low.  Is there a roadmap to better?
« Last Edit: 02/12/2024 09:48 pm by LMT »

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #50 on: 02/12/2024 10:04 pm »
Oh, now it becomes clear: this thread motivates a sense of 'need' for dust-braking.
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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #51 on: 02/13/2024 12:52 am »
Comparison of propulsion technologies.  Tweeted 5/9/18:  /bp_hutch/status/994304376058601472

Note the 4 technology mass-ratio values at 23.5 km/s, and the 3 technologies with mass ratios too close to 1.0 to distinguish at the given scale.
« Last Edit: 02/13/2024 02:36 am by LMT »

Offline TheRadicalModerate

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #52 on: 02/13/2024 05:10 am »
For ~1 month to Mars, you really only have one practical option:

Burn your fuel relatively slowly, over the entire transit, flip around halfway to slow down. Transit ships with clusters of ion thrusters or VASIMR engines could do this. Transit vehicles like this would either have a relatively slow flyby of Mars or it would enter Mars orbit, and drop off crew / cargo in separate landers.

This could result in transit times well under 100 days, heavily depending on system architecture and transfer window. VASIMR had a design concept for 40 days to Mars. If I've done the math right, a 40 day flight to Mars (near its closest approach to Earth) would require a constant acceleration of about 0.0041 g.

You could send many Cargo Starships on slow transits that take several months and only put crew on the faster ship.

Yeah, I was trying to express this quantitatively and failed.  Here's the arm-wave description of why this is hard, though:

1) Irrespective of how much delta-v your available prop and specific impulse say you have, for time-constrained problems, mass flow is an additional constraint.  This is really just a statement that there's a minimum thrust level required to burn all the prop in the allotted time.

2) As mass flow decreases, it's harder to dump all the energy into the mass to make the system self-cooling without massive heat-rejection adjuncts.  All of the exotic propulsion schemes usually come with this inability to cool themselves with the stuff coming out the tailpipe, which makes them heavy and inefficient.

3) If you do increase the mass flow, the relationship between chamber pressure, temperature, and mass flow requires that your "chamber" (magnetic bottle, fused silica lightbulb, chronosynclastic infundibulum, whatever...) becomes unreasonably strong and/or unreasonably heat resistant.  And then of course there are those pesky neutrons and gamma rays.

4) Even at relatively high thrust, the gravity drag associated with continuous thrust trajectories is... well, a drag.  I guess there's an argument to be made that, if your quasi-impulsive transfer orbit costs 60km/s of delta-v, then the 100km/s¹ that a continuous trajectory requires is less than doubling the impossibility, so what's the big deal?  But we're not getting around the tyranny of the rocket equation, and big prop loads require higher mass flows to meet the time constraint, and then we're back to problem #3.

It took about 350 years for the trip time from Europe to the Americas to drop from months to weeks.  I wouldn't be incredibly surprised if the same scaling applied to space travel.

_______
¹Rhetorical, not real, numbers.  The impulsive value is pretty close to 60km/s, but the continuous thrust version needs simulation.  I made a simpleminded "flat solar system" attempt at this and fell into the sun.  I'm debating how much I really want to know the answer before trying something closer to reality.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #53 on: 02/13/2024 12:05 pm »
the continuous thrust version needs simulation.

GMAT gives finite burns for simple sim.  You can plot and export ship velocity components.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #54 on: 02/13/2024 02:30 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?

As for Project Orion-style propulsion, Winterberg has some innovations.  1 2

Refs.

Arias, F.J., 2016. On the use of a pulsed nuclear thermal rocket for interplanetary travel. In 52nd AIAA/SAE/ASEE Joint Propulsion Conference (p. 4685).
« Last Edit: 02/14/2024 02:30 am by LMT »

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...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.
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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #56 on: 02/13/2024 03:41 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.
« Last Edit: 02/13/2024 04:18 pm by LMT »

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #57 on: 02/13/2024 06:32 pm »
It's a hard topic; many notions fall far outside the design space.  I encourage posters to experiment first with GMAT, Trajectory Browser, or similar tools, and try out core engine equations.

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Re: One Month to Mars -- Methods for Very Fast Settler Transit
« Reply #58 on: 02/13/2024 08:34 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.

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...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.

Consider NASA's DS1, which had an ion engine with a thrust of about 0.0980 Newtons. About the same force as a single sheet of paper resting on a desk.

It burned its ion engine for a cumulative 16,265 hours, expending about 73 kg of xenon, resulting in about 4.3 km/s dV with an ISP of about 3,100 seconds.

Constant acceleration, it adds up.
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