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

[MickQ]

I only have one person in my ignore list.  Guess who.

[LMT]

86.3% Fusion Propulsion Efficiency

A fusion device at the focus of a parabolic magnetic mirror is reported to have a remarkable max 86.3% propulsion efficiency, due to optimal geometry for eddy current repulsion.  Romanelli et al. 2017, after Thio et al. 1999.  (See previous post for parabolic magnetic mirror geometry.)

We can use that number in calculations:  e.g., estimating the reflector diameter needed for one month to Mars.

During [fusion] plasma expansion, azimuthal eddy currents... are generated along the wall in reaction to the changing magnetic field, as well. Therefore, the field cannot be swept across the wall and is compressed between that and the expanding plasma... Due to its solenoidal property, the amplitude of the field (and hence the magnetic pressure) increases. During this phase, part of the plasma energy is thus transferred to the field until the plasma expanding towards the chamber stops. When that happens, the system reacts and returns to its stable initial condition, plasma is pushed out of the open side of the chamber...  Best results are attained when the conductor wall has a parabolic section and the reaction is ignited at the focus of the parabola; in that case plasma is reflected along the axis of the chamber and propulsion efficiency can ideally be as high as 86.3% according to [15].

Historical note:  The fusion-drive "magnetic mirror" was first proposed in Winterberg 1971.

Refs.

Romanelli, G., Mignone, A. and Cervone, A., 2017. Pulsed fusion space propulsion: Computational Magneto-Hydro Dynamics of a multi-coil parabolic reaction chamber. Acta Astronautica, 139, pp.528-544.

Thio, Y., Freeze, R., Kirkpatrick, R., Landrum, B., Gerrish, H. and Schmidt, G., 1999, June. High-energy space propulsion based on magnetized target fusion. In 35th Joint Propulsion Conference and Exhibit (p. 2703).

Winterberg, F., 1971. Rocket propulsion by thermonuclear micro-bombs ignited with intense relativistic electron beams. Raumfahrtforschung, 15(AD-746766).

[LMT]

Breeding Tritium in Space

Only ~ 30 kg of tritium is available at any moment, at ~ $30,000 per gram.  At min fusion-drive prototype scale, that's ~ $30 per shot; cheap for prototype tests, but still rather expensive when scaled for a fleet.

Tritium must be created, and there are several methods.  But could a fusion-drive tug breed its own tritium?

Both fast and thermal neutrons breed tritium from lithium, effectively transmuting both isotopes (Marques et al. 2018).  Beryllium doubles the breeding rate from fast neutrons.  Therefore, a lithium / beryllium blanket, formed perhaps as a rugged ceramic, is commonly considered for tritium production at fusion facilities. 

-

In operation:

Notionally, on a Winterberg 3F tug, some such blanket would be inserted behind the steel reflector.  Upon detonation, fast and thermal neutrons are released.  High-explosive ISRU MOX and an ISRU LCH4 thrust-augmentation cap capture fast neutrons.  However, LCH4 thrust is optional; the LCH4 mass could be restricted.  Restriction aims to maintain adequate fast neutron flux into the blanket to supplement thermal neutron flux, for self-sufficient tritium production. 

When a synodic launch window closes, the blanket is removed and returned to Earth for processing.  Perhaps the Hydrogen-3 Advanced Technology Center (H3AT), Lithium Breeding Tritium Innovation (LIBRTI) program, ITER / DEMO breeding facility, or other facility could partner in OP timeframe to produce a suitable blanket formula and integrate it into the processes of a new or existing tritium recovery plant.

In a sense, tritium bred on tugs would be the most valuable space ISRU product, per unit mass, though not in terms of total annual revenue.

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Could other methods augment or replace the notional blanket?

Update:  6Li enrichment would increase tritium production rate, and at least one promising enrichment approach exists.

Refs.

Marques, R.V.A., Cabrera, C.E.V., Lima, C.P.B., Veloso, M.A.F. and Costa, A.L., 2018. Evaluation of a tritium breeding layer in a fusion-fission hybrid system. Semana de Engenharia Nuclear e Ciências das Radiações.

[LMT]

Cross-post:  tug concepts compared and contrasted with concepts in Elmar Moelzer's post on Project Orion / Helicity / tritium breeding.

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1.  Winterberg's 3F microexplosion engine and its performance 1 2 3

2.  Tug prototype / scaling 1 2 3 4

3.  Attempting self-sufficient tritium breeding

[acksed]

I have a proposal.

Photonic Laser Thrusters.

This is essentially a recuperation mechanism for a photon drive.  It uses a thin laser amplification medium and 99.99% reflective dielectric mirror on the powering station/spacecraft, with a smaller mirror on the target spacecraft to bounce the laser light back and forth thousands of times, essentially turning it into a very long laser cavity itself and amplifying the thrust.

As a consequence, it vastly reduces the insane terawatt power requirements of the old Beam-powered Propulsion.

It grew out of earlier research funded by NASA's NIAC, where the researcher was looking at formation-flying satellites that used laser light from the 'mother' or the 'flock' to move around in orbit. Thus one spacecraft could keep multiple others in orbit: https://www.nasa.gov/general/propellantless-spacecraft-formation-flying-and-maneuvering-with-photonic-laser-thrusters/

Our original emphasis was on propellant-free nanometer accuracy tethered formation flying, however, during our study on photon thruster demonstration, we discovered that our photon thruster (Photonic Laser Thruster, PLT) has a much larger potential in NASA mission applications than precision tethered formation flying. The potential resulted from a surprising discovery on the extraordinary stability of PLT against dynamic motions of mirrors in our unique active optical cavity, which may enable in-space propulsion for an extremely wide range of unprecedented NASA missions. For example, 10,000 times recycling of photons with 15 kilo-watt input laser power, which can be delivered by a 100 kW solar panel would produce up to 1 N of photon thrust, which is sufficient to enable these missions.

Dr. Young K. Bae has continued to research it further.

In the demonstration below a 500 watt infra-red laser, incident on a mock 750g cubesat on an air-rail, acts like a 500 kilowatt laser, with a measurable increase in thrust that pushes the satellite away and decelerates it. It's just 3.3 milliNewtons, but like a solar sail, this is continuous thrust that does not use propellant and has extremely high velocity.



Because I know people will ask, the researcher says it is "insensitive to mirror motions". I haven't been able to access the journal he cites, though. (Young K. Bae, Journal of Propulsion and Power, Vol. 37, pp. 400-407 (2021))

The slides and audio on his FISO presentation, "A Roadmap to the Multiplanetary Civilization with Photonic Laser Thruster" are available (reproduced below), and they propose that in the near-term, a small 1-ton spacecraft (50% of that payload) with a 4m mirror, 1000 times recycling and a 10 MW laser, could reach the Moon in 20 hours with a beam incidence time (i.e. boost phase) of 6.8 hours over 10,000km before it goes out of range, producing a delta-V of 11.6 km/s.

With future advances, the same mass with 10,000 times recycling, a 7.5m mirror on the spacecraft and a gigawatt laser on a 50m mirror, with an incidence time of 1.2 hours over 30,000km before it goes out of range, could reach a phenomenal 141 km/s (with an acceleration of 3.4 gravities!). That's Mars in 18.5 days.

We could easily increase the total mass to 10 tons and keep the mirror size and force applied the same to reach the same velocity, it'd just take a longer boost phase, as the force applied only drops when it's out of range. As it's accelerating slower, it's within range for longer.

My naive calculations are - assuming it's using a Lunar beaming station and it's in orbit around Luna at a speed of 1600m/s - that the force exerted on the future spacecraft is 3227N, so that 10-ton SC would have a boost phase of 43,198 seconds, just under 12 hours, and exert 0.34g of gravity on the passengers.

This all presumes you sent a beaming station ahead to decelerate at your destination, and a conventional lander, but once you do, you can keep sending them.

It also gives you a legitimate excuse to have a Moon laser. Win-win, I say.

Now of course there are practical difficulties:

You need multi-megawatt to gigawatt continuous-beam lasers, with cooling and radiators. Not trivial, but not impossible either. Clustering fibre lasers, a gargantuan CO2 laser, whatever you use is going to make a fair few people interested and/or worried. Again: Moon laser.

You have to have power - quite a bit of it. If using a beaming station in GEO, L1 or L5, a method of propulsion to oppose the force being exerted is needed. However, you already have the power station, whether it's a raft of solar panels or a nuclear reactor, and a solar-electric propulsion system with a few tons of argon is just the thing to use the excess power to return it to position. This is why I favour the Lunar station: no repositioning, relatively simple to lay out vast fields of solar panels.

You need to send the beaming station on ahead to your destination, which either needs people along to deploy it or to deploy itself.


P.S. I found a later paper, "Photonic Laser Thruster: Optomechanical and Quantum Electronical Analyses" (2022) https://arc.aiaa.org/doi/epdf/10.2514/1.B38634

Again, I was unable to access it, but I want to.

[LMT]

Photonic Laser Thrusters.

This is essentially a recuperation mechanism for a photon drive.  It uses a thin laser amplification medium and 99.99% reflective dielectric mirror on the powering station/spacecraft, with a smaller mirror on the target spacecraft to bounce the laser light back and forth thousands of times, essentially turning it into a very long laser cavity itself and amplifying the thrust...

Hi, and yes, mirrored systems do increase thrust.  Construct a long laser cavity, or alternately, as in Soliman 2023, you might augment a laser with a solar concentrator reflector and hydrogen propellant.  That's another way to increase thrust.

Mass and time are challenging in thread context.  Current notional min requirement is to cycle through ~ six 400-ton Starships, boosted 23.5 km/s, every 12 hours.  How could one hope to scale any photonic system for that min requirement?

Refs.

Soliman, M.A., 2023. A Comparison Between Laser Beamed Thruster and Solar Thermal Thruster. In AIAA SCITECH 2023 Forum (p. 1023).

[acksed]

Could we not take the "slow cargo, fast passenger transit" approach?

[LMT]

Cross-post:  "You asked about the temperature."

[LMT]

MMO Comparison

The Mini-MagOrion (MMO) concept has been developed by Andrews Space et al. (Ewig 2003), with results overlapping 3F tug notions. 

Notes:

1.  Initially, MMO was only a Z-pinch 245Cm fission concept, but subsequently a "D-T fusion driver" was added to "enhance fractional fission yield" (Lenard and Andrews 2007).  It seems the propulsive benefit of DT fusion has proven itself in MMO analysis, as in Winterberg's.

2.  MMO yields of 12-120 tons TNT equivalent are modeled, comparable to the range (3-100 tons) considered in thread for a Starship 3F tug.

3.  Their small yields and superconducting magnetic mirror are explicitly designed to remove the need for a "mechanically dampened pusher plate", which entails removal of the shock absorber.  Max acceleration of 0.26 g is calculated for a Mars crew, without a dedicated shock absorber mass budget.  Note also the absence of shock absorber in MMO concept illustration.

4.  Their 10 MA superconducting mirror cables reflect an 81-ton TNT equivalent blast.  Note that a 3F tug prototype could conceivably scale blast energy and mirror current an OOM smaller without inefficiency. 

Refs.

Ewig 2003.  Mini-MagOrion Program Document:  Final Report.

Lenard, R.X. and Andrews, D.G., 2007. Use of Mini-Mag Orion and superconducting coils for near-term interstellar transportation. Acta Astronautica, 61(1-6), pp.450-458.

[LMT]

Where's Kenny the nuclear engineer?  He could do some breeding calcs for us, catch theoretical mistakes, or...?

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Shock Absorbers and Limits

Brulle 2009 Fig. 14 plots the human tolerance for acceleration in each direction.  Tolerance is especially high over timeframes below 0.1 s: 

~ 30 g transverse (at max jerk ~ 1500 g/s)
~ 20 g parallel (at max jerk ~ 300 g/s)
~ 15 g lateral (at max jerk ~ 500 g/s)

The Gemini safe-ejection acceleration criteria we used were based on human tolerance acceleration limits...

...in the parallel positive (upward acceleration), a human body can withstand about 20 g-forces if the application time is only 0.1 second. It was understood that these acceleration limits were for human bodies restrained in perfectly contoured couches, and even then some injury was possible.

In comparison, roller coasters set a modest acceleration jerk limit of 15 g/s, and deceleration jerk limit of 0.8 g/s.  Pendrill and Eager 2020.

Pendrill and Eager 2020 Table 1 shows the harmonic frequency ranges in the body.  For accelerations spaced > 0.5 seconds apart, no harmonics issue is encountered.

Brulle's limits are consistent with the absence of shock absorber on the MMO Mars mission DRM-1 in Ewig 2003.  Pulses spaced at 1 Hz, giving max acceleration of 0.26 g, would have no harmonics issue and (transverse) jerk far below Brulle's limit.  Here the blast yield is ~ 29 tons TNT equivalent.  The blasts considered previously for a Starship fusion-drive tug ranged up to ~ 100 t equivalent, also having transverse jerk below the limit. 

Higher yields and accelerations for fast tug turnaround are of course possible, but some ingenuity would be needed to manage blast pressures beyond 100 MPa with thin steel plates and dispersive backing.  Optimized hoop materials might be especially useful.

A small HTS electromagnetic regenerative shock absorber (small relative to Project Orion scale) could be incorporated, capturing << 1% of blast energy for regeneration of the magnetic mirror nozzle.  Response might be set to reduce jerk encountered in just the first ~ 30 microseconds, when blast pressure and acceleration appear to change most quickly.  Ewig 2003 Fig. 11.

Refs.

Brulle, R.V., 2009. Engineering the space age: a rocket scientist remembers. DIANE Publishing.

Ewig 2003.  Mini-MagOrion Program Document:  Final Report.

Pendrill, A.M. and Eager, D., 2020. Velocity, acceleration, jerk, snap and vibration: Forces in our bodies during a roller coaster ride. Physics Education, 55(6), p.065012.

[LMT]

humanity doesn't have a practical energy source to make such transits

magic energy sources

there are good reasons why everyone usually proposes Mars missions with multi-month transit times.

agree that is infeasible, close this thread

exceed the usual unassisted acceleration limits

magical properties

make your ship out of unobtanium.

your "chamber"... becomes unreasonably strong

a science fiction fan discussion.

the solution set is empty

I have an idea where you could stick that pin

Wake me up when you've got it working.

don't be so condescending. But I admit I didn't go through the full math.

what you're proposing is science fiction.

no one cares

Checkpoint:

When did spaceflight enthusiasts become annoyed bureaucrats?  Posters have a lot of notions to correct, in this one thread alone. 

But it's good to see that "one month to Mars" is a solvable problem, even now.

[Twark_Main]

solvable

In other words, your standards in this thread have sunk to the lowest possible bar. Not "economical" (ie actually a better value and therefore going to dominate over other tech), not "feasible" (ie possible for a company to not die, even if it's not dominant). Just..... not impossible.  Maybe.

To be crystal clear, solvable, "even now" = TRL....  2?  3? 

Thanks.  Trying to calibrate the LMT-to-English translator. 

[LMT]

solvable

In other words, your standards in this thread have sunk to the lowest possible bar. Not "economical" (ie actually a better value and therefore going to dominate over other tech), not "feasible" (ie possible for a company to not die, even if it's not dominant). Just..... not impossible.  Maybe.

To be crystal clear, solvable, "even now" = TRL....  2?  3? 

Thanks.  Trying to calibrate the LMT-to-English translator. 

Some posters still imagine fission-fusion-fission tech to be low-TRL, despite the famous TRL-9 demo from 72 years ago.  Not one complaining poster could compare those basic diagrams and acknowledge the obvious high-TRL heritage.  That's a psychological block, and telling.

Likewise, weak rhetoric against saving $ billions annually.

[LMT]

Optimizing a Fusion Tug -- Tritium Self-Sufficiency from Thermal Neutrons

Previously:  1 2 3

Considering a Winterberg 3F fusion drive:

If tritium self-sufficiency could be obtained from fission thermal neutron waste (including intermediate neutrons, to ~ 0.5 MeV), DT fusion fast neutron waste could be reserved for thrust augmentation.

Notes:

Enrichment:
A blanket enriched in 6Li boosts tritium production significantly, with 90% enrichment sometimes considered.  Goel et al. 2023.

Cross-section:
Notably, the 6Li neutron cross-section peak is close to the peaks for 235U and 238U prompt neutron energies (images).  Given that a Winterberg 3F device holds roughly 1000x more uranium mass than tritium, the device's uranium fission thermal neutron production seems well suited for tritium breeding in 6Li.

Neutron reflector:
Additionally, a neutron reflector, such as graphite, can increase neutron path length through the blanket, to minimize the required blanket thickness and mass needed for maximized tritium production.  Hong et al. 2006.  Reflections also cool the more energetic uranium neutrons down toward the 6Li cross-section peak, for maximized tritium breeding.  The neutron reflector would be placed behind the blanket.

Exothermal transmutation and materials:
6Li tritium breeding is exothermic, heating the blanket.  A ceramic blanket and graphite neutron reflector could tolerate very high operating temperatures.  Temperature might be limited by the metal reflector plating; a tungsten-hardened steel can raise that reflector's operating temperature.  E.g., the F82H and EUROFER-97 tungsten steel alloys have been developed for the DEMO fusion reactor first wall (placed in front of the blanket), with continuous operating temperature of 550 °C.  Tanigawa et al. 2017.

Options?  Challenges?

Refs.

Fundamentals, D., 1993. Nuclear physics and reactor theory. DOE-HDBK-1019/1-93.

Goel, V., Aslam, S. and Dua, S., 2023. Optimization of Tritium Breeding Ratio in a DT and DD Submersion Tokamak Fusion Reactor. arXiv preprint arXiv:2310.00220.

Hong, B.G., In, W.K., Kim, Y., Lee, D.W., Song, K.W., Yoon, K.H., Cho, S., Ahn, M.Y., Kim, D.H., Yun, S. and Cho, N.Z., 2006, July. Progress in the Design of a Tritium Breeding Blankets for Testing in ITER. In 21st IAEA Fusion Energy Conference.

Tanigawa, H., Gaganidze, E., Hirose, T., Ando, M., Zinkle, S.J., Lindau, R. and Diegele, E., 2017. Development of benchmark reduced activation ferritic/martensitic steels for fusion energy applications. Nuclear Fusion, 57(9), p.092004.

Tao, X., Wang, J., Chen, G. and Shen, Q., 2017. Theoretical calculations and analysis for n+ 6Li reaction. In EPJ Web of Conferences (Vol. 146, p. 02038). EDP Sciences.

Wulandari, H., Jochum, J., Rau, W. and Von Feilitzsch, F., 2004. Neutron flux at the Gran Sasso underground laboratory revisited. Astroparticle Physics, 22(3-4), pp.313-322.

[LMT]

Example:  Tritium Self-Sufficiency from Depleted Uranium

Expanding on the goal of tritium self-sufficiency in a fusion tug: 

The fission-fusion hybrid reactor design of Reed et al. 2012 gives an example of self-sufficient tritium breeding from 238U (essentially, depleted uranium).

Their reactor places 6Li-Pb alloy outside a 238U fertile shell, to capture 238U neutrons for tritium breeding.  This configuration has its parallel in the tug concept, where a 6Li blanket would be placed outside a 3F device's 238U shell.

Their reactor configuration achieves tritium self-sufficiency (TBR >= 1), with capability to spare.  This suggests a path forward for a self-sufficient Winterberg fusion tug.  It would breed its own tritium from fission thermal neutron waste (both 238U and additional 235U neutrons), while leveraging its DT fast neutron waste to augment its best-in-class fusion thrust.

This work develops a conceptual design for a fission-fusion hybrid reactor in
steady-state L-mode tokamak configuration with a subcritical natural or depleted
uranium pebble bed blanket. A liquid lithium-lead alloy breeds enough tritium
to replenish that consumed by the D-T fusion reaction...

Under the conditions in Figure 3.4, we would need the lithium layer to be at least
15 cm thick to achieve a tritium breeding ratio of 1.

Refs.

Reed, M., Parker, R.R. and Forget, B., 2012, June. A fission-fusion hybrid reactor in steady-state L-mode tokamak configuration with natural uranium. In AIP Conference Proceedings (Vol. 1442, No. 1, pp. 224-231). American Institute of Physics.

[LMT]

Cross-post:  gauging "one month to Mars" with PuFF

[randomly]

Mike Shot

How many similarities can posters see between the "mini fission-fusion-fission" design (Winterberg 2004) and the primary stage of the Teller-Ulam design of Mike Shot, 1952 (Rafique 2023)? 

How many differences?

Who can find the most?  (You might team up and share notes.)

Refs.

Rafique, M.M.A., 2023. Design of Non-Tactical Deployable 20 Gwh (17.20841 Kilo Ton TNT) Fusion Device-Energy Basis.

Winterberg, F., 2004. Mini fission-fusion-fission explosions (mini-nukes). A third way towards the controlled release of nuclear energy by fission and fusion. Zeitschrift für Naturforschung A, 59(6), pp.325-336.

The Rafique diagram you posted is actually for the Castle Bravo shot, not Ivy Mike. Ivy Mike was a design using cryogenic liquid deuterium in a Dewar flask not solid lithium deuterium. The rest of the design was roughly similar though, with a boosted fission device providing the x-ray pulse to implode the deuterium and the cylindrical fission core 'sparkplug' in the center.

[LMT]

Mike Shot

The Rafique diagram you posted is actually for the Castle Bravo shot, not Ivy Mike. Ivy Mike was a design using cryogenic liquid deuterium in a Dewar flask not solid lithium deuterium. The rest of the design was roughly similar though, with a boosted fission device providing the x-ray pulse to implode the deuterium and the cylindrical fission core 'sparkplug' in the center.

Thanks; and an explanation of the second-stage liquid.

We were comparing the spherical primary stage to a spherical Winterberg device.  To confirm, each element looks correct in that basic primary stage diagram, yes?

[LMT]

Altered Carbon

Previously:

-  Winterberg's 3F microexplosion engine and its performance 1 2 3

-  Tug prototype / scaling 1 2 3 4

-  Attempting self-sufficient tritium breeding 1 2 3

If a fusion-drive tug breeds tritium, operating cost is slashed, but heat and mass increase.  Multirole materials could limit the increase, as with Type-II glass-like carbon (type-II GC).  Each layer has tunable properties.  Zhao et al. 2015, Hu et al. 2017.  Rough numbers below suggest a 3F tug for 29 km/s and 12-hour turnaround could employ a carbon-reinforced nozzle reflector with < 7 m diameter.

Common and tunable type-II GC properties include:

a.  Withstands high pressure (> 10 GPa), with high specific strength (> 4 MNm/kg)

b.  Withstands high temperature (> 2000 °C), with high thermal conductivity in sp² bonds (500-1000 W/mK at breeding 550 °C)

c.  Low, tunable density (< 2000 kg/m3)

d.  Tunable porosity (open or closed pores), gas-impermeable when closed

e.  Adjustable fullerene-like spheroid (FLS) concentration in disordered multilayer graphene (DMLG) matrix, for tunable auxetic material (negative Poisson’s ratio), which thins when compressed (superelastic reversible deformation to 50% compression)

Multirole type-II GC could reimagine a notional fusion drive structure.  How many roles does carbon play in the cartoon? 

Labels:

1.  Li2TiO3 pebbles, tritium breeding blanket for 3F detonator (15.)

2.  Metal nozzle reflector for (15.), radiator surface for (1.), notionally to 550 °C for tritium breeding

3.  HTS cabling for magnetic mirror on (2.).  Cable diameter and current increase with distance, for field uniformity.

4.  Metal hull hoop structure

5.  Porous type-II GC DMLG layers, > 4 MNm/kg.  Honeycomb loadbearing structure, neutron reflection, and helium coolant circulation for (1.), structural reinforcement for (2.).

6.  Porous type-II GC for neutron reflection and helium coolant circulation for (1.), structural reinforcement for (2.) (5.), hoop stress dispersion for (4.), superelastic shock absorption

7.  Direction of transition to auxetic layers, with increasing FLS and lower density, thinning under shock compression for hoop stress dispersion for (4.), superelastic shock absorption

8.  Direction of transition from open-pore to closed-pore type-II GC layers, to give a helium-impermeable surface (10.)

9.  Direction of pulsed helium flow from (1.) through open pores of (5.) (6.)

10.  Type-II GC radiator surface for (1.).  Heat transfers through helium circulation (9.) (11.) and through conduction at perhaps 500-1000 W/mK.  Balandin 2011, Arena et al. 2021.

11.  Direction of cooled helium flow away from (10.) and compressed FLS of (7.), toward (1.)

12.  Cylindrical regenerative HTS shock absorber for (3.), limiting superelastic kinetics of (6.)

13.  Cryogenics for (15.) and all other methalox systems

14.  Directional cryogenic radiator, e.g., V-Groove

15.  Winterberg 3F detonator, notionally 25-ton TNT equivalent

16.  Deployment tube for (15.)

Notes:

i.  Radiator: 

In the tug’s notional helium-cooled pebble bed (HCPB), helium circulation would complement GC heat conduction.  Here fusion pulse acceleration (15.) and transmutation heating in (1.), plus peripheral cooling in (10.) and FLS compression in (7.), could circulate helium coolant (9.) (11.) through open pores in (5.) (6.).  Radiator sim with type-II GC layers could be complex.

ii.  Reflector: 

Nozzle reflector (2.) with Type-II GC reinforcements (5.) (6.) could withstand 1 GPa.  Setting average acceleration to 0.2 g with 1.5-second pulse intervals for 12-hour turnaround, a tug’s reflector diameter is < 7 m.

iii.  Roundtrip Propellant: 

A tug’s methalox requirement for 12-hour turnaround increases with tritium breeding structure, to roughly 400 t. 

iv.  Earth-launch: 

Remarkably, such a 3F tug should be light enough to launch as a second-stage unit, even with tritium breeding structure.  If launched on a reusable Raptor v3 Super Heavy, the tug could burn from roughly 100-km MECO to a LEO depot.  A fractional load of tug propellant (< 50 t) + 3F is needed, with methagox RCS and no Raptors.  Igniting the fusion drive above the Kármán line while rising toward apogee, the 3F plume could be angled above horizontal to prevent even minor atmospheric contamination.

Videos: 

-  compressed glassy carbon

-  auxetic foam compression

Refs.

Arena, P., Del Nevo, A., Moro, F., Noce, S., Mozzillo, R., Imbriani, V., Giannetti, F., Edemetti, F., Froio, A., Savoldi, L. and Siriano, S., 2021. The demo water-cooled lead–lithium breeding blanket: Design status at the end of the pre-conceptual design phase. Applied Sciences, 11(24), p.11592.

Balandin, A.A., 2011. Thermal properties of graphene and nanostructured carbon materials. Nature materials, 10( 8 ), pp.569-581.

Hu, M., He, J., Zhao, Z., Strobel, T.A., Hu, W., Yu, D., Sun, H., Liu, L., Li, Z., Ma, M. and Kono, Y., 2017. Compressed glassy carbon: An ultrastrong and elastic interpenetrating graphene network. Science advances, 3(6), p.e1603213.

Winterberg, F., 2015. Thermonuclear Operation Space Lift. Journal of Spacecraft and Rockets, 52(2), pp.613-618.

Zhao, Z., Wang, E.F., Yan, H., Kono, Y., Wen, B., Bai, L., Shi, F., Zhang, J., Kenney-Benson, C., Park, C. and Wang, Y., 2015. Nanoarchitectured materials composed of fullerene-like spheroids and disordered graphene layers with tunable mechanical properties. Nature communications, 6(1), p.6212.

[LMT]

Re: the "Altered Carbon" post:

The notional HTS (3.) for the tug's magnetic mirror is closer to reality with this week's announcement:

Zenno Astronautics and Faraday Factory partner on superconductors for space

AUCKLAND, NZ and TOKYO, Japan, April 9, 2024 – New Zealand headquartered space-flight systems company Zenno Astronautics (Zenno), a developer of world-first superconducting electromagnets for space applications, and Faraday Factory Japan LLC, the world’s leading superconducting tape maker, have partnered to collaborate on the development of bespoke high-temperature superconductor (HTS) magnets for space applications.

Zenno and Faraday Factory Japan will combine their respective technologies and fields of expertise to develop a high-temperature superconductor (HTS) magnet product that uniquely suits space. Faraday Factory will manufacture the bespoke space HTS tape to conduct electrical currents with zero electrical resistance and an extremely high current density of several hundred amperes per square millimetre. Zenno will implement the new technology in its superconducting magnet platform, designed to enable reliable, scalable, and fully electric space applications...

Magnetic mirror sim of Ewig 2003, mentioned previously, used 10 MA cables to reflect an 81-ton TNT equivalent blast off a 22 m diameter nozzle.  In comparison, the notional carbon-reinforced 3F / MOX tug reflects a 25-ton blast off a < 7 m nozzle.  Here blast pressure is roughly 3x higher. 

Result:  A set of Faraday Factory space-rated HTS cables with diameters averaging around 40 cm might meet requirement near-term.  HTS tapes with much higher current densities do exist, so smaller cables seem likely in the OP timeframe.

Video:  Zenno Astronautics