Random Thoughts: New Shepard for Pop-Up TSTO NanoSat Launch

Blue Origin’s recent successful launch, recovery, and re-flight of their suborbital RLV New Shepard got me thinking about an idea that I came up with back in late November shortly after their first flight.

NS_InsertUpperStageHereIn an article written shortly after the first successful flight and recovery, Erika Wagner of Blue Origin mentioned an intriguing suggestion for a potential mission upgrade for future customers:

Down the road, Wagner said Blue Origin would offer a number of upgrades for research, including quick access to payloads shortly before launch and after landing, as well as hand-on control of experiments once the vehicle starts carrying people, something she later said was still a couple of years away. Blue Origin would even be open to replacing the entire crew capsule with a research payload. “If you have something that weighs 8,000 pounds and needs to get up to 100 kilometers, come talk to me,” she said.

This got me thinking about the possibility of replacing the capsule with an orbital payload and small upper-stage, and flying a Pop-up TSTO trajectory. Basically, the New Shepard booster would follow a similar trajectory to its current operations, but shortly after main engine shutdown, the upper stage would separate, and fire horizontally to accelerate to orbital velocity. So long as the upper stage had a sufficient T/W ratio, this would mean that the first stage will have covered all or almost all of the gravity/drag losses, meaning that the upper stage would only have to deliver ~7400m/s of delta-V to reach orbit.

While that’s a lot for a single stage, the stage would be operating entirely in vacuum. For a LOX/Kerosene stage with a conservative 320s vacuum Isp, that would require a pmf of ~92% to deliver a 100lb payload to LEO. For a LOX/LH2 stage with a conservative 440s vacuum Isp, you’d need about an 83% pmf. Both of those seem reasonably achievable for small pump-fed stages (possibly using electro-pumps such as those under development by Ventions or RocketLabs). With slightly more aggressive pmf/Isp assumptions (85% pmf and 450s Isp for the LOX/LH2, and 94% pmf/340s Isp for the LOX/Kero), you can get closer to 300lb payload. And if you don’t care about the added complexity of going to a 3STO system, with two expendable stages lofted by New Shepard, my numbers suggest you could probably push up into the 500-800lb paylaod range, assuming similar pmfs and Isps to the single expendable stage–though how you would exactly configure a 3STO to fit in anywhere near the same payload volume as the capsule is an open question.

Volume-wise, even the LOX/LH2 stage would likely fit within close to the same volume as the existing capsule–at typical O/F ratios, you would need about 10m^3 for the LOX/LH2 tankage, compared to 15m^3 of internal volume in the existing New Shepard capsule, with the LOX/Kerosene upper stage being even smaller. You might want to stretch the volume a little bit to provide more length for putting a payload on top, and you’d likely have some sort of fairing to surround the upper stage and payload on the way up, but neither of those seem like show-stoppers.

It would be interesting to see how the economics of such a NLV compared with others in its class. Blue Origin didn’t give a price for replacing the capsule, but if they have six paying passengers at $200k each (comparable prices to what VG has been claiming for a while), that would imply a price per flight of <$1M for the first stage, possibly as low as $500-600k.

I wonder if Blue Origin is already thinking about something like this themselves, or if they aren’t personally interested, if they’d be willing to partner with another company to do something like this. Food for thought.

Posted in Blue Origin, Launch Vehicles, Random Thoughts, Rocket Design Theory | 25 Comments

Venus ISRU Latest Thoughts

A friend sent me this YouTube video earlier this morning:

While this video really covers a lot of the same ground as previous posts in my Venus ISRU series, it still got me thinking about where we stand as far as closing a plan for Venusian settlement.

As I see, it here are the big questions I’d still like to see more thought put into to help validate the feasibility of the concept of building settlements on Venus, in no particular order:

  1. Water and Sulfuric Acid are very low density in the Venusian atmosphere, but is it possible to extract water or sulfuric acid in useful quantities? I have some ideas, but not enough ChemE-fu to really analyze them.
  2. Are there materials you can make readily from Venusian atmospheric feedstocks that are resistant enough to Sulfuric Acid to serve as external layers for a settlement? I’ve done a little research on this, but have questions because I don’t know how to compare low-density Sulfuric Acid aerosols to the various SA concentrations in the chemical compatibility literature. For instance, since it’s very dilute, would HDPE or PEX work? Those are some of the easiest polymers to make off-world. This might require testing in a Venus cloud environment simulation chamber (we’d love to build one if people have money and are interested!). The reason this question is important is that if you can’t make the outer boundary layer from locally derived materials, that become a lot of mass you have to ship from earth–even if most of the elements can be procured locally.
  3. What construction materials can you realistically make locally? I’m digging into this one a bit, and it looks like I see pathways forward to simple plastics (PE/LLDPE/HDPE/UHMWPE/PEX, and PP), there may be pathways to more complex plastics that only rely on local materials, but I haven’t dug that far. I’ve also seen reasonable paths to carbon fiber (via PAN), and sulfurcrete, all assuming we can extract enough hydrogen from atmospheric water and/or sulfuric acid. But that still leaves a lot of questions unanswered. The more of the mass of your colony you can make locally, the less you have to ship in.
  4. How do you do the reentry, landing, and return to orbit transportation in a way that works safely and reliably, including aborts, rendezvous/landing with the platforms, etc. If you can’t get stuff safely and reliably down and back, Venus might end up more of a one-way trip than Mars, and it will be harder for it to participate commercially with the rest of humanity. Once again, I have some ideas, but actually fleshing out working transportation concepts is going to take a lot more work.
  5. What would a Venus cloud city really look like, if you look at the required gas envelope and structures made from realistically locally-derived materials? After factoring in all the required hardware to make a new unit cell, how much mass is left over for other things (like people, water, plants, etc). What does this mean for what a Venus city might look like?
  6. Once you know the end-state of a modular cloud city derived from local materials, how heavy is the ISRU hardware to start that process? What structural volume and air/water production rate could you sustain with an ISRU plant you can land in a single mission?
  7. What materials can you not derive locally from the atmosphere? How much of them do you need if you’re creative? Are any of them ones that could be harvested robotically form the surface? Do we have any good ideas for mining architectures to make that happen?

Anyhow, food for thought. If you’re interested in the idea of Venus cloud colonies, these are some of the questions I’d be focusing on.

Posted in ISRU, Venus | 28 Comments

Long Range Stage Recovery

Recovering stages from far down range is likely to be on the agenda for several companies in the next decade or so. RTLS for the boosters or a first stage is a different proposition than collecting something from Mach several and a couple of thousand miles down range or so. The more performance that can be obtained from a stage that is economically recovered and reused, the less performance required from the upper stage. The less upper stage performance required, the less it should cost if expendable, or the more margin is available for recovery if reusable. A dense fuel upper stage released at Mach 17 should require a mass ratio of about 2 to reach LEO.

I vaguely remember discussion of an air captured stage from years ago and have sketched it out as I remember it.

aerorecoveryOn the left is the stage vertically ascending with some small fins to help stabilize it along with normal thrust vectoring. An aerospike is a prerequisite for this technique. If not for the other use in recovery, the fins would be hard to justify economically. On the right is the stage reentering with the fins operating as wings just large enough to enable a high speed glide. The tail surfaces fold out for the reentry. The fins/wings have a loading of 500 pounds per square foot to enable a glide at about 400 knots.

If this stage had a million pounds of sea level thrust and a mass of a million pounds, it should have an empty weight of about 50,000 pounds. The wing area would be 100 square feet with tail surfaces about a quarter of that. Fin/wing span would be about 16 feet plus the diameter of the stage. Mass of aero surfaces and controls estimated at 2,000 pounds.

This middle stage or parallel stage would reenter with the aerospike in the nose position absorbing the most of the heat and supplying the mass in the nose for stability. The blunt back end which was the stage front end before the upper stages will be high drag and a further stabilizing force.

A high speed tow aircraft hooks up to the  gliding stage at fairly low altitude and tows it back to the launch site. Since the fin/wings on the LV are far too small to land, and there is no landing gear either, the means of safely collecting the LV from this point I don’t recall except that a lot of the ideas were way out there. It may have been a usenet discussion from the late 90s.

Does anyone remember the discussion? Has anyone worked out details that you know of?

Posted in Uncategorized | 7 Comments

The Return of FalconV

In the last several weeks two companies have landed boosters that have been to space. The boost back RTLS with VTVL is now established as a demonstrated capability. What’s next?

The boost back gets the upper stage(s) to space with some horizontal velocity and returns the booster to the original launch site and has a substantial performance penalty in return for this capability. An expended lower stage of the same size puts considerably more mass in orbit, though at considerably higher cost.

I would suggest that a logical next step would be for the various companies to do a parallel staging of a full first stage booster with RTLS capability along with a derated first stage booster with the upper stage on to of the derated one.

falcon 5A Falcon Five Nine configuration is an obvious early entrant. The fourteen engines burning at launch is just over half the thrust of the FalconH in development. With cross feed, the Nine will stage at a similar velocity as last Mondays’ launch and RTLS. The Five continues on until upper stage separates and then tries to reach a seaborne recovery. The upper should have a payload of about half that of the FalconH. This is 25-29 tons of payload in the DeltaIV Heavy class.

This would give a capability above that of the expendable Falcon9 for the expenditure of  six Merlins instead of ten. The possibility of recovering the Five still exists and a development path similar to that taken with the Nine could be pursued. This concept is to bridge the gap between the light payload of an RTLS Falcon9, and the large payload of the FalconH.

Since capabilities spread through any competitive industry, we could expect to see Vulcan Two One, Antaries Two One, and a Blue Origin entrant within the next several years. Not to mention the other configurations by various other companies.

Posted in Uncategorized | 21 Comments

Random Thoughts: Space Debris Cleanup Funding Mechanism

Ever since that Kosmos spacecraft t-boned an Iridium spacecraft a few years back, more and more people have been paying attention to the space debris issue. We’ve been able to live with the nuisance caused by space debris for a long time, but space debris, especially in LEO, is one of those things standing in the way of us having really nice things. You want to do something that involves a lot of cross-sectional area (a 4000 satellite LEO constellation, large depots, LEO colonies like what Al Globus has been promoting, power beaming satellites for powering aircraft or microwave thermal launch vehicles, or LEO commsats with enough power and antenna gain to provide satellite-based roaming for cellphones), and all of the sudden the nuisance starts becoming a real pain in the neck or even deal-killer from a collision probability standpoint.

The problem is that while there is no shortage of good ideas for cleaning up space debris1, what has been lacking is a way of monetizing debris cleanup. Without some way to make a profit, companies won’t generally invest resources, and you generally can’t get external investment either. Now, government created most of this mess with launches for military or civilian space exploration missions, so they should be a big part of the solution, but finding ways to make space debris cleanup profitable is important (and preferably in a way that isn’t 100% tied to traditional government funding as the government is a notoriously undependable customer). But initially, even directly government funded efforts would be a huge improvement over what we have now. The challenge has always been finding ways to convince governments to cough up the money for cleaning up their mess.

Here’s my really crazy not-even-half-baked idea:

There are a lot of US corporations that make profits overseas and don’t repatriate them due to the high US corporate tax rate. In some cases these are profits that legitimately came from servicing overseas customers with overseas service centers and factories, and in some cases it looks more like clever tax avoidance schemes. While the sane thing to do would be to lower our corporate tax rate to be more in line with the rest of the developed world, that’s an idea that’s been talked about for a long time without any action.

What if the government agreed to a mechanism for allowing companies to repatriate foreign profits at a discounted corporate tax rate in exchange for donations to a fund for debris cleanup prizes/bounties for US-launched space junk? Say for every $1 a corporation donates to that debris removal bounty fund, they’re allowed to repatriate $X at a corporate tax rate quite a bit lower than the 35% they’d normally pay (say 15%). You could tweak the value X to make sure that it’s economically better for the companies to repatriate than to stay overseas (effectively varying X you can make it as though the repatriation tax rate is somewhere between 15 and 35%). For instance, if X were $5, that would be the equivalence of a tax rate of 29.2%2, at X=$10, the effective tax rate is 22.7%, and the breakeven value of X where it doesn’t save any money to use this method over repatriating at the existing 35% corporate tax rate is X=3.25.

There are several advantages I see to this approach:

  • This is revenue the federal government wouldn’t have gotten anyway without something like this, so this is actually revenue positive for the US government.
  • The repatriated profits minus tax and donation is more money being invested into the US economy.
  • The donation money provides a mechanism for filling the coffers on a bounty system for deorbiting US launched space debris (something like 1/4 of the junk up there) that I don’t think would be tied to the normal appropriation process, since the donation money goes into the fund before any money enters the tax system.

While on the surface this might appear to be very similar to lowering the corporate tax rate to be more competitive, and then using some of the increased tax revenues to fund a program like this, the key benefit is that the money from this donation isn’t appropriated tax revenue, so that may change dramatically how stable it is compared to say a normally-funded program at NASA or DoD. Because this isn’t collecting a tax and then appropriating money for a program though, they could probably setup this program to run for a longer duration that isn’t tied directly to the annual appropriations process. Maybe initially provide a 10yr duration, with the option for renewal, where the money goes into a bounty/prize fund for demonstrated removal of specifically designated US-launched space debris (and the debris of friendly countries willing to approve their debris for removal).

A couple of potential nuances you could add if desired:

  • You could limit eligibility of using this specific mechanism to profits made by aviation and aerospace companies overseas, and use similar mechanisms for other industries to funnel donations to other prize funds more directly tied to the interests of those industries (like say a fund for safe, low-cost nuclear power funded by repatriated oil profits, a fund for low-cost desalination provided by repatriated foreign agricultural profits, etc).
  • If you’re worried about too many people taking this route, or it encouraging companies to move operations overseas to take advantage of the slightly lower effective tax rate, you could set a first-come-first-served yearly cap on the amount that is eligible for this approach, like say $5-10B or something. Or you could set a yearly cap per company (say $100M or $500M or something).
  • You could have the fund either be directly government run, or run by a non-profit with strict requirements that some minimum percent (say 90%) has to be set aside for prizes themselves.

As I said, totally half-baked, and not at all meant to compete with more traditional appropriations-funded approaches like say COTS-like funding, or gov’t funded tech demos, or even gov’t funded debris removal missions, or more government-free options like someone finding a way to make a profit off of satellite recycling. But I wanted to throw this out as a potentially creative mechanism for getting funding donated for an important cause.

Posted in Orbital Debris Remediation/Mitigation, Random Thoughts | 17 Comments

SBSP for electric aircraft, cont.

Chris speaking.

Along with Jon, I’ve given a lot of thought to this specific application of space-based solar power (SBSP).

Yes, there are thousands of aircraft in the air at any one time, but I find this to be one of the strongpoints of this concept: The market is huge, and it’s not competing with grid power. Over 5 million barrels of jet fuel are burned each day, translating into an average of about 250 Gigawatts of power, about half of the US’s average electrical power. (Only a fraction of this is turned into useful thrust, of course, because jet engines are only so efficient, just like your typical piston engine… though they’re larger and are used constantly, so probably do better than your SUV… From my research, the total efficiency of a jet engine is about 20-40%. For an electric propeller with a high-efficiency motor, about 85%.)

Many of these flights are likely short-haul, and could be handled by batteries, such as advanced lithium ion and lithium sulfur batteries. (Lithium-Air batteries are also a real possibility… Some research on them is being done at NASA Langley Research Center.)

Also, there are still improvements to be made to efficiency using blended wing, larger engines, advanced alloys, higher carbon fiber usage, topological optimization of structure, etc. And many of the aircraft are older (especially in the developing world), not taking advantage of the latest improvements. Ultimately, we could probably reduce consumption to about a half, to 125GW, with almost half again being taken care of by ground-charged batteries (the ascent uses a lot of fuel, but low enough that it can be handled by advanced batteries), so let’s say 75GW average.

The market will grow substantially through the rest of this decade (a factor of 5?) as the rest of the world comes up to US standards of living. But we’ll set that aside for now.

50MW is about right, I think, for the solar power production of a SBSP satellite, each made with 1-10 launches, depending on how powerful your launcher is. A challenging figure, but possible. That gives about 1500 SBSP satellites to produce average 75GW of power, not counting eclipsing. But of course, that’s only equivalent to our average power if we can transfer that power to the aircraft and into thrust at the same efficiency that jet fuel can be transferred into thrust. Let’s say around 25% round-trip efficiency, so at least 50% efficiency transmitting and at least 50% efficiency receiving (including beam losses).

There are 2 main options for transmitting power. Microwave or laser.

But first, let’s pick an orbit. I’m going to pick a low Medium Earth Orbit altitude of about 3000km, with a total distance from transmitter to receiver of about 5000km (the receiver won’t be directly underneath very often). Geosynchronous is so far away that a mobile receiver is almost hopeless. LEO is so close that you have big slew-rate issues as well as more shadowing and a more difficult thermal environment plus atomic oxygen. (Shadowing is less of a problem than you might initially think, since there are much fewer flights at night… although for long-haul flights, this is less true.)
(BTW, if launch costs are low enough, it may actually make sense to put batteries on the satellites… but it’ll require very low launch costs… below $50/kg IMLEO.)
The drawback is higher radiation. The solar arrays will need to be self-healing (which thin-film cells are capable of, when heated).

To be economical to launch, we’d likely require 500-1000W/kg solar arrays (which is doable in a few different ways, but would require cleverness). They’ll also need to be regularly healed of radiation damage. So 50-100 tons of solar arrays. But I think the radiators will end up quite heavy as well, perhaps rivaling the solar arrays. 50 tons, because that’s a complicated problem.

The optics/antenna will be significant, too, but it’s harder just to estimate parametrically. Let’s look at the size required.

Microwaves sound great at first. High efficiency, easy conversion, just chicken wire receiver. Except none of that applies at high frequency.

With a 100m dish, 1mm wavelength, 5000km distance, you have:
5E6m/(1E2m/(1E-3m))= 50m receiver. We can just about fit that on a large aircraft.
That is 300GHz, by the way. And with 1mm wavelength, you can’t just use a mesh, you basically need a solid dish (which would need to be unfolded or pieced together on orbit).

But 300GHz sources are very hard to come by (and are often called Terahertz sources). Gyrotrons are a good option since they’re compact (200kg?) and high-power (1-2Megawatts apiece are fairly easy to come by) and fairly inexpensive (~$1/Watt), but they usually max out at around 100-200GHz for high power. In the 100GHz range, you can get around 50-55% efficiency with 1-2 Megawatt output (but usually are lower efficiency). To try to operate at higher frequency causes huge reductions in power and efficiency, and especially as you push to 300-500GHz, you may need to operate just in pulsed mode. The reason why is because they need a high magnetic field strength. About 28GHz per Tesla. So a 100GHz gyrotron needs just a relatively modest 3 Tesla. A 500GHz gyrotron needs 15Tesla, which is super difficult to come by except pulsed or using a superconductor. So realistically, we’re limited to around 100-200GHz until superconductor technology significantly improves and becomes cheaper. And I don’t think that receiver rectennas in this frequency range are much better than 50% efficiency.

So we need more like a 150m diameter antenna… even at just 3000km altitude and 25% round-trip efficiency. That’s pretty horrible.

What about lasers? Next post. Lasers are also a good option, and direct diode lasers are available at 100kW and fairly inexpensive ($1-10/Watt?) and efficient (wall-plug of about 50% or more), but they really need to wait until diffraction-limited weapon-class (>100kW) lasers are available and become more efficient since direct-diode lasers have high divergence and horrible beam quality, necessitating optics nearly as big as our microwave sources.

A good rule of thumb, by the way: The laser satellite will need to be close to the same price as the aircraft it powers, so around $150-300 million, including launch costs. That is really, really challenging, but not quite impossible.

Posted in Uncategorized | 7 Comments

Random Thoughts: SBSP for All-Electric Aircraft?

I haven’t blogged about Space-Based Solar Power (SBSP) much. At least not as much as Chris and some of my other cobloggers. While I try to be non-dismissive of ideas like this, I’ve never been able to convince myself that SBSP is going to make sense for terrestrial ground-power applications. As I’ve observed Liquid Flouride Thorium Reactor fission work pushed by former co-blogger Kirk Sorensen at Flibe Energy, and “new fusion” concepts being pushed by my friends at Helion Energy (and other groups at Lockheed, TriAlpha, and several other companies), I can’t help but think that at least one of these concept is going to pan out. If it does, I just don’t see how space based solar power is going to compete with small, local reactors, especially if the Helion guys make their concept work. It would be awesome if there were markets for SBSP though, because those are potential big drivers for all the sorts of in-space infrastructure (RLVs, depots, lunar mining, ISRU, etc) that us space fanboys like. So, I was trying to think of places where SBSP would actually be useful in a world where “new nuclear” (fission or fusion) pans out.

After thinking about it for a bit, the conclusion I came to is a similar one to what Chris Stelter (and frequent commenter Paul Dietz as well) thought of independently–using beamed power to enable all-electric aircraft.

Here’s my thoughts on why this might be a better market for SBSP than ground power:

  1. As safe as LFTRs are for ground-based power, I doubt anyone is ever going to be happy with a flying fission reactor1. And new fusion power systems, even if they can be made to work, look unlikely to have a high enough power density (W/kg) to make them practical for aircraft use anytime soon.
  2. Aircraft typically cruise at altitudes above 30kft, where you’re above clouds, and also above enough atmospheric moisture that many power beaming wavelengths become feasible that won’t work for ground power applications. Both high frequency microwaves and various laser frequencies as well. The fact that the lower atmosphere absorbs them efficiently means that any misses just heat up moisture in the air on the way down.
  3. While electric motors can often be more power-dense than gas motors, batteries tend to be much lower energy density than typical aviation fuels, which means that for a comparable flight, you’re going to have a much heavier aircraft, that’s going to need more peak power and thrust than a conventional aircraft.
  4. With SBSP beamed power, you only need enough batteries for the initial climb to cruising altitude (and some contingency power for emergency landings)–most of the flight can be under beamed-power, meaning the aircraft can potentially be much lighter, enabling either more cargo weight to be carried, or lighter engines and lower overall power consumption.
  5. With SBSP beamed power, you could potentially stay cruising fast for super long durations, possibly even indefinitely, even in local nighttime conditions. Imagine an AWACS or other similar military jet that could stay flying at 50kft around an area for days on end, without needing refueling.
  6. SBSP might even enable all-electric supersonic passenger aircraft. Those tend to be very fuel inefficient, and power hungry, but tend to have decent vertical cross sectional areas for power reception. Getting rid of the fuel might enable a much lighter and more cost competitive supersonic business jet. Once again, you use subsonic battery-power to get to cruising altitude, then kick it into high speed using the beamed energy.

A couple of other considerations:

  1. If my google-fu is working, it looks like a 777 produces ~150Mw of max power in its engines, and a 737 should be roughly ~1/3 of that. While 50MW is still a lot of beamed power, this is much less than the 1GW power levels usually discussed for SBSP applications.
  2. Since the target is moving, there’s no longer a big advantage to putting your SBSP spacecraft in GEO–MEO might make a lot more sense. Even a supersonic aircraft isn’t moving that fast with respect to a MEO spacecraft, so slew rates should be reasonable. There’s more radiation in MEO than GEO, but it’s also easier to get to delta-V wise.
  3. Using MEO spacecraft (4x closer) and higher frequency microwave or IR, you should be able to cut the spot beam size down to something quite reasonable. You’d need to make sure it wasn’t near a frequency used for communications, but because you’re focusing on higher-frequency radiation that doesn’t penetrate the atmosphere well, this should be solvable.
  4. For MEO you could initially get away with a few (say 6-9) equatorial birds, and just focus on markets below say 40 degrees North or South, or go with a few planes of satellites with modest inclination to enable covering most of the useful globe.
  5. For microwave based SBSP concepts, especially if you do a large reflector, you could theoretically use the same satellite for both power beaming (high power, very high frequency), and telecoms (lower power, lower frequency). You’d have a lot of power and aperture available, which might enable satellite roaming for normal sized/powered cellphones.

A 50MW transmitter is still no small project, but is probably not a multi-billion dollar demonstration either. Definitely need to dig more into the technical and economic details to see if this would close, but if it did, this is the kind of market that might enable large-scale space industrialization.

Posted in Random Thoughts | 14 Comments

XCOR Thoughts

For those of you who read my blog but somehow hadn’t heard the news already, three of XCOR Aerospace’s four founders left the suborbital rocket startup this past week. I got a notification last week on LinkedIn about Dan DeLong leaving, but found out today via Twitter and a press release from XCOR that Jeff Greason and Aleta Jackson had both left as well. For some reason this news feels like a little bit of a gut punch, so I feel like sharing some of my rambling, semi-coherent thoughts on the subject.

I’ve been following XCOR since they first started in 1999. I was barely 19 at the time, and had been interacting with (arguing with) Doug Jones, Jeff Greason, and Dan DeLong on the sci.space.* usenet groups for about three years by that point. I remember John Hare sending me an email while I was in the town of Bolinao on my mission (shortly before 9/11) with pictures of the EZRocket’s first flights. I remember crashing on XCOR’s hangar floor the night before watching SpaceShipOne’s first flight (and being terrified by how much that building creaks in the Mojave winds). I remember being grateful for all the legwork XCOR (particularly Randall Clague) did in trying to help shape the reusable vehicle experimental permit and launch licensing process in a way that protected the uninvolved while enabling the industry to learn and grow. I remember XCOR encouraging Masten to move down to Mojave, and later helping us in our successful Lunar Lander Challenge attempts. I remember watching the flights they did of the X-Racer, and being impressed by how technically competent their team always felt. I remember Jeff Greason serving on the Augustine Committee, acting as the voice of reason, and a sort of “Elder Statesman1” representing the commercial space industry. I could probably go on.

It just feels weird thinking of the idea of an XCOR without Jeff, Dan, and Aleta there.

While we don’t know the cause of their departures from XCOR, I’m not sure whether them being booted, or things getting bad enough that they would rather leave than stay another few years is worse. I know one of my fears as a founder has been the idea of eventually losing control and being kicked out of my own startup. That would be almost as awful as having my family disown me. I hope that’s not the situation, and that it was more one of the three of them decide that they needed a change of pace and/or seeing new opportunities they needed to pursue. I also hope for the sake of my many friends still there at XCOR that the company will manage to soldier on and make it to flight with Lynx. I also hope that Jeff, and Dan, and Aleta will soon find new projects that can use their skills, and that they can yet see all the hard work they put into XCOR pay off for them and for the industry.

I’m not sure if I’ve really added anything, but this news just has me in a bit of a funk. Good luck, my friends!

Posted in Random Thoughts | 9 Comments

Anti-radiation Biological Countermeasures: Amifostine

Amifostine (image rights: Ganfyd-licence user Mlj)

Amifostine (image rights: Ganfyd-licence user Mlj)

Whenever human spaceflight comes up, inevitably someone mentions radiation. Personally, I think the radiation risk is WAY overblown. “Compound conservatism” is rampant, I believe, and gets worse as time goes on and people keep recycling the same sources, adding some safety factor each time. (see here for a slightly longer explanation) Being extra conservative with radiation risk assessment eventually can cause an estimate for the tolerable risk that’s completely detached from reality, leaving very little budget left to deal with the other, much bigger risks if there’s even any money left to do the mission at all!

If we followed EVERYONE’s conservative advice for radiation risk, we’d be asking astronauts to fly in a giant sphere of polyethylene with no windows, hardly any room, and no EVAs ever (no “one small step” moment because of the risk of radiation, let alone a colony). We certainly wouldn’t be flying to ISS as we are now.

That aside, we can look at what IS a reasonably feasible and low-mass approach to dealing with radiation. Instead of the usual water or polyethylene or regolith shielding or magnetic shielding, I will look at a somewhat over-looked option: biological countermeasures. Radiation is, of course, often used to treat cancer. As such, there is a sizable body of work and several possible treatments that limit the toxicity of radiation to normal (non-cancerous) cells (thus allowing a higher dose to be used against cancerous cells, which are protected less). The most studied drug is, I believe, Amifostine. “Amifostine is the only approved radioprotective agent by FDA for reducing the damaging effects of radiation on healthy tissues.” (Cakmak et al)

While most such studies look at the ability of Amifostine to protect healthy cells from cell death and other damaging effects of radiation (such as damage that may lead to neurodegeneration), which seems to be effective (according to Cakmak and friends), what is most relevant to us in this discussion is the effect on a specific type of radiation-induced toxicity: carcinogenesis. People have suggested that stopping cell death may actually increase tumor-related toxicity (I see their argument, but it is much more likely that, due to Amifostine’s free radical scavenging, the total damage to the DNA is reduced) but is that actually true? No. No it’s not:

Paunesku et al:

Amifostine protected against specific non-tumor pathological complications (67% of the non-tumor toxicities induced by gamma irradiation, 31% of the neutron induced specific toxicities), as well as specific tumors (56% of the tumor toxicities induced by gamma irradiation, 25% of the neutron induced tumors). Amifostine also reduced the total number of toxicities per animal for both genders in the gamma ray exposed mice and in males in the neutron exposed mice.

(note: neutrons have a high quality factor, sort of like GCRs)

However, there is the argument that long-term use of a radioprotectant is not very effective, since it could reduce the body’s natural defense mechanisms.

As an aside, these very natural defense mechanisms are exactly why I think the threat posed by long-term chronic low doses of radiation is actually quite low… The body adapts to the constant radiation by increasing its natural repair/scavenging mechanisms… But with a short, very large acute dose, the body does not have time to adapt and its repair mechanisms are over-whelmed. It is these large acute doses that the general risk of cancer is actually based off. I find that extrapolating down from acute doses is incredibly unrealistic (on the ultra-pessimistic side). Aside over.

So, it may be that Amifostine and similar drugs are really most effective against acute doses of radiation. You might want to inject a little Amifostine when you learn a flare is on its way (once you get inside your radiation shelter). BUT I am not entirely convinced that there’s no benefit at all to Amifostine for chronic low-dose radiation. Even so, this whole field has tremendous potential. Imagine, you can potentially reduce the tumor toxicity of a really bad solar flare event by 25% with just a few grams of extra mass! And that’s on top of the benefit you might get from shielding and fast transit. One a per-mass basis, biological countermeasures are essentially unbeatable. This is why I think that if we’re going to spend any resources on solving the radiation problem, it probably should be to maximize whatever benefit we can get from drugs like Amifostine and, say, finding out if we can maximize our bodies’ built-in repair mechanisms through, say, targeted gene therapy. There are examples of extreme radiation tolerance and gene repair in nature that put even some rad-hard electronics to shame, so the ultimate potential (on the physics side) of biological countermeasures is pretty high as well. Biology may be a lot messier and frustratingly complex, but the potential gains make this path toward radiation mitigation worth it. Once developed, a drug or treatment would be very cheap, while shielding your transit craft with tens of tons of polyethylene or something will always be fairly expensive (even with space mining) or at least cumbersome.

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Simple SPS ??

Reading about the drawbacks of conventional Solar Power Satellites and the comments in response to Chris eventually triggered an idea, or perhaps a memory of something hinted at in something I read once. I’m somewhat less certain of complete originality of my ideas than I used to be.

The standard concept SPS has a few drawbacks that Chris brought out quite well. I hadn’t really given it that much thought and found the problems to be more interesting than the idea itself. There are several conversions in getting from sunlight to the terrestrial power  grid. Each conversion has some efficiency loss which increases the required SPS size. The four or five conversions times efficiency of each jacks up the SPS size to several times the value that one would think of without doing the trades. The kilowatt per meter making a square kilometer a gigawatt facility becomes several square kilometers of SPS to net a gigawatt on the grid.

Another problem is the heat that must be disposed of to keep the solar cells and transmitters cool enough to work properly. The mass on orbit doubles again to net your gigawatt on the grid. A couple of unsettling problems if you happen to be an SPS fan.

In comments it was suggested that it would be better to simply orbit a mirror to reflect sunlight to the desired location. That doesn’t work because the sun is not a point source of light. Sunlight is converging at about 1% to a mirror in GEO and will diverge at the same 1% when reflected to Earth. The reflected sunlight would cover a disk of well over 300 kilometers on the ground so that a one km mirror would light the ground at 1/90,000 of solar power.

So my thought is to use one mirror to focus solar radiation on a hot spot that would then be a point source of radiation for a second mirror to send to Earth. The hot spot could be thought of as similar in intent to the tungsten filament in a light bulb in a flash light. This cartoon is not to scale and is meant to show the intent only.

sps

This possibly could reduce beam spread to something reasonable at the expense of the beam being smeared across many frequencies. The visible light SPS could serve a few functions sometimes suggested by reflected sunlight advocates. If one km of sunlight could be focused such that 50% of the light was in a 10 km diameter, 1/200 of sunlight would be considerably brighter than a full moon. City lights for a large city without any conversion at all, and both storm and strike proof. Battlefield illumination as desired out of reach of interdiction. Operations lighting in the arctic for commercial and military uses. Night search and rescue. And so on for illumination as the beam would be too weak for power collection.

If a full sunlight focus is possible, then zenith solar cells would be considerably more productive than current usage.

The main advantage of a scheme like this, if feasible, is that it would be relatively light, cheap, and simple with a real likelihood of being implemented with mostly ET sources such as asteroids or the moon.

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