LANTR LTV/LEV: A New Way to the Moon

lantr-lev-side-by-side-comparison

Two versions of the LANTR LTV/LEV. On the left is one suggested for a SSTO launcher that could carry 20 tons to orbit and had a 13.5 meter payload bay. The one on the right could fit in a 9.5 meter cargo bay, at the cost of using less efficient methane for lander fuel, a smaller crew capsule, and a fiddly tank-within-a-tank to hold some of the craft’s liquid oxygen oxidizer. Public domain image composited from two separate diagrams in NASA’s Human Lunar Mission Capabilities Using SSTO, ISRU and LOX-Augmented NTR Technologies A Preliminary Assessment. Click for a larger view.

What it was: A mid-90s proposal for a lunar mission using an innovative rocket engine for the trip to the Moon and some basic lunar industry to refuel its chemically-driven lander for the trip back. It was one of the first proposals for a Moon mission to try and move away from a brute-force Apollo-style mission that was impossible to fund.

Details: The core difficulty with a Moon mission, or a mission to much of anywhere really, is that you need such massive vehicles. The Saturn V, for example, was 2950 tonnes when fueled, and was 111 meters tall. It was accordingly expensive: approximately US$700 million in 2016 dollars. Reusability was the route taken in the decades since to try and bring this down, but the Space Shuttle ended its life costing US$450 million per launch and for a considerably smaller payload being taken to orbit too.

By the early 1990s, in-situ resource utilization (ISRU) was seen as the next coming thing for making missions cheaper. This is to say, don’t haul all the mass you need up into space, take advantage of whatever mass is already there wherever you’re going. The difficulty here is that that mass is useless rock and, to a much lesser extent, water ice. The most obvious thing to do would be to refine cryogenic rocket propellants from it, as both rock and ice can be sources of oxygen and hydrogen. By the mid-90s people had been thinking for several years about how to do that, and what what would be possible once it could be done.

The most famous fruit of this effort was planning for Mars missions, partly because the vehicles for a traditional flight there would be ridiculously large even by Saturn V standards and partly because Mars’ carbon dioxide atmosphere is almost trivially easy to turn into methane (a decent rocket propellant) if you bring along some hydrogen from Earth. Less well-known is a lunar mission using ISRU which was developed at NASA’s Lewis Research Center.

In the early 1990s Lewis had been involved in the development of a nuclear rocket of an unusual type, what they called a LOX Augmented Nuclear Thermal Rocket (LANTR). A regular nuclear thermal rocket like NERVA runs on pure hydrogen, not burning anything at all and simply relying on nuclear power to heat the propellant and produce a high specific impulse. Unfortunately liquid hydrogen is very low density, and so the tank to hold it has to be large—and it doesn’t matter how light something is if you literally can’t fit it into the cargo bay of the Space Shuttle, or however else it is you’re planning on getting it into orbit.

The LANTR solved this problem by using liquid oxygen along with the hydrogen. After being heated by the reactor, the hydrogen was mixed with oxygen, which would then burn. This had the paradoxical effects of reducing the engine’s specific impulse, but also radically reducing the amount of hydrogen needed and making the necessary hydrogen tank much smaller. Liquid oxygen is seventy times denser than LH2, so its tank would be small too. The usual mix of oxygen to hydrogen is near 1:2 (as the chemical formula “H2O” would suggest), but even when mixed 5, 6 or 7:1 with the hydrogen the reduced specific impulse of the LANTR was still considerably better than you got with a conventional LOX/LH2 rocket while also being smaller than a pure-hydrogen nuclear rocket..

artist

“Artist’s Illustration of a Self-Contained, Modular LUNOX Production Unit”, plus an astronaut apparently taking a selfie. Public domain image from A Revolutionary Lunar Space Transportation System Architecture Using Extraterrestrial LOX-Augmented NTR Propulsion. Click here for a larger view.

The leap to lunar ISRU came with the realization that oxygen was a major component of the Moon’s soil. For example, the orange soil famously (and excitedly) discovered by Jack Schmitt during Apollo 17 contained hydrated iron oxide, and was rich in oxygen and water. At Lewis, the combination of LANTR and ISRU for a Moon mission crystallized in a flurry of papers spearheaded an engineer there, Stanley Borowski, in combination with a variety of colleagues. Rather than go with an already compact Moon mission using entirely Earth-sourced oxygen, why not use the Moon’s native oxygen for oxidizer on the way back? The result would be smaller and cheaper still.

The result was a proposal to build a Moon landing ship that was embedded in some basic Lunar industry that would be set up prior to the crewed landing. The first step would be to send an automated lander with a teleoperated mining equipment to a site where ilmenite or some other oxygen-rich rock had been pinpointed from orbit. Also included would be a 35-kilowatt nuclear reactor, which would provide the heat to break down the lunar rock with the hydrogen that would be brought along too, producing water. The water in turn would be broken down to oxygen and hydrogen, the former being stored and the latter recycled to start the process again on the next batch of rock.

Once 10.5 tons of liquid oxygen had been built up (a process which would take a year), the LANTR LTV/LEV (Lunar Transfer Vehicle/Lunar Excursion Vehicle) crewed mission would begin. Here a little bit of variation appears. When first suggested in 1994 the craft was assumed to be using a Shuttle-C, a derivative of the Shuttle for cargo only, to get to orbit—the LANTR wasn’t powerful enough to lift the whole works by itself (and no-one was very keen on firing a nuclear engine at ground level in any case). The Shuttle-C was already a cancelled project, however, and by 1995 NASA had been pinning its hopes on the VentureStar or some similar SSTO. At the time the LANTR LTV/LEV was being bruited about, the size of the SSTO’s payload bay hadn’t been nailed down and while NASA had specified 20 tons to LEO it was unclear how long the cargo it carried could be, Accordingly Lewis Research Center came up with two LANTR LTV/LEV configurations, each of which would be lifted in three pieces and mated in orbit.

If the SSTO gave them 13.5 meters to work with, the result was a 58.8-ton, 26.2 meter-long craft. Compare that with roughly 140 tons and 35 meters for the Apollo LM/CSM/S-IVB that launched the Apollo astronauts to the Moon. This version of the LANTR LTV/LEV would have be entirely fueled by LOX and LH2, excepting (presumably, as none of the sources say) hydrazine for the RCS thrusters as usual. On top was a curiously inverted command module; the author could find no discussion of how that was handled when time came for re-entry, so one presumes rotatable seats for the crew.

The longest part of this variation was the joint LH2/LOX tank for the transfer vehicle, while the widest was the bulbous hydrogen tanks on the lander. Both had to go to get into the smaller 9.5-meter SSTO payload bay suggested. The lander was switched to a more-compact but less efficient fuel, liquid methane, while one of the two oxygen tanks for the LANTR was moved to inside the LH2 tank, and outfitted with a double wall that would keep the supremely cold hydrogen from solidifying the oxygen within. The resulting craft was slightly lighter at 58.5 tons and definitely shorter at 24.2 meters, but in return they had to come up with some way of shaving 700 kilograms off of the crew capsule. Both variations of the capsule were approximately the same size as the Apollo CM, though the first’s was slightly larger than the second.

profile

The LANTR LTV/LEV mission profile. Note the direct descent and direct return. Public domain image via NASA from Human Lunar Mission Capabilities Using SSTO, ISRU and LOX-Augmented NTR Technologies A Preliminary Assessment. Click here for a larger view.

There was no LM, though, because the LEV was a direct-descent, direct-return vehicle. This did mean that if the stay on the lunar surface was to be of any length, a third mission, automated like the LOX plant, would have to be sent beforehand to give the astronauts a habitat. The LEV itself was inadequate otherwise.

What happened to make it fail: Though the mission was considerably cheaper than an Apollo-style trip to Moon—Johnson Space Center was looking at the time to spend less than US$1 billion on a Lunar return mission—not even that amount of money turned out to be available in NASA’s budget, particularly after the decisions were taken to continue with the Space Shuttle and build the International Space Station around the same time as the proposed first flight of a LANTR LTV/LEV’s, around 2001.

It also didn’t help that the craft came to an unwieldy size. It was intended to be launched on the VentureStar, and that never came to fruition. A comparable mission restricted to launch vehicles that actually existed needed one Shuttle mission and one launch of a Titan IV (which could lift longer payloads than the Shuttle could), a peculiar and expensive combination.

Something like it still could have begun as late as the about ten years ago, but then a discovery about the Moon put the final nail in its coffin. From 1994 through 2009 it became increasingly clear that the Moon had ice in some of its South Polar craters, with the case being settled by the Chandrayaan-1 probe. This changed the game for ISRU, since ice is a lot more useful raw material than lunar soil. Essentially all serious planning for a Moon mission since then has reflected this, and lunar rock has fallen by the wayside.

What was necessary for it to succeed: Much like the First Lunar Outpost, the LANTR LTV/LEV’s best bet would have been at the time the Clinton Administration was trying to decide how to help occupy the former Soviet Union’s rocket scientists so that they wouldn’t end up designing missiles for who knows what country. The decision to go for an joint space station rather than a joint lunar mission or base was a relatively easy one, given the USSR’s experience with stations, but it’s not too difficult to see the US deciding to go for the public relations spectacle of the Moon over the more staid ISS.

Otherwise the LANTR LTV/LEV is a sound concept if the promised Isp advantage holds, to the point that (by the standards of this blog) something much like it still would be worth building and flying. The primary difficulty with it in 2016 might be, oddly enough, that it’s too small. Sixty tons falls into the “between two stools” range that we discussed in the entry on the R-56, too big for something like an Ariane 5 or Delta IV Heavy, but too small for the upcoming SLS. Given that you’re going to have to use an SLS and that rocket will quickly outstrip 60 tons by a lot, why not design a spacecraft that uses up the extra payload capacity? Fans of SpaceX’s Falcon Heavy effort might want to take some notes, though.

Sources

A Revolutionary Lunar Space Transportation System Architecture Using Extraterrestrial LOX-Augmented NTR Propulsion. Stanley K. Borowski, Robert R. Corban, Donald W. Culver, Melvin J. Bulman, and Mel C. Mcilwain. 1994

Human Lunar Mission Capabilities Using SSTO, ISRU and LOX-Augmented NTR  Technologies A Preliminary Assessment. Stanley K. Borowski, 1995

High Leverage Space Transportation System Technologies for Human Exploration Missions
to the Moon and Beyond. Stanley K. Borowski and Leonard A. Dudzinski. 1996

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