Sometimes the ideas I throw out are obscure and hard to communicate, and sometimes they are so blindingly obvious that they have been rehashed many times in the decades past. Since I have no feel for which is which, sometimes I throw an idea to the wolves (that’s you) to see which it might be, and also to see if some of the follow ons are equally obvious, or not as it may be.

In my last post on the small tetherocket, the idea was somewhere in the middle. This is one of the possible follow ons that I have thought about before, but only decided to write out as my reaction to some of the feedback in comments.

turbinozzleOnce again my cartoon may be a bit difficult to read. The black lines at 3,000 m/s are expansion nozzles spinning on a tether complex as in the last post. The small rocket inside the curves is fixed relative to the ship with a suggested exhaust velocity of 4,000 m/s retrograde to the ship. The 4,000 m/s exhaust encounters the spinning nozzles with a closure rate of 7,000 m/s. The exhaust bends nearly 180 degrees in the moving nozzle with the gasses retaining the 7,000 m/s velocity relative to the turbinozzle. The gasses exit the turbinozzle at 7,000 m/s nozzle relative which is 10,000 m/s ship relative.  Net Isp just over 1,000.

I suggested 4,000 m/s exhaust velocity for the H2/O2 rocket as it would likely have a low expansion ratio to fit the envelope. I suggest that the near 180 degree turn in the turbinozzle channels would cause shock losses that would cancel any expansion gains from the nozzles.  I believe that the thrust/weight ratio would remain in the 1 m/s range for the bare engine.

This would retain the capability of using any fluid reaction mass available in the solar system from CO2  to water to impure methane if that is available ISRU. The rocket engine would need changing out to a steam engine using whichever fluid is available for a likely Isp in the 500+ range if I see the reactions correctly.

One of the comments suggested beamed energy and hydrogen only as a superior alternative. Nothing any of us said would preclude using beamed energy to drive the reactions. It would solve a number of problems with onboard power if available. Hydrogen may or may not be the reaction mass of choice. The tankage mass and handling properties may well make it second best even if it happens to be available at a particular location.

On one end of the conceptual capabilities is the possibility that I am pessimistic in the capabilities suggested. A larger envelope for the fixed expansion nozzle may make it  possible to get 4,500 m/s exhaust velocity from the fixed rocket which would add 1,000 m/s to the final exhaust for a total of 11,000 m/s exhaust velocity. It may also be possible to recover the energy from the turbinozzle heating to a higher velocity exhaust which could possibly add another 1,000 m/s to the final gas velocity then totaling about 12,000 m/s.  Hopefully the speculative possibility of Isp=1,200 will have a qualified person of two running a few simulations for the entertainment value.

Posted in Uncategorized | 5 Comments

Small Tetherocket

Some years ago I posted an idea for combining tethers, rockets, and nuclear power to build an engine with very high Isp for orbital transfer operations. 750-850 seconds Isp with LH2/LO2 seemed and still seems possible. I am posting a follow on to that idea with some modifications suggested by commenters and some of my own.


The original post is at     http://selenianboondocks.com/2008/10/tetherocket/


In this cartoon, the spiderweb structure is a connected tether complex inside a pressure disk. It is powered up to 3,000 m/s by the onboard nuclear or solar power with the spin maintained by continuous power input.  The little angles on the tether tips are the throatless diverging combustion chambers. The LH2 and LO2  are sprayed into the path of the combustion chambers and both mixed and ignited by the 3,000 m/s impact. The pulse detonation burn will exit the combustion chambers at close to the 4,500 m/s characteristic of an LH2/LO2 engine in vacuum relative to the moving combustion chamber.

The small combustion chamber  expansion ratio is partially compensated by the impact heating and pulse detonation in the initial chamber. The near 4,500 m/s exhaust velocity is relative to the moving combustion chamber though and has a vehicle relative velocity of nearly 7,500 m/s. The exhaust then enters the single fixed high expansion ratio nozzle to finish expanding to as near vacuum as possible. The net exhaust velocity should be in the 8,000-8,500 m/s range.

I still see the T/W as being about 0.1 or 1 m/s acceleration with the bare engine and probably about a tenth of that including  vehicle mass. 100 cm/s would go from LEO to Lunar Transfer Orbit in about half a day using about 1/3 of IMLEO to do it.

Higher T/W than ion engines. No radioactive exhaust as in nuclear thermal and higher propellant densities to boot at similar net Isp. Switch to methane/Lox or any other bi-propellant  by changing two low pressure injector elements. Able to use inert gasses from ISRU anywhere in the solar system for Isp in the 450-500 range using the mechanical drive alone.

Posted in Uncategorized | 31 Comments

Precursor to Station

The comments on the torus station post made it clear that I had skipped steps in suggesting a specific station type. One step is the investigation of exactly how many RPMs are acceptable for a working orbital facility. The acceptable RPMs dictate the length of the station arms to achieve a given gee level. The required arm length or torus diameter needs to be known before design starts, and long before construction starts.

I suggest that an initial investigation on the ground could be a 10 meter radius unit with a 3 meter inclined floor around the circumference. There would be a bit over 190 square meters of available floor area when the unit is spun up. The floor area would be divided into offices, bathrooms, kitchens, bedrooms, and other requirements as needed.

The unit would be spun up to the design RPM for that particular design. There would be one acceptable RPM for any fixed floor angle with several different units for different investigations. This would be terrestrial construction and relatively cheap compared to anything launched which would make multiple units on the ground affordable compared to launching sub-optimal stations.

The unit is spun up to design RPM with the intention that it will spin continually for several months at a time. Several shifts of investigators work in the facility on short and long term ‘missions’. Some works 8 hours plus lunches and go home at night exiting through the hub without stopping the unit. Entry, exit, and transition through the spokes would be part of the experiments. Others stay for 30 days straight, while yet others do business visits of minutes to hours.

The purpose would be to determine whether 10 or more RPM can be adapted to in a working environment. I would see the experimenters as being  perhaps one department of a company or government facility totaling between 20 and 100 people on a near continuous basis plus many visitors. The quality of an individuals work compared to their normal performance in regular environments would be a good baseline to prove/disprove the possibility of very short radius stations for people in high stress and workload environments as during a space mission.

The long term investigators would exit fairly often for family functions and such which would be the  spin equivalent of going to the microgravity sections of the station or an EVA. The ability to conduct the investigation without missing the kids recital or your wedding anniversary would make it possible to get long term volunteers.

After determining the acceptable RPMs for people in a real working environment it wold be much easier to design a real working station. A 5 meter spin radius in orbit is a totally different animal than a 1000 meter radius. If the ability to adapt to high RPMs eliminates 99% of the population, then there would still be about 3 million people in this country to select from plus several times that number world wide.

Posted in Uncategorized | 17 Comments

YHABFT: MIT MarsOne Analysis — Alternative Solutions to the Excess O2 Problem

A few months ago, a group at MIT did an analysis of the MarsOne mission that was fairly critical of the concept, on technical grounds. This week’s FISO telecon featured an update on the analysis they performed, particularly on the issue of the excess oxygen problem, and I wanted to make a few comments on what I read that were a little too long for twitter.

First, before I get to some alternative solutions to the excess oxygen problem, I did have one thing I noticed that surprised me–the caloric requirements. On page 19, they talk about a 3040 calorie per day diet per person. That seemed a bit on the high end. The only people I know that consume that many calories without getting fat are people with really active lifestyles (mountain climbers, marathon runners, etc). I wonder if those numbers came from ISS experience, where the effects of microgravity force them into a very strenuous exercise routine in the hopes of not having too bad of bone/muscle loss. This is one of those areas where knowing how much hypogravity we need would be really important. If you didn’t need anywhere near ISS-like exercise requirements to maintain health in a Mars-gravity environment, that would likely cut down significantly on the required calories, and might shift the ratio of carbs/proteins/fats from what was assumed on this page.

Now, moving on to the “excess oxygen problem”. Basically, using plant-based life support, and using the biomass numbers MarsOne estimated would be required to supply the required amounts of carbs/proteins/fats, they found that the plants produce too much oxygen. This leads to venting atmosphere overboard, and trying to make up with other constituents, leading to hypoxia. Their suggested solution was to isolate the plants, and come up with some sort of oxygen scrubber for storing the oxygen elsewhere for later use on EVAs and such.

But I think they may be overthinking this a bit. Here’s a few suggestions of alternative ways of solving the problem:

  1. Small animals (pets or food): The assumption in this analysis is that you only have humans and plants. Plants consume CO2 and produce O2, and humans produce CO2 and consume O2, and some fraction of the plants. What if you brought small animals along? Something that could eat parts of the plants inedible to humans. Could you increase the effective O2 consumption enough that way to counteract the rising O2 levels? If you picked something small that was edible (chickens? Cornish game hens? fish? etc.) it might allow you to replace some of the vegetable biomass dedicated to protein and fat production. I don’t know if this would completely solve the problem, but whether you eat the animals, or keep them as pets, it seems like you might have at least part of a solution there.
  2. Just Burn It: When you have an excess of O2 and a deficit of CO2 it seems like a combustion process might be in order. It would be relatively easy to take some Martian air, split it into CO and O, vent or store the O, and then run the process in reverse to combine CO with excess oxygen inside the habitat. This could be done to provide extra power at night using a solid oxide fuel cell. If this produces too much CO2, that’s easier to scrub using existing technology than O2 is. If you don’t feel safe handling CO in the habitat, turn it into CH4 using a Sabatier reactor, and burn that and recover the excess water from the combustion to put back into the Sabatier reactor.
  3. Mixed Food Sources: It might also be possible to pick some mix of food sources (some of it dehydrated pre-packaged food from earth, some locally grown) so that you optimize what you’re growing locally. For instance, if it turns out that your carbs are taking up the most area and generating the most surplus O2, maybe you can have more of those come in dehydrated ingredients from Earth for a while.

Ultimately, I don’t want to look like I’m ripping on the MIT team. They’ve done a very thorough analysis, and it’s almost always easier to point at potential flaws in an existing analysis than to create one from scratch. I just wanted to suggest some potential solutions. I particularly like #1. The whole idea of space colonist having to go 100% vegetarian always struck me as somewhat nutty. There definitely should be additional research to see if you can strike a balance with primarily biologically-closed life support in this way, but it seems like an obvious angle for further research/development

Posted in ISRU, Space Settlement | 15 Comments

Random Thought: Simulating Mars Speed-of-Light Delays on ISS

[Note: I’m almost positive this isn’t a new idea, but I figured I would share it, just in case.]

One of the many concerns with deep-space human spaceflight missions, is that all of NASA’s experience has been with missions close enough to Earth that mission control could be in direct and nearly instantaneous communication with astronauts. Worst case round-trip speed-of-light delays for the Apollo missions were on the order of 2.5-3s or less. But for missions even to NEOs, let alone Mars, you quickly end up with round-trip speed-of-light delays on the order of tens of minutes, to over 3/4 of an hour worst-case at Mars. It is a legitimate concern that NASA’s practical experience in mission ops has been so used to mission control being able to provide almost instantaneous feedback when problems crop up.

One very cheap way to fix this might be to intentionally start simulating speed-of-light delays between ISS astronauts and mission control (and visa versa). There are very few problems on ISS that really need real-time mission control help, and getting used to dealing even with problems with a built-in time lag would be useful experience for deeper space mission. If NASA can’t stomach the risk of doing this with a big space station like ISS, I can’t imagine they’ll ever have the intestinal fortitude to really do a crewed deep-space mission.

If desired, NASA could theoretically start with only a small delay, and then gradually build to longer times. For instance, once commercial crew starts flying, maybe they intentionally swap the whole USOS team out at once (instead of staggering the replacements like they do right now). Then start initially with no lag, but adding lag little by little, at about the rate that it would build up for a mission to an asteroid or Mars. You might make some exceptions for communication between astronauts performing research on the ISS and the ground subject matter experts they’re working with (since ISS isn’t just about preparing for deep space, but also about performing useful research), but make sure that they’re not allowed to talk with other NASA folks except via the speed-of-light delay simulated communications link. Also keep communication with friends and family on the ground routed through that delayed link as well.

My guess is that if they started performing this experiment on a regular basis, they’d likely learn all sorts of new things. Some things they’ve done in the past may translate fairly well to a delayed environment, while large parts of how they do things now may have to change. But really, if NASA wants to prepare for deep-space missions, why not start now when the risks are low? Not only would the mission control people learn better how to interact with crews at long distances, but those crews will also be gaining experience with interacting with mission control at long distances. My guess is even if a commercial deep space mission isn’t done exactly as NASA would (fairly likely), that there’ll still be all sorts of useful lessons learned that could be passed on.

And really this experiment wouldn’t cost hardly to add to the ISS manifest.

Does anyone know if NASA has thought of doing something like this? Any suggestions on how to run the experiment better?

Posted in Lunar Exploration and Development, NASA, NEOs, Space Exploration | 15 Comments

Launching a Torus Station

A torus space station is one of the staples of future thinking. Launching it in one piece from the ground is not. It has however, been occasionally mentioned in various comments. Here are a few numbers on the idea a few of us have kicked around from time to time.

Assume a 100 meter torus with a 10 meter minor diameter. 23,000 net cubic meters of internal volume should do for a start. A 50 meter radius that is not enough for an Earth gravity at under 4 rpm is sufficient for lower levels of artificial gravity that may mitigate some of microgravities’ harmful effects. It is getting it up there that seems to be a bit of a problem.

10,000 square meters of station surface would mass 540 tons if we assume the skin has the equivalent mass of 2 centimeters of aluminum including insulation, braces and such. Internal structure and furnishings would have to go up in subsequent flights. 540 tons in orbit would require something over 10,000 tons GLOW. This is heavy launcher country. Fortunately the internal volume of the torus is sufficient for fuel tanks.

Bladder fuel tanks inside the station could mass under half a percent of fuel mass, while LOX could be carried with simple bulkheads in the appropriate sections. Sooner or later an F1 equivalent will become operational at reasonable prices. 15 of them could push the torus edge on with a sled take off to avoid building launch towers and such. This would be a near SSTO with just engines dropped off for recovery at designated velocities when they are no longer required. Final push to LEO could be with just one engine with all the others recovered from various suborbital velocities.

How the station would be equipped and used would be up to the parties that paid for it.

Posted in Uncategorized | 30 Comments

PhD Dissertation Idea: NEO Trade Network Analysis

Frequently I wish I could afford to have a personal orbital dynamics minion that I could have run analyses for me whenever I have a complex orbital dynamics question and would like to test my intuition. Unfortunately, Altius isn’t successful enough yet for me to afford that luxury, and while we have friends and advisors with the relative skills, I don’t have enough money to have them run every analysis I’m interested in. So I figured maybe I could toss this out as a potentially useful topic for someone’s PhD dissertation instead.

Short version: I’d like to do an analysis to see how hard it is (in delta-V, travel time, and launch window frequency) to get from one NEO to another, versus going from a NEO to a planet and back.

I’ve got a lot of friends (bless their hearts) who like to turn up their nose at poor benighted planetary chauvinists, and who go on and on about “once you’re out of a gravity well, why would you want to go back into one?” My intuition suggests they may be wrong, but I’d like to see numbers to either back up my opinion, or shoot it down. My intuition suggests that without atmospheres or the Oberth effect, you’re going to pay big delta-V penalties at each end of the trip, unless the synodic periods are really really long.

My thoughts on how to run the analysis:

  1. Before you start, pick some subset of the NEO population above a certain size (say 100m diameter, or maybe 1km depending on available computing resources). Gather ephemeri for all of them.
  2. Pick a random NEO in that group.
  3. Depending on computing resources pick some specific number of other asteroids to analyze (say 50 or 100 or 1000), and randomly select them from the population.
  4. Calculate pork-chop plots for travel from the initial asteroid to the other asteroid, and then back again.
  5. Automatically extract from the pork-chop plots say the minimum delta-V for both legs of the trip, and the associated trip times, stay times, and estimated revisit frequency.
  6. Run steps 4 and 5 using the Earth, Mars, and Venus as destinations, both with and without aerocapture/aerobraking at the planet end.
  7. Repeat steps 3-6 a bunch of times (say 100 or 1000 times depending on how much parallel computing capability you have, and how thorough you want to be).
  8. Analyze the data.

My hunch says that especially if you have aerocapture, you’re going to find that most of the time it’s easier for a given NEO to regularly “trade” with planets than it is with other NEOs, because you can both take advantage of the Oberth effect on departures from the planets, and you can take advantage of aerocapture on the way in. But I’d love to see someone run the numbers–I could be completely wrong.

An analysis like this would be really useful for figuring out what trade networks might look like in the future between NEOs and other solar system entities. If it turns out it’s much harder to get from one NEO to another on a regular basis, as my intuition suggests, it would suggest planets may remain the trade hubs with NEOs being more mining bases. If my hunch is wrong, maybe I owe my NEO-chauvinist friends an apology. :-)

Bonus points if you can run the analysis using both high-thrust impulsive maneuvers as well as low-thrust, high-Isp maneuvers (like Solar Electric Propulsion systems can provide), to see if SEPs noticeably change the equation–I really have no intuition on how realistic SEPs change or don’t change the equation for NEO transportation.

Posted in NEOs, PhD Dissertation Ideas | 11 Comments

Top 10 Reasons Why Something ARM-like Is Worth Doing

[Up-front Disclaimer: My space startup is being paid under the Asteroid Redirect Mission BAA to do a study contract on one possible way to do the Option B mission. Even though we’re not dependent on follow-on work, I figured it was worth stating up front my potential biases.]

I’m not sure I’ve ever seen a major NASA program as nearly-universally disliked as the Asteroid Redirect Mission. Some people hate it for ad hominem reasons like the fact that the Obama Administration has been pushing it, or the supposition that Darth Garver (as they see her) came up with the idea. I’ve also heard a few anti-SLS/Orion people refer to it as a “pathetic attempt to reengineer the Solar System to make it handicapped-accessible for SLS and Orion”, or to come up with something for SLS and Orion to do that is more inspiring for them than endless Apollo-8 rehashes (but without the subsequent Apollo missions to follow). Ironically, I think some of the pro-SLS/Orion fanboys on the internet who hate it are afraid that ARM doesn’t really need SLS or Orion (which is true to some extent–in a NASA where Human Spaceflight was done more with PI-led, competitively selected, not-overly-politically-driven missions, I bet few PI’s would be suggesting SLS or Orion for this mission). Some of the Small Bodies scientists seem to hate it because they see it coming from the human spaceflight side, and think the whole thing could be done better without humans involved, and it wasn’t invented there anyway. All told, lots of people find lots of reasons to hate this mission.

But I wanted to provide 10 reasons why a mission like ARM might be actually be worthwhile:

  1. Adding a new, even more accessible moon to the Earth-Moon System: A lot of people fixate on the fact that we’re going to spend all of this money for a couple of astronauts to go out to a rock in lunar orbit, climb over it for a few days, and bring some samples back. What they conveniently ignore is that >99.5% of the material brought back will still be there, orbiting the moon for the next several hundred to several thousand years, in a fashion that is easily revisitable for a long time (docking adapter pre-attached, and at least for a while still attitude stabilized). And this new moon would be about as hard to get to as L1/L2. Which means that yes, future missions to it using a lightly modified CC vehicle are totally possible.
  2. Providing an ideal testbed for Asteroid ISRU development: Many people, including many of my friends, see the asteroids as the premier source of vast quantities of off-world resources. But while there are no shortage of low-TRL concepts for how to extract resources from asteroids, actually testing those out isn’t going to be easy. I think testing will be much easier when you have the ability to send people and robots, when you’re close enough that teleoperation of robotics is an option, and when you have frequent repeat visit opportunities where you can try new approaches, and where you can do your testing in a microgravity or near microgravity environment, like you would have at an asteroid.  I’m sure prospective asteroid miners like DSI or Planetary Resources wouldn’t complain about having one or more easy-to-access testbeds to work with.
  3. Providing a much larger sample quantity to work with than other existing or proposed missions: While scientists may be happy spending $800M to return 60g of material from an asteroid (OSIRIS-REx), and can likely tease out all sorts of information from that two Tablespoons worth of material, ISRU development needs a lot more material to work with. Even the smallest of Option B concepts I’ve seen brings back tens of tonnes of material, both rocky and regolith, which should be plenty to work with for ISRU development.
  4. Providing a good way of testing out a man-tended deep space habitat: As was reported by Jeff Foust at SpaceNews, one of the ideas NASA is looking at incorporating into ARM is attaching a prototype deep space habitat (possibly commercially derived if the NextSTEP BAA leads somewhere useful). This would allow visits of up to 60 day duration by crews of up to 4. While there are other ways you could test something like this (such as L1/L2 gateways), testing it in an operational environment would be useful. As would demonstrating the ability to do long-term habitation in close proximity to an asteroid.
  5. Demonstrating large-scale Solar Electric Propulsion (SEP) systems: This is one of NASA’s main interests in the ARM mission–in the land of expensive launch vehicles, very high Isp propulsion like you can get with SEPs can make many missions a lot more affordable. Even with low-cost earth-to-orbit transportation, SEPs probably make sense for a wide range of missions. Demonstrating the ability to use large-scale SEPs for tugging huge objects in heliocentric space, and performing precision injection maneuvers, etc. might be very useful. We already have a fair deal of experience with small SEP systems, but doing these sort of missions with 100kW+ class SEP systems can be pretty useful.
  6. Demonstrating Planetary Defense Techniques: If something like “Option B” (the grab a boulder option) is selected, NASA is interested in demonstrating the Enhanced Gravity Tractor method for deflecting the parent asteroid (see slides 27-29 of this presentation on Option B). Learning how to deflect potentially hazardous asteroids is probably one of the more worthwhile things NASA could be spending money on right now, and providing a way of getting real hands-on experience applying those techniques would be very useful. We have lots of theory on how this would work, but getting experience with a real, lumpy, non-idealized asteroid of significant (>100m) size would be really useful. And contra some of their critics, using a “Rube Goldberg arcade claw” to pick up a boulder and increase your spacecraft mass by 5-10x is a great way of allowing you to get measurable results in a reasonable amount of time.
  7. Developing Technologies for a Phobos/Deimos Large Sample Return: One of the keys to affordable exploration and settlement of Mars will be determining if Phobos and/or Deimos have water in them, and if so, figuring out how to extract it efficiently. Having a large source of propellant feedstocks available in Mars orbit (for supersonic retropropulsion on landing, hydrogen feedstock for surface ISRU, and earth-return propellant) could significantly reduce the amount of propellant needed for both round-trip and one-way Mars missions. If Option B is selected, and if it designed properly, it would be possible to use the same hardware (with slightly modified CONOPS) to capture and return a decent sized (>1 tonne) sample to lunar DRO for evaluation and hopefully ISRU process development/debugging. A manned Phobos and/or Deimos mission is something I strongly support in the future, but if they had enough info that they could be setting up a propellant extraction facility while they’re there (that we’ve already pilot-tested in cislunar space so we know it has a high probability of working), that would just be awesome.
  8. Providing the Beginnings of a Lunar Gateway?: It turns out that getting to and from Lunar DRO, and getting to/from the lunar surface from a Lunar DRO aren’t massively different from getting to/from Earth-Moon L1 or L2. The orbital dynamics is a bit more complex, but the propellant and travel times are relatively similar. And some lunar DROs can be long-term (centuries or millennia) stable without active stationkeeping. While if we were ready for going straight to the Moon (I’m actually a bit of a Moon-Firster believe-it-or-not), L1 or L2 might be slightly preferable to a lunar DRO as a location for a lunar gateway, if we did something like ARM, with the habitat module, you’d already have a de-facto start to a lunar gateway. One that will likely be setup (by NASA or follow-on efforts) with ISRU hardware, which would likely include at least rudimentary LOX/LH2 and/or LOX/Methane storage and handling capabilities (after all, if they’re going for a carbonaceous chondrite sample, extracting water will be a key part of what they’d be trying to prove). While this wouldn’t likely provide anywhere near enough fuel storage for a Constellation-class mission, it might provide enough propellant to refuel a “Golden Spike” class lander. And even if the asteroid itself only yields a mission or two or three worth of propellant, the tanks and handling equipment would be there and it could make shift as a miniature depot for earth-launched and eventually lunar-derived propellants. Lots of details have to be done right to make this feasible, but it’s possible that ARM could be done in a way that make future lunar missions easier.
  9. Providing More Experience with On-Asteroid Operations: If the Rosetta/Philae mission should tell us anything, it’s that there’s still a ton to learn, from an engineering standpoint, about how to operate successfully on the surface of large, low-gravity objects like asteroids or comets. While we’ll continue to get some small-scale experience using other robotic missions, and while a manned mission to a free-range asteroid will also provide a good way to get more data, ARM will likely extend our knowledge about how to do operations like these safely with large objects, and would likely provide good data increasing the likelihood of success of future manned missions to free-range asteroids.
  10. Leaving Something Permanent: One of the saddest things about the Apollo missions is that they didn’t leave anything permanent that made future missions any easier. When they were canceled all that was left was museum pieces, pictures, and a few hundred kg of rocks. But the nice thing about ARM is that once the asteroid sample has returned to lunar DRO, it’s there. It doesn’t require continued expenditures from NASA for it to stay there. Until we’ve mined every last kg of it, it’s going to be there orbiting the moon, close enough that almost any spacefaring country or business in the world can reach if it wants to. It doesn’t need an ongoing standing army that can be defunded. It doesn’t need a mission control to watch over it 24×7. It doesn’t need some sustaining engineering contract that’s going to suck up significant portions of NASA’s limited human spaceflight budget on an ongoing basis. It’s just there. Ok, if there’s a hab there or a more sophisticated node, it could require ongoing mission support when being used. But if for some reason they decided to stop visiting that node for a while, it would still be there, waiting to be restarted whenever someone cares again, or ready to be handed off to private companies or international partners once NASA is done with it. At least for a few centuries. Having something that accessible and that permanent out there is worth something, at least to me.
Posted in Commercial Crew, International Space Collaboration, ISRU, Lunar Exploration and Development, NASA, NEOs, Space Development, Space Exploration | 19 Comments

Wild Data-Free Speculation on the SpaceX Landing Attempt

First, before saying anything else–congrats SpaceX! Both on the successful launch, and on coming so close to a successful recovery as well! If SpaceX’s competitors aren’t feeling the heat yet, I don’t know when they will.

As most of you have probably seen, Elon tweeted that while the rocket made it back to the drone ship, it hit at too high of a speed for successful recovery. But as of the time I started this blog post, they haven’t provided any more details.

So, in the grandest tradition of the intertubes, I would like to wildly speculate with almost no data on why they hit too fast. Being a former VTVLer, I have a few theories:

  1. GN&C Failure Modes: This category of failure modes relate to either the rocket not knowing precisely where it was/how fast it was going, or making the wrong decision on how to bring it in for a successful landing.
    1. Navigation Error: In this failure mode, the rocket either thought it was in the wrong place or going at the wrong speed. Ie it thought it was higher up and still had time to decelerate, or thought it was going slower than it really was when it hit. I think these are relatively unlikely–in order to stick a landing with a minimum throttle setting near 2G’s, you need to know very precisely where you are and how fast you were going. SpaceX knew this, so they probably put tons of resources into making sure this was done right. I wouldn’t be surprised if they had some sort of differential GPS “ground station” on the drone ship, combined with accelerometers to back out a very good estimate of GPS errors that could be sent to the rocket. Unless they made some implementation error (I doubt it based on the fact that they got the rocket all the way back to the drone ship), the rocket probably knew where it was to within a couple of cm, and probably knew how fast it was going to within a few cm/s.
    2. Guidance or Control Error: In this failure mode, the rocket knew where it was and how fast it was going, but made some poor decision about how to command the engines in order to bring it to a stop on the pad. For instance, not turning the engine on when it should’ve, or going to gentle on the throttle at first. Once again, I think this one is pretty unlikely, especially with the experience they have with regular F9 flights, previous ocean landings, and F9R dev1 flights.
  2. Engine Issues: This category says that the engine for some reason or other didn’t produce the thrust desired at the time desired.
    1. Relight Failure: In this failure mode, the engine either didn’t relight at all for the final burn, or relit too late (maybe after an unsuccessful first relight attempt). While SpaceX knew this was important, making a complex rocket engine that can relight correctly every single time, on-time, is really hard. That said, knowing this, they probably had some sort of backup plan in case the engine didn’t relight (maybe light two outer engines, and do a faster hover-slam?). This sort of failure mode is why I’ve started gravitating back toward helicopter landing in my rocket philosophizing.
    2. Engine Underperformance: In this failure mode, the engine was behaving sluggishly, underthrottled, or something else. I think this is somewhat unlikely, but possible.
    3. Engine Shut Down Inadvertently by Computer: In this failure mode, the computer saw a sensor reading it didn’t like, and shut the engine down. Knowing that this would doom the vehicle, and that the odds of false-positives is high, if I were SpaceX, I would’ve either disabled this capability entirely, or made it really hard for the vehicle to decide to shut the engine down in this situation. The engine should give its life for the vehicle, not the other way around.
    4. Engine Ate Itself: In this failure mode, the engine had a failure. Either a hard start, or a failure after ignition. Totally possible, and hopefully this is something they’ll have data to easily determine whether or not this happened.
  3. Premature Propellant Depletion: In this category (the most likely one IMO), the rocket ran out of propellant or the engines were starved of propellant before successfully nulling out all of the velocity. I.e. They ran out of gas and hit fast–seeing as how Elon didn’t give a lot of details about how fast the impact was, this is my guess.
    1. Less Landing Reserves Than Planned: In this case, for some reason the F9R first stage used more propellant either during the flight itself (due to off-nominal engine performance or something else–I didn’t get to watch the flight yet), or during the two burns prior to the final landing burn. Basically they got to the burn, but just didn’t have enough gas left.
    2. Too Big of a Divert Burn: This may be a variant on 3A, but it’s possible the grid fins got them only so close to the drone ship, and they had to do a big divert burn in order to get back to the drone ship, resulting in having insufficient fuel to finish the maneuver. This one seems the most likely to me, since getting back to the drone ship always seemed like the hardest part of this mission. The good news is if this is the case they can solve this by refining the grid-fin controls, adding more propellant margin, or some other combination of solutions (maybe an extra divert burn a little higher up where it does more good?). As I said, I think this was the most likely failure mode.

There are probably tons of other possible explanations, but those were the ones that popped out to me. Once again, I was doing this as a total fanboy, wanting to speculate about what happened, not in any way a diss on SpaceX or their team. Hopefully once SpaceX has reviewed the data, they can share the conclusions with the rest of us. It’ll be fun to see if any of my guesses were right.

In the other grand tradition of the internet, feel free to speculate in the comments as well!

Posted in SpaceX | 16 Comments

Random Thought: Dragon V2 as an xGRF Platform?

Review: How Much Gravity Do We Need, and Why Do We Care?
One of my hobby horses that I’ve blogged about a few times is the question of how much gravity do humans need to be healthy? As I’ve pointed out in the past, we know microgravity is awful for people long-term, and 1G is fine, but we really don’t know what number between 0 and 1G is the minimum that a typical person needs to avoid unacceptable health degradation.


Which Curve is Right? How Much Gravity Do We Need? Is There a Knee In the Curve? If So, Where?

Why do we care? This topic came again today in the context of a twitter conversation about Mars colonization. Obviously, if the magic gravity level people need is higher than 0.38G (Mars gravity), Mars colonization is going to be harder than if it is lower than 0.38G. If the minimum required gravity level is more than Mars levels, you’ll need to come up with some sort of countermeasure on Mars, or face potentially severe health issues over time. This may involved a mix of biochemical countermeasures (drugs), exercise, and even small centrifuges–all of which have large potential drawbacks. If it turns out that the minimum required gravity level is less than 0.38G though, life becomes a lot easier. You might want to do something about the microgravity on the flight out, but wouldn’t have to deal with countermeasures after you landed. And if you went the artificial gravity route on the way out, you wouldn’t need to provide as much of it, which would make the system size a lot smaller and more manageable–for a given max spin rate, the centrifuge radius is inversely proportional to the gravity level required–for lunar gravity you’d only need 8m radius at a 4rpm rotation rate.

But while many in the space community have pointed out the importance of answering this question, and joked about how nice it would be to have a national space program to answer questions like this, no real progress has been lately. Most of the potential solutions have been too expensive to raise the money for. What is needed is a way to start getting the data as cheaply as possible. Hopefully well less than $50M if we want a realistic chance of getting NASA or private donors to fund it.

Dragon V2 as an xGRF Platform
While the idea came to me based on some of the ISS visiting vehicle post-mission reuse ideas we’ve been looking at at Altius, when doing a little research for this blog post, I found that this idea was originally suggested by A. M. Swallow and googaw in comments to my previous post. I usually blow those two off, but they were right a lot sooner than I this time around. The idea is basically using the pressurized volume of a repurposed Dragon V2 as the habitat for a 1-person (and many mouse) version of Kirk Sorensen’s xGRF concept. While there is a lot more info in the previous blog post on xGRF, the high level version is you start with a habitat connected via a variable length tether to a counterweight. As shown below, when the tether is all the way out, the system will settle into a gravity gradient orientation, completing one rotation per orbit. By winching in the cable in the right manner, conservation of angular momentum causes the rotational rate to spin up, creating higher levels of artificial gravity up to a peak level with the tether at its minimum length.


xGRF in a Nutshell

For my Dragon V2 variant, here are some key points in the concept (which is still only partly baked):

  1. You would use a Dragon V2 after it has launched a crew to the station. All but one member would board the station while the last crew member stayed aboard for the experiment. After successful completion of the mission, the Dragon V2 would return to the station to pick up the crew for return to Earth.
  2. You’d have mice on board as well as the human for two reasons: to give the person something useful to do so they don’t go crazy being by themselves for a few weeks or months at a time, and because you can get a lot more data points in a small volume with mice than you can with humans. While the human data is very useful, the mice might give you a better idea of the variability of the effects.
  3. You’d try to locate the Dragon V2 xGRF experiment as close to the station as you could get while still factoring in the risk of tether breakage–Kirk’s paper shows that at station altitudes a tether break wouldn’t lead to immediate reentry, but you’d want to make sure it also had a negligible probability of hitting the station. But if possible it would be great if you could find a position close enough that you could visibly see the station. Being alone for long periods of time might not be so bad if there are people within easy visual range that you can communicate with with no delay.
  4. You’d probably leave the xGRF kit attached to the Falcon 9 upper stage, but tucked into the trunk volume. This kit would include a dumb docking port, the tether/winch system, any required solar panels (if the solar panels on the trunk of Dragon V2 aren’t enough), and possibly inside the docking port some extra life support equipment/consumables (if you can’t cram enough into Dragon itself). Once the crew going to ISS were offloaded (along with most of the launch couches, and the vehicle configured with the mouse and its one human inhabitant for the experiment), it would leave ISS, re-rendezvous with the upper stage, dock with it, maneuver it to the experiment flight position, and then deploy the xGRF system.
  5. You’d probably want to make sure you had way to do a lot of telepresence, to keep the volunteer from getting too lonely. Two people might be psychologically nicer, but a lot harder to cram into a 10m^3 room for long periods of time. Telepresence coupled with being in visible range of ISS might mitigate the issues with having one person flying solo on a mission like this.
  6. You’d probably want to pick an astronaut who had flown a few times before, so you’d have pre-existing data on how their body responds to microgravity, to use as a comparison point. It’s not perfect–long term you’d want as many human data points as you can get, but at least with someone who has flown a few times already, you wouldn’t be dealing with a completely unknown quantity as far as space physiology reactions. If they weren’t already burned out, having twins like Mark and Scott Kelly do the experiment, with one on ISS and one on the xGRF platform might also be an interesting way of screening-out some potential genetic effects.
  7. The most interesting data is if the minimum required gravity level is less than Martian gravity levels (less than lunar would be even better). If it turns out it’s higher than that, Venus and Earth become the only realistic solar system destinations that you could live at without expensive countermeasures. So it might be worth intentionally designing the system for a maximum of say 0.4G at say a little under the 4RPM max limit that people like to stay under. With the upper stage potentially being a larger chunk of the xGRF mass than Kirk’s original paper suggested, this would greatly reduce the required tether length, making the system lighter and more manageable.

Some potential concerns include:

  1. Is 10m^3 enough volume for one person for long durations? I don’t know for sure, but if I’m not reading it wrong, a quick skim of this reference suggests that 10m^3/person might work for durations up to 3-6 months.
  2. Can the Dragon V2 without structural mods handle the loads in question? I would think so–the docking port is usually resisting a pressure load on the order of 15psi acting on the cross-section of the passageway, which is at lest 30in in diameter–yielding pressure loads of >10,000lbf they’re designed to resist. My guess is the loading would be similar in this situation, so probably something that could be handled without modification.
  3. Can Dragon V2 support 1 person on-board for long durations (>3wks) without expensive modifications? I’m less confident on this one, especially since the life support would be acting in partial gravity instead of zero-gravity. Everything I’ve heard suggests that even a little bit of gravity (for fluid settling and natural convection) can make many things a lot easier, but I’m not sure how much could be done leveraging existing hardware and interfaces, and how much would have to be done semi-custom. The less mods the less development cost. Fortunately, aborts to Earth or aborts to ISS can likely be done quickly enough that if life support stuff starts breaking down, there are plenty of quick rescue options.

The biggest question of all is how much would something like this cost, and would it be cheap enough to cross the line into something that could actually get funded? By reusing a Dragon V2 that’s already going to the ISS, you might only have to pay the delta-cost of operations and of the xGRF module. Could you keep that below $50M? Below $10M?

My guess is you’re almost positively above the $2M-ish limit of what is demonstrably crowd-fundable. But are you too high for a wealthy philantrocapitalist? I’ve heard that part of why Dennis Tito suggested Inspiration Mars was that he wanted a way to invest some of his money into making a lasting difference in the development of humanity in space. Could you get the cost low enough that Dennis, or someone like him, could chip in the money? Could you do this with NASA as a partner without NASA’s safety culture turning this into something so expensive it never flies?

Anyway, it’s some food for thought.

Posted in Commercial Crew, Commercial Space, NASA, Space Development, Space Settlement, Space Tethers, SpaceX, Variable Gravity | 30 Comments