Author Topic: Oxygen-fueled SEP stage for XEUS  (Read 3353 times)

Offline Nydoc

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Oxygen-fueled SEP stage for XEUS
« on: 12/24/2015 12:15 PM »
The Problem

The plan for XEUS is to make it the workhorse of a lunar-sourced propellant depot. The delta-v budget is very critical for this scheme to work. XEUS would burn hydrolox propellant to take off from the lunar surface with its payload. When it arrives at the orbital fuel depot, XEUS would transfer some of its payload (propellant) to the fuel depot. Enough propellant needs to be left in XEUS for an empty run back to the lunar surface. However, the propellant required for the trips to and from the lunar surface will cut into the payload delivered with every trip.

The Realization

If you are separating water by electrolysis then you will have an O/F ratio of 8 by mass. Hydrolox engines generally run fuel rich with a typical O/F of about 4.8 in a vacuum. This means that for every metric ton of usable hydrolox propellant produced you would have about 0.55 metric tons of excess oxygen.

What do you do with the excess oxygen?

My thought is you could have a SEP stage for XEUS using oxygen-fueled Hall-effect thrusters Lorentz Force thrusters (see Jon's comment below). The stage would be parked in a frozen low lunar orbit while XEUS is on the surface. This stage would use the common payload adapter to mate with XEUS. It would complement XEUS with high ISP and would be sized specifically to provide the transfer for XEUS between low lunar and high lunar. The idea is to maximize the delta-v and allow XEUS to deliver more payload to high lunar. A trades study would be needed to determine if there is an overall benefit to this scheme.
« Last Edit: 12/24/2015 11:09 PM by Nydoc »

Offline jongoff

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #1 on: 12/24/2015 06:31 PM »
The Problem

The plan for XEUS is to make it the workhorse of a lunar-sourced propellant depot. The delta-v budget is very critical for this scheme to work. XEUS would burn hydrolox propellant to take off from the lunar surface with its payload. When it arrives at the orbital fuel depot, XEUS would transfer some of its payload (propellant) to the fuel depot. Enough propellant needs to be left in XEUS for an empty run back to the lunar surface. However, the propellant required for the trips to and from the lunar surface will cut into the payload delivered with every trip.

The Realization

If you are separating water by electrolysis then you will have an O/F ratio of 8 by mass. Hydrolox engines generally run fuel rich with a typical O/F of about 4.8 in a vacuum. This means that for every metric ton of usable hydrolox propellant produced you would have about 0.55 metric tons of excess oxygen.

What do you do with the excess oxygen?

My thought is you could have a SEP stage for XEUS using oxygen-fueled Hall-effect thrusters. The stage would be parked in a frozen low lunar orbit while XEUS is on the surface. This stage would use the common payload adapter to mate with XEUS. It would complement XEUS with high ISP and would be sized specifically to provide the transfer for XEUS between low lunar and high lunar. The idea is to maximize the delta-v and allow XEUS to deliver more payload to high lunar. A trades study would be needed to determine if there is an overall benefit to this scheme.

Interesting idea. I had noticed before the Oxygen/Hydrogen imbalance from water derived ISRU propellants. A few thoughts though:

1- I think RL-10 runs leander than 4.8:1 these days. I can't remember the exact number but I thought they were trying to push into the 5.5:1 to 6:1 range. That reduces the amount of LOX excess you have to deal with.
2- With thrust augmented nozzles, you could theoretically run the nozzle section LOX-rich, making the overall mixture ratio closer to stoichiometric. The overall Isp goes down a bit, but your T/W ratio gets better (this is similar to the LANTR concept from about a decade back that added LOX to afterburn with the hot GH2 from a NTR to give it a higher T/W ratio).
3- If you are going to use the excess oxygen for a SEP system, a Hall Thruster is probably a poor choice--you'd likely get all sorts of erosion issues from the highly reactive oxygen plasma. You might be better off using something like what MSNW is working on. Those Lorentz Force thrusters transfer energy to the propellant electrodelessly using inductive forces, instead of hall thrusters, which I think are more resistive in nature. MSNW has already run their thruster on O2, CO2, and several other typically reactive species, without noticeable wear (though I don't know if they've done a lifetime test yet). MSNW also has been working on a neutral entrainment technology that should allow them to get the overall efficiency of an oxygen-based system up closer to what you could get with a Xenon Hall Effect Thruster.

~Jon

Offline JamesG123

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #2 on: 12/24/2015 07:49 PM »
Or you could just use a mass driver to fling blocks of ice into LLO and use the excess O2 from your in-orbit, round the clock solar, electrolysis plant  in the OMS of the "catchers".

There will be plenty of work for XEUS (what is the plural form of Xeus?) with cargos that can't tolerate high G accelerations.
« Last Edit: 12/24/2015 07:57 PM by JamesG123 »

Offline nadreck

Re: Oxygen-fueled SEP stage for XEUS
« Reply #3 on: 12/24/2015 08:34 PM »
Or you could just use a mass driver to fling blocks of ice into LLO and use the excess O2 from your in-orbit, round the clock solar, electrolysis plant  in the OMS of the "catchers".

There will be plenty of work for XEUS (what is the plural form of Xeus?) with cargos that can't tolerate high G accelerations.

minor nit to pic - you can't throw anything into LLO from a mass driver, you have to go to an EM-L point since any course with less energy ends up intersecting the surface in a single orbit.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Nydoc

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #4 on: 12/24/2015 08:45 PM »
Interesting idea. I had noticed before the Oxygen/Hydrogen imbalance from water derived ISRU propellants. A few thoughts though:
[...]

Those are some valid points Jon. I'll check out the Lorentz Force thrusters and crunch some numbers. I notice that even with O/F at 6:1 you are still getting 0.29 tons of oxygen per ton of hydrolox propellant. This should be plenty to work with. Another thought is you might have 2 XEUSes (XEUSi ?) per SEP stage. Any idea what the per-trip propellant savings might be from using a SEP stage?
« Last Edit: 12/24/2015 08:50 PM by Nydoc »

Offline JamesG123

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #5 on: 12/24/2015 11:26 PM »

minor nit to pic - you can't throw anything into LLO from a mass driver, you have to go to an EM-L point since any course with less energy ends up intersecting the surface in a single orbit.

Well,  yes I imagine the best place to put your storage depot and plant will be EM-L, so that is where they ultimately will wind up.

Offline Nilof

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #6 on: 12/25/2015 12:39 PM »
Well, very high lunar orbits may work as well, you just need to leave the domain where orbits are Keplerian. The EML points have the advantage of being stationary relative to the lunar surface though, which eliminates launch window issues if you have a catcher there.

One thing worth mentioning is that lunar ice is not just water ice. The LCROSS data told us that the three main volatiles were (in order): Carbon monoxide, water, and hydrogen sulfide. So ice mining would give a much larger surplus of oxygen and carbon than you'd get from electrolysis, not including additional oxygen surpluses that may be obtained from refining oxides into metals.

Lunar poles ISRU may favor Methane over Hydrogen just like Martian ISRU, since that makes use of the available carbon and makes better use of the available oxygen. Hydrolox is better for the lunar equator though, where only LOX ISRU is available.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline georgesowers

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #7 on: 12/30/2015 02:11 PM »
We have thought about the oxygen excess issue (I hesitate to call it a problem).  Not a lot but a little.

Jon is right.  Today's LOX/LH2 engines can run at a mixture ratio of 6 or a little higher with little degradation of Isp. 

My initial thought was excess oxygen might be useful somewhere else in the logistics stream, ECLS for example if there are humans about.

Other potential uses, as you suggest, could be as a monopropellant in some kind of electric propulsion, like the Lorentz force thrusters MSNW is working on.

Or it is simply waste.

Our initial idea for propellant logistics is XEUS lifts off from the surface of the moon with 70mT of propellant in it's own tanks as well as 70mT of propellant as payload (in a tanker designed for low boiloff).  It that configuration XEUS can deliver the entire payload to L1 or L2 and return with the empty tanker to the surface of the moon.  If you are mining water on the moon, you will need the infrastructure there to convert it to LOX and LH2 to fuel XEUS.  It's not clear if it is more efficient to transport water to L1/L2 and convert it to propellants there or transport propellants.  There are pros and cons each way.

Offline gin455res

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #8 on: 12/30/2015 04:37 PM »
I read it was possible to gel hydrogen with methane.

would it be possible, to run stoichiometric, but to gel the hydrogen with water till an acceptable mixture ratio is achieved?

Offline Nydoc

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #9 on: 01/04/2016 02:07 AM »
We have thought about the oxygen excess issue (I hesitate to call it a problem).  Not a lot but a little.
I don't consider excess oxygen to be a problem either. My concern was more with maximizing the propellant delivered with each run or maximizing the percentage of harvested materials which is delivered to EML1. Also, another use for an ELF thruster might be efficient station keeping of the propellant depot.

Our initial idea for propellant logistics is XEUS lifts off from the surface of the moon with 70mT of propellant in it's own tanks as well as 70mT of propellant as payload (in a tanker designed for low boiloff).  It that configuration XEUS can deliver the entire payload to L1 or L2 and return with the empty tanker to the surface of the moon.  If you are mining water on the moon, you will need the infrastructure there to convert it to LOX and LH2 to fuel XEUS.  It's not clear if it is more efficient to transport water to L1/L2 and convert it to propellants there or transport propellants.  There are pros and cons each way.
If you can transfer 50% of your loaded material to L1/L2 then this is a lot better percentage than I was expecting! I ran some numbers. These are just guesses so they are probably way off:

Assumptions:
RL10C-X Isp = 448.5s
XEUS + Tanker Dry Mass = 7 tons
Delivered Fuel = 70 tons

Delta V Req.Starting MassEnding MassDelta-V Achieved
Surface to EML12.5214982.82.52
Deliver Payload-82.812.8-
EML1 to Surface2.5212.872.65

I ran the numbers again with a SEP stage. Again these are very rough guestimates.

Assumptions:
RL10C-X Isp = 448.5s
Oxygen-Driven ELF Thruster Isp = 3000s
XEUS + Tanker Dry Mass = 7 tons
SEP Stage Mass = 1 ton
Delivered Fuel = 100 tons

Delta V Req.Starting MassEnding MassDelta-V Achieved
Surface to LLO1.9177114.91.9
Dock SEP-114.9115.9-
LLO to EML10.65115.9113.360.65
Deliver Payload-113.3613.36-
EML1 to LLO0.6513.3613.060.67
Undock SEP-13.0612.06-
LLO to Surface1.912.0672.39

This shows material delivered per run might be increased by about 43%. However if we look at the percentage of all material loaded onto XEUS which gets delivered, the increase is only from about 50% to about 59% with the SEP stage. The usefulness of the SEP stage is debatable. Presumably the business case for a SEP stage would also depend on the intended scale of the operation.
« Last Edit: 01/04/2016 02:59 AM by Nydoc »

Offline RocketmanUS

Re: Oxygen-fueled SEP stage for XEUS
« Reply #10 on: 05/11/2017 07:02 PM »
What about having Zeus deliver cargo to SEP in LLO from Lunar surface and then Zeus return to surface.
Have SEP take cargo from Zeus and deliver cargo to EML1/2 or GSO, SEP return to LLO.
No need to take mass of Zeus to EML1/2 or GSO ( refuel satellites with noble gas from Lunar ).

( SEP or other type that could use any excess material such as oxygen left over from water electrolysis. )
SEP takes longer travel time, however for this concept the cargo could be on constant delivery ( multiple tankers )and unlike people can handle the longer travel time.
Mars and beyond, human exploration
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Online Steven Pietrobon

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #11 on: 05/12/2017 05:36 AM »
What about using Magnesium or Aluminium as your propellant, which is abundant on the Moon? A company here in Adelaide called Neumann Space is developing an ion thruster using a wire-triggered pulsed cathodic arc system. Uses only 160 V, is solid fueled and gets Isp's up to 110 km/s!

http://neumannspace.com/science/

« Last Edit: 05/12/2017 05:55 AM by Steven Pietrobon »
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline spacenut

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #12 on: 05/12/2017 06:18 PM »
Lunar sand is 40% oxygen, and the silicone can be used for 3D printed silicone products manufacturing on the surface.  Or, as suggested, pile driver could deliver lunar sand to orbit, to get excess oxygen. 

The excess oxygen can be used to refuel a metholox Mars transporter at say L2 with a lunar lox station at L2. 

Getting the lunar gateway stations in orbit around the moon, one can use lunar resources instead of Earth launched resources.  Solar power can operate a pile driver, lox SEP tugs could move resources from low lunar orbit to an L1 or L2 station for lunar flyby refueling needs, or manufacturing needs. 

Online Steven Pietrobon

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Re: Oxygen-fueled SEP stage for XEUS
« Reply #13 on: 05/13/2017 07:26 AM »
I've started a separate thread on the Neumann Drive in the Advanced Concepts section.

http://forum.nasaspaceflight.com/index.php?topic=42920.0
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

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