Starshot's speed limit

Centauri Dreams exists to tout the Breakthrough Starshot, a mission to Alpha Centauri at 0.1-0.2C. Unfortunately this didn't get a dedicate team at the time, which was, what, the late 1990s such that we'd be getting data now. The fact of it taking 20 years tends to keep Congress (and any republocratic system) from greenlighting such a thing, like those Uranian missions we don't got (Voyager was politically-feasible only because it promised to relay data from Jupiter and Saturn en-route).

The one bit of tech that did get some work was the solar sail itself, having some local applications as it does. IKAROS went to Venus in 2010; ACS3 is in operation in NEO now. This blog has recommended solar-sails for 50ish AU targets, like Eris.

Suppose we could iterate that tech toward something faster. 0.75C (... like Passengers) means 5.66 years (Einstein & Copilot say 3.75y on-board). Senators - in particular - might be talked into that one, especially new Senators.

Engineering issues abound: for instance, light gets redshifted as the sail vanishes away, but we might fix that with a fleet of boosters, which expend their last enthalpic battery pulses punching out one last laser. Up to Jupiter, these expendable boosters can be "3U Plasma Magnets".

There's also dust. I expect that's more a worry at Oort and not interstellar (at present).

The real issue is a background photonic speed limit. We've discussed that here for cosmic rays and, uh. Something like it applies to Starshot too.

Assuming complete unobtainium and space-magic for everything short of warp, the sail is only getting to 0.75C. At that point diffuse scattering begins acting as drag. That's cutting it thin for our Senate.

Shuttle shenanigans, III

And here I was thinking that Sierra had forgotten about the Dream Chaser. Turns out, they've scaled it. If you have grams you've created in orbit, here's how to get them back. At "1.5 g" which I take to refer to accelerational pseudograv 14.7 m/s². (I'd hate for g to refer to grams here.)

In other spaceplane news, there's Dawn, filling in for the "New Shepard" - which (by contrast) Bezos has forgotten. That's testing Astral Materials' microgravity furnace, for four hours at a clip. So it won't need full orbit; it is doing some freefall before gently returning. Dream Chaser would be in orbit or near-enough, so can take longer before drag pulls it back to us. If Astral wanted it, the 'Chaser could test their furnace for that much longer.

I don't know if Dream Chaser ever intends to dock with one of those orbital stations we're getting next year. That would allow the furnace to be used for many days, with the return cargo being much larger. We might have ideas for shrinking that furnace, at that.

Alla'dhat's not nothing for unmanned cheap returns, but... doesn't Elon already have Starfall? That's a metric tonne in a a 2.5× 1.5× 0.5m container.

Maybe the issue here is that these grams being returned are just that fragile.

LANTR to Mars

The nonenthalp LANTR has been evaluated for Mars, against classic NERVA: Daria Nikitaeva and L. Dale Thomas. They studied both the more-or-less direct Conjunction Class and the Venereal fryby. We join the Mars Society to consider Conjunction only, until/unless we have first-aid stations at Venus.

Nikitaeva-Thomas calculated a LOX-assisted NERVA could cut propellant volume but - refer to the periodic-table - raises the mass (by 32.4% so - 132.4% the baseline). The mass buys 540% the thrust and 122% the impulse. Raising mass is a major problematic as long as we are delivering the LOX from Earth; but from our Moon, which may even get to use a electric-powered sling: that wouldn't be an issue. More at issue is this LOX mass takes from the "dry mass" of the rest of the mission.

When other planets be considered for humans, I suspect better options shall be at hand.

Shockwave exhaust

Ryan Gosse, a bunch of Floridians, and Tony Colozza of NASA offered "Nuclear Wave Rotor Bi-Modal Cycle for In-Space Propulsion" in, I think, 2024 (pdf). They fill out "wave rotor" with "enhanced nuclear" to make the catchy 'cronym WREN. h/t ToughSF on Twitter 30 August who summarises: A wave rotor uses shockwaves to compress the exhaust of a nuclear thermal rocket to 4765 K, achieving 1430s Isp at 2.27 kW/kg, or turn a generator to feed 3000s Isp thrusters at 0.47 kW/kg. The target is Mars(/Deimos).

I_sp is, recall, really a measure of exhaust velocity, so just x9.8. NERVA-mode 14 km/s just like it says on Table 1 (page 15, relayed at ToughSF).

As we've learnt from ramjets, usually this much temperature melts ... everything. We again want liquid-scale results from a still-solid core. They're saying the WREN can use the shockwaves to keep the hydrogen plasma off the chassis, heat it even more (Boyle's Law, bitchez) and ram it out to that bespoke velocity. So an afterburner except not burning.

The NΘP mode instead offers 34.7 kN of thrust, twice the beryllium option of fusion pinch (also skiffy). Which does indeed put Mars into the frame.

The NΘP propulsion cycles run a total irradiation time of only two days for a 45-day mission and less than 10 hours for a 65-day mission. (67 days is where Princeton's direct-fusion is at, now "Starfire".) To follow that - I suppose - they... don't run. The assumption is a one-way mission, perhaps short enough they don't need to spin the wheel nor, one assumes, build to a wheel. At Mars, the areonauts find their own gravity until return.

The paper notes that pre-VASIMR, I_sp had been chemical and rather unconstrained. The high impulse allowed by electric propulsion permitted variable propulsion, for mass savings over the trajectory burn. Mass of propellant, that is; which means more for the mission, albeit likely to be taken by the NEP mode's own subsystems.

The paper must admit to wasting heat, defined as heat not going to propulsion. They hope to recover some of that as electricity: life support for the crew, or just filling up batteries (thus relieving strain on the radiators if nothing else). Once at Mars solar-cells get only, what, 43% flux and on Mars that's at high noon with no dust. That's for a later day.

Also deferred is the propulsion cycle, dependent on thrust/weight of the NΘP. The authors had touted the 10-48 hours as short. Classical NERVA, for its 900s, aimed for 30m-2h of burn. WREN must settle on NΘP 8-15 hours. That's not so short; it's comparable to the issues identified with NTR to our own moon (that paper recommended LOX for a true afterburner, so "LANTR"). The WREN authors suggest they drop the NEP components to focus on thrust; this would jack up the time of the mission and maybe require that spinwheel after all.

Soft landings

LANTR seems to stop short of landing a craft upon the Martian surface. As to that L1 Kevlar rope, as it is under construction it might leave a megameter or two over the landing spot. Bouncing cargo onto bare regolith is going to kick up some dust. Launching off again will kick up still more, especially if they use the Al-LOX mix.

A week ago Space Startup News pointed to xARC, on the southwest edge of the Texas Triangle, concerning Lunar mission architecture. They were going to drop off some 'bots to roam the regolith and cook up hexagonal bricks, to ensure dust-free landing pads, roads, other pavements. (Lunar Forge is on that tech.) Also supplied would be berms to keep dust from drifting back in, from all the industry hopefully happening around the site.

Meanwhile in June, Anderson, Saxena, Metzger [ht Toughsf 31 August] came up with another idea. The Solar-System has already built some berms for us, in the form of small craters. Land craft with the fragile cargo in one of those. Schlep cargo over rim with Lunar Terrain Vehicle. Assemble base around crater (some material here could have been dropped less-gently in other missions).

Zubrin has long noted is that bricks are good for pushing down, but not for pushing out. We are here talking about the base camp, even its airless robot-only stage. It might be a trick to find a suitable berm-crater reasonably close to a human-friendly environment, like a lava cave.

LANTR

Whilst we await Firestar's nuclear afterburner, Borowski et al. a decade ago had proposed a chemical one. The subtext, best I gather, was that solid-core HALEU is all we're allowed in Earth's orbit (bought from Centrus, say). That buys us NERVA at 900s I_sp. This has never been that great for thrust. Most Mars missions assume NERVAlike NΘP. Or big punches to orbital trajectories, like The Martian to Hohmann.

Borowski's crew was pondering a high-thrust solution as may ignore much I_sp and Earth won't care what is burnt near us. This has a use-case in delta-V punches: a mission to the Lunar poles. I'll bite; even that spaceline only got to equatorial Sinus Medii.

The promise is to achieve liquid-core-temp exhaust whilst still keeping the (fissile) heat-source, solid. That'd be a safer reactor in HEO, for our legal lower-grade fuel. In 2024 the plasma-core was still SDI scy-fy (and in 2026; also none of us knew or know what Howe is doing).

NERVA, it happens, assumes hydrogen for propellant. As the Hindenburg passengers can tell you, it reacts with oxygen. (This blog takes no sides on the cause of that one's initial blowup.) The paper suggests to bring along liquid oxygen - LOX - with that superheated hydrogen. The LOX is introduced after the burn which raises the exhaust velocity. This is thereby an "LANTR". Oxygen is cheap on our moon, maybe the only cheap volatile.

As to why to do this: that is because the N part of NΘP is a nuclear reaction, and not - like Orion - a bomb. As a reaction, it is supposed to run for longer. NERVA hasn't been tested for many hours in, what amounts, a thermos the size of the Universe. "Ultra Safe" was supposed to test this which is now General Atomics' job. Earth might not want a reaction running long in our shared well.

For short bursts of heat one might even swap those nucular roggets for enthalpy. Which segues...

The antiquity of Liber Antiquitatum Biblicarum

If you had read (ye) "Old(e) Testament Pseudepigrapha": one of the texts herein was a pseud' on Philo. Of course Philo had nothing to do with the Antiquitatum Biblicarum. Western Christians, who copied and have edited this work, have rated it a "Pharisaic" document: close to Josephus' halakha against, say, Judith's (or anything else of the pre-Revolt Diaspora). Zeev Safrai posted an essay on "LAB" in 2018. Safrai puts this in the Jamnia / Javneh era after 70 CE, when the priests didn't matter.

As far as I know a post-Qumran date wouldn't have ruffled feathers at any point since the late 19th century relieved it from Philo. LAB was composed in Hebrew but Qumran shows no awareness of it (we'd be looking to Jubilees, which relation is reversed). It hasn't been picked up by Septuagintal literature the way Judith, say, was picked up in the Greek Esther. As noted Philo didn't know it. It is antiSamaritan, inasmuch as the old shrines at Gerizim / Shechem are condemned. The mutual relationships with 2 Baruch and 4 Ezra / 2 Esdras are famous.

Safrai dates LAB rather after Bar Kochba. It seems of interest LAB sides with Shammai on whether children should fast, where the Mishnah gives to Hillel the last word against Shammai. LAB also retains Jubilees' interest in maintaining the names of Israel's matriarchs, although I cannot call that "feminist" (this reflects a tradition of matrilinealism). These intra-Jewish disputes seem early to me. So far I admit this is a matter of taste.

I don't see where Safrai's essay has raised skub in the past eight years. So... I'll do it:

Within the earlier decades, came the Judaeans' Tumult in Cyrene, Cyprus, and Alexandria. Vridar, when he was poasting, deemed this revolt an important event. LAB - as Safrai knows - argues for the general eretz-Israel as holy and under G-d's protection. Perhaps this nods to a time when Jews were persecuted everywhere but in the Judaea proper, where peace maintained. I don't think that Jews could honestly say this after Bar Kochba and Hadrian.