Waiting for a Pump-Fed Methalox Lunar Descent Engine

[sdsds]

The thesis of this discussion is that all the many designs which have been proposed recently for crewed lunar landers are hobbled by the lack of an appropriate engine for the braking phase of lunar descent.

Do you accept the premise that the "tyranny of the rocket equation" won't be broken soon? (That is, that propellant sourced on e.g. the lunar surface is too far into the future to consider for the next major lander design?) If so, your design for the descent burn just has have better efficiency (Isp) than hypergolic propulsion provides.

ULA wants to use hydrolox for an ACES-derived dual thrust axis lander, but RL-10 isn't cheap and they have yet to fly even a demo like the CRYOTE testbed. (No offense to Goff et al. but their proposal was made 8 years ago....)

Blue might also be pushing hydrolox (because BE-3) but their vision is unclear to me. (Could their be a  BE-5 on the drawing boards?)

Musk still expresses no interest in the lunar surface per se.

Pump-fed propulsion for a single string mission critical (and possibly life critical) application is a hard idea to sell. So personally I envision a hypergolic ascent stage which provides abort to LLO as a descent contingency capability. This implies the descent stage isn't reused, but perhaps it could be repurposed on the lunar surface.

How hard could it be to down-scale Raptor or BE-4 to make an engine appropriate for lunar descent?

[Russel]

One technology that is worth keeping an eye on is battery powered, electric pump fed engines such as the Rutherford:

https://en.m.wikipedia.org/wiki/Rutherford_(rocket_engine)

For a smaller engine such as a lander or ascent vehicle (I was more interested in a Mars single stage lander/ascent vehicle) it could make a lot of sense.

You don't need very high pressures to get enough Isp out of methalox. Plus it has advantages in terms of simplicity/reliability. No hot turbine. Fewer parts. Good restartability and dynamics. You even have precise control of mixture. So basically most of the advantages of a pump fed (turbine) engine but simpler, robust, possibly lighter (medium not high pressure) and easier throttling. Makes it look like a nice fit for a smaller craft.

[savuporo]

One thing your adventurous lunar landing spacecraft will appreciate is a substantial bank of batteries to keep it alive for 2 weeks.
Electrically pumped engines suddenly make even more sense, because batteries can be designed to have dual use.

[RocketmanUS]

For the horizontal lander concept were the main engine(s) on the back to do the deorbit burn and once close to the surface do a burn to slow the craft down? Then have the side engines do the final landing burn? This would mean the side engines would not have a long burn time ( not much propellant to burn ) so a lower ISP could do. Side engines would be used for the first part of the ascent but most of the delta v needed back to LLO would be done by the rear engine(s).

[sdsds]

For the horizontal lander concept were the main engine(s) on the back to do the deorbit burn and once close to the surface do a burn to slow the craft down? Then have the side engines do the final landing burn?

I'm fairly certain the concept you are describing has frequently been called a "dual thrust-axis lander" (DTAL) design. It is a cool idea if you intend your descent to involve hovering (as opposed to a "hover-slam" landing).

Much depends on the amount of mass a lander leaves behind of the surface when (or if) it ascends again to orbit. In a different thread clongton put forward a design that involved drop tanks discarded on the lunar surface. Another approach is to discard the entire descent propulsion system on the surface, as was done for Apollo.

I wonder: "If an architecture designer had available a reasonably high Isp propulsion system with the "right" mass and thrust characteristics for the lunar braking burn, what other burns does it make most sense to give that same propulsion system?"

Ascent? Cis-lunar arrival (i.e. lunar orbit insertion) propulsion?

And does it say anything about the fully reusable vs. partly expended question?

[Space Ghost 1962]

The key advantages of an electrically powered pump fed engine are:
 * solar rechargeable pressurization works for the short burns and long coasts/on orbit/on land
 * less consumables/gas to restore
 * lower weight propellant tanks over pressure fed
 * higher engine performance over pressure fed.
 * higher reliability - no gas generator/close cycle
 * faster startup/shutdown/tail-off due to the high on demand torque of electrically powered pump.

[GWH]

Ursa Major Technologies is working on small scale ORSC engines, starting with a 5,000 lbf kerolox engine and then moving on to a 35,000 lbf LOX/"hydrocarbon" engine.  The wording there suggests methane fueled could be a potential development path.
http://www.ursamajortechnologies.com/#engines2

Then there was the Darma Technologies' Chase-10 pump fed (cycle?) 22,000 lbf methalox 321s ISP Vac engine, but they seem to be defunct.
http://www.darmatechnology.com/chase-10.html

I had thought there might be more out there but in reviewing it a bit seems most everything in development is pressure fed.
Blue Origin is developing a 11,000 lbf methalox thruster for their proposed Blue Moon lander, although that is probably pressure fed.
Xcor has developed 7500 lbf pressure fed methalox thrusters. I wonder how adaptable their piston pumps are that are in development for their hydrolox RL-10 replacement?
Masten Space has the "Broadsword" 25,000 lbf pressure fed methalox engine as the half scale to the 60,000 lbf "Cutlass" engine.

[savuporo]

MSFC is developing additive manufactured, turbopumped 25klbf engine.

Edit: of an pretty full score of all recent efforts, thru 2016:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160006972.pdf

9. CHALLENGES FOR FUTURE HUMAN EXPLORATION
Considering the advanced development efforts conducted by NASA (and industry partners) over the last 15 years, the overall development risk for LOX/LCH4 in-space propulsion has been significantly reduced. While these efforts have provided a strong foundation for the pursuit of an initial flight capability, some challenges still exist, requiring additional investigations/risk reduction testing. These remaining challenges include the following:
- Integrated Storage testing with 90-Kelvin cryocoolers
- Reaction control thruster design maturation
- Design maturation for regeneratively cooled main engines
- Design of low-leakage, long-duration cryogenic valves

More advanced in-space capabilities (landers, ascent stages, depots, etc.) require additional technology maturation for:
- Pump-fed LOX/LCH4 engines with deep throttle capability
- Leak detection
- Zero-G mass gauging technology maturation
- Automated fluid couplings for space cryogenic systems
- Zero-G demonstration of cryogenic liquid acquisition devices

[TrevorMonty]

If going for 2 stage lander, ACES could act as 1stage do deorbit burn then separate and crash or return to orbit. Assuming we are operating from DSH, 2nd stage needs 2.5-3km/s.

20t methane 2nd stage + 30t ACES could do deorbit and still return to DSH. Not as efficient as XEUS but ACES wouldn't need any modifications.

[oldAtlas_Eguy]

If going for 2 stage lander, ACES could act as 1stage do deorbit burn then separate and crash or return to orbit. Assuming we are operating from DSH, 2nd stage needs 2.5-3km/s.

20t methane 2nd stage + 30t ACES could do deorbit and still return to DSH. Not as efficient as XEUS but ACES wouldn't need any modifications.

If you need prop for each landing delivered from Earth a throw away ACES is not as bad as it seems because you need to get the prop to the DSH somehow. Delivering using an ACES and then reusing it as a crash stage does two things eliminates the need for a big lander stage and gets rid of the buildup of US that arrive at the DSH. Now combine that with a completely reusable last mile lander that is also the ascent stage that is pump fed (whose batteries are kept charged by the ACES ICE). Then on the surface an ICE that can charge batteries so that electric pump fed actually re-purposed high pressure fed engines from a central pump can be used for last mile landing and ascent.

The problem why so little is being done on engines for this application is that there is no demand for this application.

[GWH]

Just land the ACES and retrofit as a surface hab down line with the kit IXION is proposing.

[TrevorMonty]

Just land the ACES and retrofit as a surface hab down line with the kit IXION is proposing.

If IXION system works out, then cargo XEUS has lot appeal. Cargo is habitat module loaded with equipment to fit out LH fuel tank as habitat. Use LOX tank for waste disposal or water storage.

The something can be done for ACES in LEO or at DSH.

[Rei]

You don't need very high pressures to get enough Isp out of methalox.

Methalox is fairly low density, which means a poor thrust to weight ratio. Pressure improves the thrust, not just ISP (at the cost of increasing stress on the chamber, as well as erosion).  For a given design, pressure has a lot more of an effect on thrust than ISP.  Helping counterbalance that is the moon's low gravity, so you don't need a ton of thrust,  but on the other hand, every kilogram you add that far down the dV chain adds a lot of kilograms further up.

I think a simplicity/reliability argument might be better, since reliability is very important out there (hence Aerozene 50/MON for the LM).  Assuming that you can actually demonstrate reliability. Need to make sure that cryogenic LOX storage is also simple/reliable, of course.

[Russel]

To keep the liquid methane tank light weight means low pressure. Electric pumps would greatly assist in this. If you're really worried you can use drop tanks.

An electric pump fed engine would sit somewhere in the medium pressure range. Enough to get far enough up the performance curve without having a highly stressed engine.

[Russel]

My main concern is keeping the refrigeration light weight.

[oldAtlas_Eguy]

We are now back to the same technologies that ACES (auto-genesis pressurization, ICE, and IVF) that lower weight, increase duration, re-usability and other advantages for a reusable vehicle.

The ICE in a methalox system would burn gasses of methane and O2 to then drive a pump to re-liquefy the gas methane and O2 to manage boiloff and tank pressure. The prop loss in the system would be just the methalox used for the ICE, no gasses are dumped overboard.

This is the holy grail for not only ULA's ACES (hydrolox) but also for BO's NG (methalox) and SpaceX's ITV (methalox).

If you have this system then you have power until you you run out of methalox. So surface stay time is a function of available prop.

Also in a small engine size vehicle such as a cis-lunar tug or Lunar lander the ICE either charges batteries that are then used to drive electric pumps for pressure fed engines or the ICE drives them directly with a pump.

This tech has been thrown around for decades by Boeing mostly for lowering costs and for making long deep space mission possible with high ISP engines  hydrolox and a low or "0" boiloff.

[spacenut]

Why seperate and crash?  You can land it all together and used the empty fuel tanks for habitat or storage later.  Let residual methane and lox boil off, purge with nitrogen.  Methane is lighter and can vent out a top valve while purging.  Left over oxygen can boil off and mix with nitrogen for earth type atmosphere.  The DTAL lander using an ACES stage modified for metholox instead of hydrolox would work, but may have a little less payload capability depending on how the stage is landed. 

[Jim]

Why seperate and crash?

many reasons.  Too much mass, Too large of stage, etc

[Robotbeat]

Why seperate and crash?  You can land it all together and used the empty fuel tanks for habitat or storage later.  Let residual methane and lox boil off, purge with nitrogen.  Methane is lighter and can vent out a top valve while purging.  Left over oxygen can boil off and mix with nitrogen for earth type atmosphere.  The DTAL lander using an ACES stage modified for metholox instead of hydrolox would work, but may have a little less payload capability depending on how the stage is landed.

If you really wanted to reuse the ACES, you could do an "uncrasher" stage (edit: as TrevorMonty hinted at). Separate ACES with enough propellant for it to fly back to orbit and to the depot.

So you can start with a crasher stage with the option to do uncrasher down the line. Sounds like a cost-optimal way to do things if you don't want to do single-stage.

[Space Ghost 1962]

Why seperate and crash?  You can land it all together and used the empty fuel tanks for habitat or storage later.  Let residual methane and lox boil off, purge with nitrogen.  Methane is lighter and can vent out a top valve while purging.  Left over oxygen can boil off and mix with nitrogen for earth type atmosphere.  The DTAL lander using an ACES stage modified for metholox instead of hydrolox would work, but may have a little less payload capability depending on how the stage is landed.

If you really wanted to reuse the ACES, you could do an "uncrasher" stage (edit: as TrevorMonty hinted at). Separate ACES with enough propellant for it to fly back to orbit and to the depot.

So you can start with a crasher stage with the option to do uncrasher down the line. Sounds like a cost-optimal way to do things if you don't want to do single-stage.

Similarly, by time reversal ala Feymann, you could with perfect timing, launch from the lunar surface suborbital, match velocities with an ascending vehicle, dock, and power the ascent. You could even budget in contingencies.

Gets the lander coming and going. On two separate trips.

add:

And, if that isn't crazy enough, you could "exchange" landers, have one coming and one going, sharing an uncrasher stage's propulsion all at once on the same trip. No room at all for contingencies, however, for cargo/resupply this might be possible albeit quite a tour de force.

Lunar juggling?