Should we send humans or robots to Mars?

artursk · 2017-04-17 13:25:14

Hi all.

I'm new to this forum, was great to find it - looks like I'll have a lot of reading to do over the next week, so much good stuff here.

I have recently co-authored a simple educational book on space exploration that looks at the key pros and cons of manned missions to Mars, and thought it would be great to hear some thoughts from the community here on the topic.

Some of the key arguments mentioned in the book are:

PROS:
1) Scientific value
-Finding life
-Development of cutting edge technology
-Increasing efficiency
-A Step Towards commoditized interplanetary travel and deep space exploration
2) Economic value
-Developing cutting edge Technology
-Mining for valuable natural resources
-Boost for global cooperation
3) Inspirational value
-Inspiring children
-Restoring the 'Can Do' spirit in society
4) Growing as a species
-'Safety' planet
-Drive to expand beyond the solar system

CONS:
1) Unsustainable costs
-There are bigger problems that we need to solve
-Creation of huge budget deficit
2) Limited scientific value
-Poor value for money (could send many more unmanned missions for the same cost)
-Planetary Protection issues
3) Health and safety risks
-Risks on the way
-Risks after landing
-Limited possibility of a rescue mission
4) Incentive to carelessness on Earth

Would be really eager to know where most of you stand.

Oh and in case any of you are interested in the book itself, you can find it on Amazon by searching:
"The Two Sides of a Manned Mission to Mars"

louis · 2017-04-17 13:47:25

Taking the cons...

"1) Unsustainable costs
-There are bigger problems that we need to solve
-Creation of huge budget deficit"

There is no reason to suppose a Mars mission will be uneconomic.

Musk is largely funding his effort through huge Space X profits. In addition a Mars Mission would attract huge sponsorship and TV rights payments. Regolith, meteorites and precious metals would all have huge sale value. Mars will be highly attractive to researchers who will pay fees for transit and life support on Mars. Within a few decades Mars will be able to make its own rockets.

Mars ISRU development will help people on Earth survive better in harsh climates.

The idea that Earth can have a world GDP of $75 trillion but Mars's GDP will be zero, is clear nonsense.

"2) Limited scientific value
-Poor value for money (could send many more unmanned missions for the same cost)
-Planetary Protection issues"

Putting humans on Mars makes exploration and inquiry much, much easier. Sending slow moving robots is a false economy.

"3) Health and safety risks
-Risks on the way
-Risks after landing
-Limited possibility of a rescue mission "

All these risks can be addressed and overcome. Exploration always carries some risk. Lots of people die climbing Mt Everest.

"4) Incentive to carelessness on Earth"

No. The Earthrise picture, made possible by the Apollo mission, did a huge amount to encourage pro-environmental thinking. Creating a new civilisation on Mars will only improve our understanding of environmental issues.

Last edited by louis (2017-04-17 15:13:26)

RobS · 2017-04-17 14:13:46

The "waste of money" argument is complicated and subjective in many ways. Americans spend more on pizza, on pet food, and on cemetery upkeep than on the Space Program. The real question is, how much will it cost, and we still don't know. Musk says he can build his transportation system for 10 billion dollars. He created the Falcon 9 rocket for $300 million and a NASA study indicated it would have cost NASA something like ten times as much to do the same. He developed the first Dragon capsule quite cheaply as well. So I have confidence that if Musk can get his hands on 10 billion dollars, he can do what he says he can do (well, maybe it'll cost him 15 billion, but not 100 billion!).

So the cost issue, to me, is moot. In this case, it is being developed by private capital, and private capital can be spent any way the owner(s) want.

We have an Antarctic research program and it costs a few hundred million a year. No one says, why are we spending a few hundred million studying ice and penguins, let's spend it on rebuilding our cities instead. The reason for this is because it is obscure. You wouldn't do that much rebuilding anyway, and the research is valuable (climate change, geology of an entire continent, ecology of the Southern Ocean, ozone depletion, astronomy at the South Pole where there's no water vapor to block the infrared, etc.)

In 1970 I was attending a program on NASA and the cost came up. The NASA spokesman noted that in medieval Europe, they spent a significant fraction of GDP building cathedrals when people were starving, and the inspiration value has endured ever since. Similarly, space exploration will have enduring, historic impacts on terrestrial culture and civilization.

Regarding the limited scientific value, human beings could have done all the research the rovers have done on Mars over 12 years in a few months, and the direct observations would be much richer. If we want to find the history of life on Mars, we need people there crawling around and whacking rocks (I speak as a geologist here).

Regarding the health issues: There are no show stoppers, where the flight to Mars and back is concerned. There are issues of cardiovascular health, for example. Research will probably develop drugs and other ways to ameliorate the issues. The only way to find out is to go, though. Similarly, on the Martian surface there will be issues of radiation and possibly of low gravity. The radiation can mostly be protected against by burying the living spaces, and people won't venture outside more than a few hours a week anyway. I suspect if we ever have 100,000 people on Mars, they will have BETTER health than the average American, because they won't smoke, they won't drink excessively, they won't get hooked on drugs, they won't be obese, and they'll have access to advanced medical technology. Maybe they'll get cancer twice as often, but it'll be caught and cut out before it is serious (and I speak as someone with two minor skin cancer procedures and a missing cancerous prostate).

I don't see moral hazard as a consequence of Mars exploration. We're already raping the Earth excessively. The Marsians may teach us how to preserve our ecology; they'll have to be very careful with theirs.

Oldfart1939 · 2017-04-17 15:01:54

I love taking on the "cons!"

(1) Unsustainable costs. A total fallacy, based on the model currently favored by the U.S. Government in form of Cost Plus Contracts. Properly negotiated and awarded contracts should obtain much larger returns for the money than they do. I see Private funding increasing, as well as public-private partnerships paving the way for Mars exploration and subsequent development. More money is spent on Beer than space exploration. Major League Baseball is a multibillion dollar industry, as is the NFL. Space doesn't come close!
(2) Limited scientific value? There is not much more easily attainable by robotics. I've had this argument on several other websites, especially those not space centered. Robotics are great at doing certain types of analyses, but are limited by choice of samples for analysis. The "interesting stuff" is often in hard to access terrain, and impossible to roam far enough in the life of a rover to get widely representative samples.
(3) Health and safety risks. There are folks who go out of their way engaging in "risky activities." You mentioned Mt. Everest? That's a relatively mundane thing to do these days. I've been a technical rock climber for 57 years and haven't given it up--even after a few hair raising incidents and more than a few serious injuries. There are people called "adrenaline junkies," of which I'm one--get my highs by risk taking.
(4) Incentive to carelessness here on Earth? That's what I call "grasping at straws."

SpaceNut · 2017-04-17 20:18:29

The "cutting edge technology" is not a con as its and added expense to make as it cares nothing about cost to achieve or how long it will take to create.

The pro side of the coin for "science" only those that want to go for scientific exploration would put this on that side as its a non profit generating expense and is to benefit mankinds knowledge.

The "inspirational" is also a mankind benefits but is other wise seen as an expense.

The "economic" justifications has mining as a pro but only if what you find is a value greater as an export but must are just for on site useage.

The "growing as a spicies" is in question as we have seen what happens in microgravity to staying in space and that becomes a negative for man.

Dook · 2017-04-17 22:47:43

I think everyone here, except me, wants a human mission to Mars as quickly as possible.

I'm all for waiting until we can come up with better solutions for some of the problems. I've seen enough astronauts/teachers die.

We have to test a Moxie on Mars long before we attempt to send humans there.

So, my vote is robot rovers and satellites for a long time.

kbd512 · 2017-04-18 03:43:33

Sending humans to Mars presents three problems that haven't been adequately solved.

1. Life support systems need to be far more reliable, far lighter, and use far less power than what ISS is currently using. NASA has refined versions of OGS, new CAMRAS atmospheric scrubbers, and new IWP waste water processors in advanced stages of testing. CAMRAS has already flown aboard ISS. The new OGS and IWP systems should fly this year or next. This hardware is absolutely critical to the success of the mission and nobody is going to Mars unless and until these technologies are refined to the point of being mature flight hardware.

2. Electrical power production and storage technologies are equally important for mission success since they provide power to life support systems. The new 250Wh/kg Lithium-ion batteries and 250W/kg solar arrays are required to reduce the mass associated with electrical power provisioning. To NASA's credit, a sustained and concerted effort to maximize the power storage and output capabilities of batteries and solar panels is well underway and once again, in advanced stages of testing.

3. Substantially better in-space propulsion is required than what we have today. Using chemical rockets or even nuclear thermal rockets, the mission is right at the edge of practicality, if not economic feasibility given NASA's essentially flat budget to work with. The breathtaking cost associated with high performance rockets generally limits their usage to whatever is minimally required to deliver payload X from point A to point B.

The crux of the problem is the enormous tonnage that must be delivered to Mars (point B) at fantastic cost. Our chemical rocket technology is very near to or already at its zenith. No significant advances in chemical rocket are expected, apart from new and refined manufacturing techniques that can lower cost manufacturing costs. Even for partially reusable rockets like Falcon, manufacturing cost remains the most substantial impediment to delivery of significant tonnage figures as it relates to exploration missions to other planets. The first stage reusable boosters may account for 70% of the cost of the rocket, but the upper stage still accounts for 30% of the cost and getting the upper stage back is not possible for missions that achieve escape velocity since there is no orbital propellant depot available to service and refuel the upper stage at its destination.

The solid core nuclear thermal powered upper stages can roughly double the delivered tonnage that the best LOX/LH2 chemical powered upper stages can deliver, but nuclear thermal rockets are even more expensive than chemical rockets to develop and government space agencies are the only customers. The payload mass fraction doesn't improve substantially until exotic fission technologies like liquid and gas core rockets are available. The performance increment achievable is entirely related to the temperatures that the core can achieve without destruction and the ideal propellant is liquid hydrogen, so the largest and heaviest propellant tanks are required for optimal propulsion efficiency.

Although solar electric propulsion is a more recent and much more efficient in-space propulsion technology that can deliver significant tonnage to the destination, given enough time, there remains significant tonnage associated with the portion of the mission hardware set that contains the humans who require prompt delivery to their destination to reduce radiation exposure and consumption of food / water / oxygen. The more people involved and/or the longer the mission duration, the more significant the tonnage associated with the consumables becomes, therefore transit time reduction remains as a driving force to "get there faster". The favored (as a function of the low electrical requirements for ionization) xenon propellant for electric thrusters is also extraordinarily expensive. This technology serves best in use cases that involve high delta-V increments and time-insensitive payloads to deliver.

There are two potential solutions for swiftly delivering humans to other planets, either of which could achieve delta-V capabilities that exotic gas core rockets can't match in terms of payload mass fraction. The first is the fusion driven rocket. The fusion driven rocket uses the problems associated with electrical power production using fusion for propulsion. Physicists are still struggling with keeping superheated plasmas required to achieve fusion inside electromagnetic bottles. Meanwhile, MSNW LLC is intentionally allowing that superheated plasma to escape through an electromagnetic rocket nozzle. As a result, there are no significant power generation, storage, or thermal problems to contend with. The propellant is Lithium metal, comparatively dense and easier to store than any cryogenic propellant that conventional or nuclear thermal rockets would use. This means the propulsion system and propellant tanks can be exceptionally light. Although fusion still produces radioactive waste products, the shielding required is minimal as a function of the types of radiation produced and their prompt expulsion from the rocket, whereas nuclear thermal rockets require more substantial shielding to stop the intense gamma rays and neutrons from killing the humans or destroying the microchips in the computers onboard. Thus far, this program has encountered nothing but success and has received continuous funding every year since its inception. Fusion tests to confirm the basic function of the Lithium foil liner, D-T pellet injection, and electromagnetic nozzle have already been completed. Current work centers around improving the net propulsion efficiency or gain from fusion and integrating the hardware set into a representative ground test demonstrator unit. This technology enables trips to Mars in 30 to 90 days.

The second promising new technology is Robert Shawyer's EM-Drive. There are two fundamental problems with this technology. The first and perhaps most significant is that if it actually works in space, then it violates what physicists previously thought were immutable physical laws of the universe since it does not use reaction mass to achieve propulsion. Changing the world is generally a rough process, and so it seems that while there is an endless supply of critics lined up to lambast the handful of brave souls willing to stake the scientific careers on this poorly understood branch of physics, very few knowledgeable researchers are actually trying to figure out how and why this technology works. NASA's Eagleworks laboratory is steadily upping the level of power input since the propulsion output doesn't seem to scale linearly with the power input. If it works in space as well as it does in the lab, then there will be a shift away from chemical, nuclear thermal, fusion, and even ion engines. Who wants to spend money delivering reaction mass when all you need is electrical power?

What will be really funny is seeing upper stages using reciprocating internal combustion engines that provide electrical power since the transit durations will be so brief, with trips to Mars measured in days and orbital phasing of Earth and Mars becoming meaningless figures. In other words, we can come and go as we please to any planet in the solar system. There's one minor problem with the incredible velocities achievable. You'd better have the best micro meteoroid shielding money can buy, since a piece of space debris the size of a grain of sand can hit with the force of an artillery shell. The navigation computer required to accurately adjust course at the velocities involved must be exceptionally precise. If thrust reversal is not done precisely when and where required, then you could overshoot your target entirely, which is the best possible outcome of a navigational error. Alternatively, you could very well fly into your target at velocities that would vaporize the best heat shields available. The fastest spacecraft ever launched is only moving at a tiny fraction of the velocity achievable with EM-Drive.

The propulsion section required quite a bit of explanation, but these three problems are the "real" issues with sending humans to Mars right now. Once resolved, the only issue remaining is committing to a multi-decadal exploration program that's likely to kill a few more irreplaceable men and women. All progress has a real price tag attached to it and that's the way it's always been.

louis · 2017-04-18 04:13:24

Interesting observations. Here are my comments:

1. I am surprised that progress on life support systems has not been more solid. I think that has partly been due to a lack of focus and adequate resourcing. In a 10 year Mars Mission programme, this would be a top priority and would get the sort of investment and attention required.

2. We do know PV can work well on Mars. Whilst I still favour using lightweight PV, that is less of an issue now that Musk's Rocket Revolution is in full flow. As the price of launches drops, so we can be a little more relaxed about mass. Likewise with batteries - yes want efficient batteries, but they will no longer "break the bank". We can take a tonne of batteries - 250 KwHs. Maybe 2 tonnes.

3. I am not sure radically better propulsion is a requirement. Desirable yes, but Musk's plans do not depend on a radical improvement. With our improved knowledge of zero G, exercise requirements and space medicine, I think we can get people to Mars in a reasonable condition, then give them a couple of sols to recover function in 0.38 G (wearing weighted suits). I think they will be good to go (or stay rather).

4. I think it is reasonable to assume Space X can get launch costs down to $2000 per kg to LEO. They can probably go lower - to $1000 per kg, but let's stick with $2000. On that basis, a mission requiring 300 tonnes to LEO would cost for that part of the project $600 million. A lot, granted but not something that will dominate overall mission costs. Most of the costs come from development. The good thing about development is that once you've done it, that's not a repeat cost. Some of the Rover projects have cost around $300 million in development, and I think that's probably the sort of costs you are looking at for each major development item (e.g. your transit craft, habs, lander, humanned rover etc). However, as you are pointing out re ISS, a lot of development work has already taken place in terms of other projects. I am struggling to see why, these days, it should come in at over $10 billion. You could maybe pull it off at $5 billion. A lot of the cost allocation would be a moot point.If you use the Falcon Heavy you can't claim all the development costs relate to Mars, as it's a rocket that will be used in a lot of different contexts (e.g. lunar tourism, satellite launches and wider solar system exploration).

kbd512 wrote:

Sending humans to Mars presents three problems that haven't been adequately solved.
1. Life support systems need to be far more reliable, far lighter, and use far less power than what ISS is currently using. NASA has refined versions of OGS, new CAMRAS atmospheric scrubbers, and new IWP waste water processors in advanced stages of testing. CAMRAS has already flown aboard ISS. The new OGS and IWP systems should fly this year or next. This hardware is absolutely critical to the success of the mission and nobody is going to Mars unless and until these technologies are refined to the point of being mature flight hardware.
2. Electrical power production and storage technologies are equally important for mission success since they provide power to life support systems. The new 250Wh/kg Lithium-ion batteries and 250W/kg solar arrays are required to reduce the mass associated with electrical power provisioning. To NASA's credit, a sustained and concerted effort to maximize the power storage and output capabilities of batteries and solar panels is well underway and once again, in advanced stages of testing.
3. Substantially better in-space propulsion is required than what we have today. Using chemical rockets or even nuclear thermal rockets, the mission is right at the edge of practicality, if not economic feasibility given NASA's essentially flat budget to work with. The breathtaking cost associated with high performance rockets generally limits their usage to whatever is minimally required to deliver payload X from point A to point B.
The crux of the problem is the enormous tonnage that must be delivered to Mars (point B) at fantastic cost. Our chemical rocket technology is very near to or already at its zenith. No significant advances in chemical rocket are expected, apart from new and refined manufacturing techniques that can lower cost manufacturing costs. Even for partially reusable rockets like Falcon, manufacturing cost remains the most substantial impediment to delivery of significant tonnage figures as it relates to exploration missions to other planets. The first stage reusable boosters may account for 70% of the cost of the rocket, but the upper stage still accounts for 30% of the cost and getting the upper stage back is not possible for missions that achieve escape velocity since there is no orbital propellant depot available to service and refuel the upper stage at its destination.
The solid core nuclear thermal powered upper stages can roughly double the delivered tonnage that the best LOX/LH2 chemical powered upper stages can deliver, but nuclear thermal rockets are even more expensive than chemical rockets to develop and government space agencies are the only customers. The payload mass fraction doesn't improve substantially until exotic fission technologies like liquid and gas core rockets are available. The performance increment achievable is entirely related to the temperatures that the core can achieve without destruction and the ideal propellant is liquid hydrogen, so the largest and heaviest propellant tanks are required for optimal propulsion efficiency.
Although solar electric propulsion is a more recent and much more efficient in-space propulsion technology that can deliver significant tonnage to the destination, given enough time, there remains significant tonnage associated with the portion of the mission hardware set that contains the humans who require prompt delivery to their destination to reduce radiation exposure and consumption of food / water / oxygen. The more people involved and/or the longer the mission duration, the more significant the tonnage associated with the consumables becomes, therefore transit time reduction remains as a driving force to "get there faster". The favored (as a function of the low electrical requirements for ionization) xenon propellant for electric thrusters is also extraordinarily expensive. This technology serves best in use cases that involve high delta-V increments and time-insensitive payloads to deliver.
There are two potential solutions for swiftly delivering humans to other planets, either of which could achieve delta-V capabilities that exotic gas core rockets can't match in terms of payload mass fraction. The first is the fusion driven rocket. The fusion driven rocket uses the problems associated with electrical power production using fusion for propulsion. Physicists are still struggling with keeping superheated plasmas required to achieve fusion inside electromagnetic bottles. Meanwhile, MSNW LLC is intentionally allowing that superheated plasma to escape through an electromagnetic rocket nozzle. As a result, there are no significant power generation, storage, or thermal problems to contend with. The propellant is Lithium metal, comparatively dense and easier to store than any cryogenic propellant that conventional or nuclear thermal rockets would use. This means the propulsion system and propellant tanks can be exceptionally light. Although fusion still produces radioactive waste products, the shielding required is minimal as a function of the types of radiation produced and their prompt expulsion from the rocket, whereas nuclear thermal rockets require more substantial shielding to stop the intense gamma rays and neutrons from killing the humans or destroying the microchips in the computers onboard. Thus far, this program has encountered nothing but success and has received continuous funding every year since its inception. Fusion tests to confirm the basic function of the Lithium foil liner, D-T pellet injection, and electromagnetic nozzle have already been completed. Current work centers around improving the net propulsion efficiency or gain from fusion and integrating the hardware set into a representative ground test demonstrator unit. This technology enables trips to Mars in 30 to 90 days.
The second promising new technology is Robert Shawyer's EM-Drive. There are two fundamental problems with this technology. The first and perhaps most significant is that if it actually works in space, then it violates what physicists previously thought were immutable physical laws of the universe since it does not use reaction mass to achieve propulsion. Changing the world is generally a rough process, and so it seems that while there is an endless supply of critics lined up to lambast the handful of brave souls willing to stake the scientific careers on this poorly understood branch of physics, very few knowledgeable researchers are actually trying to figure out how and why this technology works. NASA's Eagleworks laboratory is steadily upping the level of power input since the propulsion output doesn't seem to scale linearly with the power input. If it works in space as well as it does in the lab, then there will be a shift away from chemical, nuclear thermal, fusion, and even ion engines. Who wants to spend money delivering reaction mass when all you need is electrical power?
What will be really funny is seeing upper stages using reciprocating internal combustion engines that provide electrical power since the transit durations will be so brief, with trips to Mars measured in days and orbital phasing of Earth and Mars becoming meaningless figures. In other words, we can come and go as we please to any planet in the solar system. There's one minor problem with the incredible velocities achievable. You'd better have the best micro meteoroid shielding money can buy, since a piece of space debris the size of a grain of sand can hit with the force of an artillery shell. The navigation computer required to accurately adjust course at the velocities involved must be exceptionally precise. If thrust reversal is not done precisely when and where required, then you could overshoot your target entirely, which is the best possible outcome of a navigational error. Alternatively, you could very well fly into your target at velocities that would vaporize the best heat shields available. The fastest spacecraft ever launched is only moving at a tiny fraction of the velocity achievable with EM-Drive.
The propulsion section required quite a bit of explanation, but these three problems are the "real" issues with sending humans to Mars right now. Once resolved, the only issue remaining is committing to a multi-decadal exploration program that's likely to kill a few more irreplaceable men and women. All progress has a real price tag attached to it and that's the way it's always been.

kbd512 · 2017-04-19 02:35:08

louis wrote:

Interesting observations. Here are my comments:
1. I am surprised that progress on life support systems has not been more solid. I think that has partly been due to a lack of focus and adequate resourcing. In a 10 year Mars Mission programme, this would be a top priority and would get the sort of investment and attention required.

I've been talking about this problem for quite some time. Let's be clear here. There is a serious effort to improve lift support systems, but the funding is insufficient. Any failure of life support systems, even a partial failure, is an absolute show stopper. There's a reason this was #1 on the list of reasons we can't send humans to Mars right now. There's no doubt in my mind that we'd have more dead astronauts to do administrative stupidity reports on if we insisted on using current generation hardware. This problem will not going away until we commit to making this a top priority and adequately fund the requisite programs.

While we're on the subject, current generation space suits like the EMU are a disabling technology for Mars exploration. NASA already knows this, but instead of solving the problem with technology they had during the Apollo Program, they've turned it into another jobs program for their vendors. A Mechanical Counter-Pressure or MCP suit will always provide better range of motion and require lower caloric expenditure to move around in than a gas bag suit, no matter how well-designed the articulating joints are. That said, the new Boeing-designed suit is much closer to what's needed than the Z series suits.

If there was no insistence on maintaining a partial pressure similar to what the EMU provides for the entire body, then the MIT biosuit would easily provide the required mechanical counter-pressure for astronauts bound for Mars. The costs of manufacturing such suits would not be $12M per copy, either, nor anything close to it.

louis wrote:

2. We do know PV can work well on Mars. Whilst I still favour using lightweight PV, that is less of an issue now that Musk's Rocket Revolution is in full flow. As the price of launches drops, so we can be a little more relaxed about mass. Likewise with batteries - yes want efficient batteries, but they will no longer "break the bank". We can take a tonne of batteries - 250 KwHs. Maybe 2 tonnes.

PV doesn't work all that well on Mars for mobile applications, but the Lithium-ion batteries are the problem. That's why Curiosity is RTG powered. Mars is so cold that Lithium-ion batteries require radioisotope heater units or RHU's to survive the Martian nights. If the battery is volumetrically small enough and fairly well insulated, then RHU's are sufficient. If not, then you have a problem. For stationary applications, the mildly cryogenic nights are less of a problem since the battery can be surrounded by more insulation to keep it warm.

For continuous electrical power requirements ranging into the tens of kilowatts, NASA is presently focusing more effort on small fission reactors like the Kilopower series of reactors. I think fission reactors producing less than 100kWe make little sense from a mass perspective for a mission involving humans. For deep space probes on missions that take them beyond the main belt, it makes more sense. For human missions to Mars, NASA has a stated requirement to provide continuous electrical power output of around 40kWe. I have no idea what they're running that needs that much power, but that's the stated requirement.

Solar panels and batteries are not yet competitive at that output level. If substantially better batteries were available, say 500Wh/kg, then there's less of a case to be made for using nuclear power. The energy density of our Lithium-ion batteries would have to double for that to happen. Power density is already sufficient.

louis wrote:

3. I am not sure radically better propulsion is a requirement. Desirable yes, but Musk's plans do not depend on a radical improvement. With our improved knowledge of zero G, exercise requirements and space medicine, I think we can get people to Mars in a reasonable condition, then give them a couple of sols to recover function in 0.38 G (wearing weighted suits). I think they will be good to go (or stay rather).

I guess it depends on what you think is affordable. For the exploration campaign to be economically sustainable, the costs need to come down to levels never achieved. The $1,000/kg or $2,000/kg sounds fantastic until you realize that we're not going to LEO. We're going to Mars. Mars is more like $50,000/kg. Radically better propulsion is an economic requirement from that perspective.

Solar Electric Propulsion, or SEP, seems like it solves the high delivered tonnage requirement until you realize that the Xenon fuel commonly used costs $1,200/kg, you need tens of tons of that expensive propellant for the types of payloads we want to send to Mars, and the cost of the solar arrays makes the propellant seem affordable. A fusion rocket using Lithium propellant that costs $270/kg, with similar specific impulse to SEP using far less expensive fuel and less solar power, is quite preferable if both high thrust, high specific impulse, and low cost are required. If SEP starts using comparatively inexpensive Argon, at just $5/kg, then there's a much better economics argument to be made for using SEP. You may need larger panels to ionize the Argon, but the propellant prices are so low that the marginal cost increase for the larger panels is worth the associated cost.

louis wrote:

4. I think it is reasonable to assume Space X can get launch costs down to $2000 per kg to LEO. They can probably go lower - to $1000 per kg, but let's stick with $2000. On that basis, a mission requiring 300 tonnes to LEO would cost for that part of the project $600 million. A lot, granted but not something that will dominate overall mission costs. Most of the costs come from development. The good thing about development is that once you've done it, that's not a repeat cost. Some of the Rover projects have cost around $300 million in development, and I think that's probably the sort of costs you are looking at for each major development item (e.g. your transit craft, habs, lander, humanned rover etc). However, as you are pointing out re ISS, a lot of development work has already taken place in terms of other projects. I am struggling to see why, these days, it should come in at over $10 billion. You could maybe pull it off at $5 billion. A lot of the cost allocation would be a moot point.If you use the Falcon Heavy you can't claim all the development costs relate to Mars, as it's a rocket that will be used in a lot of different contexts (e.g. lunar tourism, satellite launches and wider solar system exploration).

We've spent more than $10B to develop SLS and hardware fabrication still hasn't been completed for the first test flight. If you're struggling to understand why it costs so much, you need to read up on "cost plus" contracts. There's no incentive to control costs on the part of the contractor and Congress keeps throwing money at them because it generates jobs in their districts. If we had better in-space propulsion, we wouldn't need rockets like SLS or ITS. Reusable rockets like Falcon Heavy, Vulcan Heavy, and New Glenn would easily fulfill the heavy lift requirements. All of NASA's DRM's have IMLEO requirements ranging from 700t to well over 1000t and flying SLS will cost every bit as much as a STS mission. Given SLS development costs, even that ITS behemoth is looking more affordable with each passing year.

GW Johnson · 2017-04-21 09:53:03

Dealing effectively with microgravity diseases and with radiation exposure requires that we use the best facts we have, not just assumptions. The assumption with microgravity had been that exercise is a cure-all. It is not; we already have evidence for that. The assumptions about radiation are either (1) we can ignore this like we did with Apollo, for those who want to go, or (2) that we cannot tolerate the exposures no matter what, for those who do not want to go. Both are bad assumptions, especially with respect to solar flare events.

The microgravity diseases (plural is deliberate!!!) problem:

This is multi-faceted with multiple effects, and I really doubt we have identified them all. Attention gets paid only to those we have known about the longest, but to ignore the rest is a serious mistake. The entire suite of effects is avoided with artificial gravity, simple as that.

There is bone decalcification and weakened muscles from disuse – this can be offset with exercise to one extent or another, and perhaps there are drugs that will help, although that has yet to be shown. The kind of exercise that benefits bone density and muscle size is resistance.

There is damage to the heart and circulatory system, again from disuse. Perhaps exercise can offset this to one extent or another, and maybe there are drugs that could help. The kind of exercise that benefits the heart and circulatory system is aerobics, although we currently know less about whether that would really be effective in zero gee.

We now know there is damage to vision. There is no exercise for this, it is due to a maldistribution of internal fluid pressures, precisely because there is no gravity gradient to distribute them. It is entirely unclear as of yet whether or how much recovery is made upon returning to a gravity environment.

Most recently, we have found that there is also damage to the immune system. The mechanism for this is not yet understood at all, but there is very likely no exercise for this. Whether there might be aid from drugs is also as yet entirely unknown. It is also entirely unknown whether or how much recovery is made upon returning to a gravity environment.

With so many unknowns, with the knowledge that exercise is not the “cure” for all the problems that we do know about, and with the distinct possibility that we have not yet uncovered all the problems, does it not seem wiser to avoid the microgravity diseases problem in the first place?

We don’t know how much therapeutic effect there might be to 0.38 gee or to 0.16 gee, but it cannot be zero! The real problem to solve is zero gee in transit. If that were instead nearer 1 gee, the crew would stay a lot healthier with a lot less interventions. Simpler is better. If you have near 1 gee during the transits to and from Mars, with the kinds of spacecraft that we can really build, there’s 6 to 8 months available to become fully fit for whatever stresses there are to face upon arrival.

Plus, spin gravity has life support benefits: water/wastewater handling is more conventional, including settling processes completely unavailable at zero gee. And you can do free-surface cooking in a pot on a stovetop. That makes frozen food much more attractive, since it can also serve as radiation shielding, and it lasts longer than any other known preservation method.

The radiation exposure problem:

This is two-fold and they are distinct. There is the galactic cosmic ray (GCR) problem, with an intensity that varies with the solar cycle. It maximizes at 60 rem/year at solar minimum, and minimizes at 24 rem/year at solar maximum. When it is stronger, the solar wind outflow restricts GCR entry into the solar system more. Variation is approximately sinusoidal, with a period of about 11 years.

This GCR is extremely high-energy particulate radiation, mostly protons, but there are heavier nuclei. It creates secondary scatter when it strikes solid material, so you want either several cm of shielding, or you want many meters of shielding. The secondary scatter will kill you in between those extremes.

The other threat is solar flare events, which occur erratically in timing and intensity. These are short events, measured in hours of duration, and it is composed of far lower energy particles that only several cm of material will pretty much stop. Intensity is the problem: worst case intensities outside can be ~10,000 rem in those few to several hours, which is far beyond a lethal dose.

NASA’s radiation exposure standards allow more exposure than a nuclear worker, but are said to increase the risk of cancer late in life by something around 3%. So, these are fuzzy limits, not hard and sharp, this-or-that. There is an annual accumulation limit of 50 rem per year. There is a limit of no more than 25 rem accumulated in any given month. Career exposure limits vary by age and gender, but tend to peak out at 400 rem lifetime accumulated for older persons of both genders.

There is some slight shielding effect of spacecraft structures on GCR, more if you put a few to several cm of hydrogen-rich materials around the hull. There is also a timing effect: if you sleep in a partially-shielded area, you get some knockdown factor for about 1/3 of the daily cycle. The net effect is that 60 rem/yr outside is more like 50 rem/yr inside, and that’s worst case. It could be a lot lower, if you don’t go in a peak GCR year.

You simply will need a shelter for the solar flare events, because over a 2.5 year mission, the probability of getting hit by one is far, far higher than it was for a 1.5 week trip to the moon. 15-20 cm of water is adequate, according to NASA’s own data.

Put water/wastewater handling tanks and piping, and any propellant tanks that you can, around your designated shelter zone. Myself, I’d make the flight control station the designated solar flare shelter, so that critical maneuvers can be made regardless of the solar weather.

Put some of the rest of the propellant tanks around the sleeping quarters, and you get that knockdown factor that gets you down to 50 rem annual when it’s 60 rem annual outside. See how this drives you toward a cluster of docked items for your design, instead of just simple cylindrical “tuna cans”? Thinking outside the usual box is just required to do this effectively.

You’ll violate the exposure rules in the month (and possibly the year) when the solar flare hits (if it hits), but not by very much. So, who cares? It’s a fuzzy limit anyway. Just do it.

While at Mars, exposures are halved by the presence of the planet, even for those people in orbit. On the surface, the atmosphere shielding effect cuts the exposure further. Those effects act to reduce your annual accumulations. For having babies, you want to cut it further: use buried habitations, or put a meter or so of regolith on top of a very strong roof or cover. But, until you are having babies, you don’t really need that.

If we had some sort of active magnetic shielding, that would be even better. But we don’t. There is no tested hardware anywhere that we could install. So, you go with what you have: passive shielding. It comes from the shell construction, the water/wastewater gear, the propellant tanks, and any frozen food that you have. Add the magnetics later, once such hardware becomes available.

One of the reasons I like the Bigelow B330 inflatables is the near half-meter thickness of the “shell”. This is layer upon layer of various fabrics and sheet materials, nearly all of them hydrogen-containing polymers. It definitely has some radiation-shielding effect, and also considerable meteoroid protection. Some version of the B330 module design would make a very good crew cabin module (more than one is needed for the living space to be adequate) for a Mars spacecraft. If they can be landed, they might work for surface habitations as well.

GW

GW Johnson · 2017-04-21 09:55:31

Propulsion? What's science and what's science fiction?

I see too few correspondents on these forums distinguishing clearly between these two categories. It is very important to understand what is available right now to use, versus what might become available in future decades. Perhaps future centuries. The list below is not comprehensive.

Propulsion items that are “real” (meaning they are in actual use) ---

Liquid rocket propellants

LOX-LH2, LOX-kerosene, LOX-ammonia, NTO-MMH/DMH/hydrazine/Aerozine-50 (basically “any hydrazine”), IRFNA-any hydrazine, IRFNA-kerosene, and solid propellants are all real. For the monopropellants, “any hydrazine” and monopropellant hydrogen peroxide are both real.

The IRFNA-oxidized systems are very old, and generally the lower-performing items, but they do offer storability advantages over NTO-oxidized systems. They are still quite real.

IRFNA-any hydrazine, and IRFNA-kerosene are hypergolic ignition, as is NTO-any hydrazine. The rest require some sort of vacuum-qualified ignition system to restart in space, above and beyond the ullage problem common to all the liquids and hybrids, but not the solids.

All of the cryogens offer very large difficulties with long term storage in space conditions. Hydrogen is the very worst of these. The biggest problem is boiloff versus time. The second biggest problem is density, showing up in the “typical” density x specific impulse rating, something extremized with hydrogen, and afflicting methane to a lesser extent. LOX is reasonably dense, as are the other liquid fuels.

LOX-ammonia is what propelled the X-15 once it began flying its “big engine” in the 1960’s. Not many have used this combination since then. But it does perform well. Ammonia needs to be both cold (not cryogenically cold) and pressurized substantially, for effective storage. Anhydrous ammonia is the proper form, very similar to what farmers spread on fields, and similarly toxic to handle.

Electric propulsion

Electric (ion) propulsion items like Hall effect thrusters are quite real, but use expensive xenon fuel and provide vanishingly-small thrusts for the engine-plus-power-supply mass. These have flown in space on real spacecraft, usually (but not entirely) as solar-powered items.

Hybrid Rockets

Hybrid rockets have flown, but only in smaller sizes, and only suborbitally (most notably in Spaceship One and Spaceship Two). These take the form of liquid oxidizer / solid fuel implementations. These include LOX-rubber, LOX-wax, and LOX-“other polymer” systems. Acids are also possible oxidizers, and may offer hypergolic ignition. Solid fuel “burn” rates in a hybrid are typically very low. If you add solid oxidizer to the fuel to achieve burn rate, you just created a solid propellant, thus entirely negating the safety advantage of the hybrid (abortability). Mixing and combustion completeness are still very, very significant design issues. This is an immature technology, in spite of having flown.

Solid Propellant Rockets

There’s gunpowder-like fireworks stuff, and there’s “real” solid propellants for rocket propulsion applications. The solid propellant families include double-base, composite-modified double-base, and both AN and AP-oxidized composites. Of these, the AN-oxidized composites are obsolete and pretty much the lowest-performing. (AN = ammonium nitrate, AP = ammonium perchlorate.) These solids tend to have the highest thrust potential of all the rockets, by far, but mostly because the nozzle dimensions are simply the largest fraction of stage diameter.

Double-base is solid nitrocellulose dissolved into liquid nitroglycerin, which then reacts and gels back into a solid. As a finished propellant, it reacts fairly benignly in fuel fire scenarios, but not very benignly in bullet and fragment impact. Performance is a little better than a typical AN composite. Depending upon additives, it can be quite free of primary (particulate) smoke, and of secondary smoke (no chlorine for atmospheric water vapor to condense upon). Handling neat nitroglycerin is very dangerous, but inherently cannot be avoided making this kind of propellant.

Composite-modified double base is double base with significant solid additives mixed with the nitrocellulose before the nitroglycerin addition. The two most significant additives would be a dry oxidizer powder (usually AN) and a powered metal (aluminum). These both add performance, and in the case of aluminum, primary smoke. The other properties are similar to plain double base. Handling neat nitroglycerin is very dangerous, but inherently cannot be avoided making this kind of propellant.

Composite propellants disperse solid ingredients into a curable polymer binder. The two principal solid ingredients are oxidizer powder (can be AN or AP), and powdered metal (usually aluminum). Limited by processing hazard, low-percentage solid ingredients can include the explosives RDX and HMX. Trace ingredients include carbon black as an opacifier, and either solid or liquid iron compounds as burn rate catalysts. Metallization produces primary smoke. AN-oxidized formulations are free of secondary smoke, but AP-oxidized formulations usually produce quite a bit of it, in humid conditions.

Common binder polymers include PBAN, CTPB, and HTPB. These are all merely flammable materials. There are some propellants that use GAP as the binder, but these are quite a bit more hazardous to process, as GAP is a very friction-sensitive monopropellant liquid explosive all by itself.

The composite propellants usually perform rather benignly in bullet and fragment impact, not very benignly in fuel fire scenarios. AN formulations perform about like double-base, not quite as good as composite-modified double base. AP formulations generally outperform both double-base and composite-modified double base.

Propulsion items that were once “real” but are no longer –

Hydrogen peroxide-kerosene

Hydrogen peroxide-kerosene is hypergolic ignition, but has not ever actually flown, except experimentally. Lots of ground tests have been done.

Hydrogen peroxide monopropellant

This was the thruster design that provided attitude control for the X-15 and some other early vehicles.

Problems with rocket-grade hydrogen peroxide

With both bipropellant and monopropellant hydrogen peroxide systems, the problem is storability of the hydrogen peroxide because of its inherent chemical instability. It takes about 90+% strength peroxide to make an effective rocket oxidizer or monopropellant, but its long term safe storage is in serious doubt above 50% peroxide, maybe 80% if the stability additives are as effective as claimed.

The outcome to be avoided is spontaneous explosive decomposition, which is very violent and destructive. There are additives that are said to promote better stability, but these have never, ever been used in flying systems. Hydrogen peroxide use was restricted to short times in monopropellant decomposition thrusters.

Alternatively, some claim that extreme purity confers stability. These viewpoints have yet to be reconciled. Until they are, rocket grade hydrogen peroxide must be used within only a few hours of its distillation. You dilute it down for safe storage, and distill it back up again right before use.

Solid core nuclear thermal

Solid core nuclear thermal was well-demonstrated in ground tests in the 1960’s and 1970’s in the US, but was never taken to flight demonstrations, although it could have been. About 1990, this concept may have been ground tested in the old Soviet Union, but it was never actually flown there, either. Most of the US people who actually worked testing this technology are now dead.

Propulsion items that were never “real”, but did show interesting lab experiment results ---

Nuclear thermal rocket items

There are permutations of the basic solid core nuclear thermal rocket design that improve max core temperature and engine thrust/weight ratio, and perhaps delivered specific impulse, but these have never been tested on the ground as engines, much less in flight. These include fluidized particle beds, something called “Timberwind”, something different termed “Dumbo”, and the liquid core concepts. Performance estimates are paper calculations, only partially-supported by experiment.

There are two “gas core” nuclear thermal approaches: the nuclear light bulb, and the open-cycle engine concept. Certain non-comprehensive small-size lab experiments have been performed to confirm some basic physics notions. No engine or engine-component tests have ever been done on the ground. Nothing has ever flown, of course. All performance predictions are nothing but paper calculations unsupported by appropriate experiment.

Nuclear pulse propulsion

Nuclear pulse propulsion was extensively explored with paper calculations and one subscale flight experiment, in the 1950’s. It appears to work well enough, although the nuclear technology upon which it is based, is a permutation of early-1950’s second-generation fission weapon technology. The subscale flight test verified with chemical explosives that explosion propulsion really does work within the atmosphere, and against one full gee of gravity.

It requires a sort of “shaped charge” nuclear explosive that is unsuitable as a blast weapon or as an EMP weapon. The EM radiation blast is strongly “spindle-shaped”, with one “prong” pointed at the vehicle’s pusher plate. In the vacuum of space, there is no blast wave of any significance at all, only this EM radiation spike. It is very intense, leading to high vehicle accelerations at high specific impulse, something no other known form of propulsion offers.

According to the paper calculations, this type of propulsion is far more efficient in very large vehicle sizes, those far-exceeding 5000 tons. But nothing ever flew, except for the one small 1-meter model propelled with small chemical explosive charges. It does seem likely that this approach would work, if developed and flown. It could use an update from the obsolete 1950’s fission weapon technology.

“Breakthrough EM drive”

“EM drive” has to do with an unbalanced force seen in some experiments with microwaves resonating in a cavity. This is a series of lab experiments, the results of which are still disputed within the science community. If the effect is real, the laws of physics as we know them will likely have to be re-written to some significant extent. This effect violates conservation of momentum.

No ground or flight tests of this concept as an engine have ever been done, although they could be. If it does prove to be a real effect, the thrust seems (so far) to be vanishingly-small in comparison to the mass of the test equipment. This concept definitely borders upon science fiction in its “reality”. It needs a basic scientific feasibility demonstration to proceed.

Propulsion items that could become “real” very soon --

VASIMR

VASIMR is an alternative means to produce electric propulsion thrust without immersing electrodes into the flow that otherwise have limited life because of erosion. It suffers from the same vanishingly-small thrust compared to the weight of the engine equipment, that afflicts the Hall effect thruster, and all the other ion engine concepts. It does use the far less expensive argon as its propellant.

VASIMR has seen numerous ground tests, but has yet to fly, unlike the Hall effect thruster. All these electric concepts need an electricity supply that does not yet exist in a high power/weight form ready to use. Right now there are only solar panels or radioisotope generators, which are both heavy.

“Green (non-toxic) propellant”

“Green propellant” is a relatively non-toxic monopropellant thruster propellant option that is much safer to handle than the current thruster “standard” MMH (or the other hydrazines). The leading version is a USAF-developed formulation named AF-M315E, now slated to fly in orbit sometime during 2017.

This is a water-based solution of hydroxyl ammonium nitrate (HAN) salt, plus one or two other components in the solution. It requires different thruster construction because it burns hotter, a different catalyst to decompose it, and it has about 50% improved density impulse over hydrazines.

The NASA “GPIM” flight test of this combined monopropellant and thruster technology has been delayed for about 4 or 5 years now. Details on AF-M315E composition are lacking, as are any indications that this material could be used in a bi-propellant engine.

“Safe-400 nuclear electric power supply”

This is the first US nuclear fission reactor power supply meant for space since the SNAP-10. It uses a core whose fuel elements are also heat pipes by which the fission thermal energy is removed from the core. These dump that heat through heat exchangers into a working-fluid gas that runs gas turbine-driven electric generators. At rating, the reactor core produces 400 KW thermal energy, and the generators produce 100 KW electrical energy. The remaining 300 KW waste heat must be radiated to space, using radiator wings of significant size.

This is the kind of power source that the electric propulsion items need for operation further from the sun than Mars, including VASIMR. It is very hard to find accurate sizes and weights for the heat exchangers, the turbine generators, and the radiators. The reactor core itself is fairly lightweight at a published 512 kg. Otherwise, the complete system is likely a few to several tons, and that includes no radiation shielding whatsoever.

This system has received a lot of component and reactor ground tests. The next step is flight test in space. That has yet to happen, but it seems likely that such a test would work fine. Even unmanned, some shielding may be required to protect spacecraft systems from radiation damage. It could also serve as an electrical power supply for a crew on Mars, if properly shielded.

Propulsion ideas that are nothing but concepts (meaning they are nothing but science fiction so far) ---

Fusion rockets

There is no controlled thermonuclear fusion yet. So, how could there possibly be fusion rockets? To claim otherwise, or to claim that such technology is near-term, is utter nonsense.

Magnetic confinement fusion has been “in work” without success since about 1950. Laser inertial confinement fusion is about 3 decades newer, but has also been worked-on for about 3 decades without success. There are also two “dark horse” candidates: Polywell fusion, and cold fusion.

Polywell has been funded for several years now at low levels by the US Navy. Success is still in doubt.

Cold fusion was mostly-discredited for experimental irreproducibility years ago, yet something unidentified was definitely going on. There has been very little activity since then, which is a mistake.

Laser sail propulsion

This could work if (1) materials and construction techniques were available for the lightsail, and (2) lasers of sufficient size and power actually existed. Both are currently lacking, so this is still science fiction until those technologies do exist. This lightsail must resist destruction by the powerful laser illumination, quite unlike a solar sail. The laser emitter needs to be of size 10’s to 100’s of meters, not a few mere millimeters. There is no known fundamental reason why the two supporting technologies could not be developed, but these will take considerable time and considerable expense.

The Bussard ramjet

This concept calls for scooping up interstellar medium (mostly hydrogen) with a gigantic magnetic field, let it ram-compress to the thermonuclear fusion point while confined by that magnetic field, and then expanding the products to very high speeds for thrust, using the confining magnetic field as a nozzle.

We have no ways to develop magnetic fields that intense and large (~100 miles scoop width). We do not have controlled fusion with deuterium/tritium, much less the more demanding ordinary hydrogen. We do not have the fusion rockets needed to boost the Bussard ramjet up to its minimum operating speed (on the order of half the speed of light).

This concept is still completely science fiction, and will be for a very long time to come.

Alcubierre and other “warp” drives

These things are only science fiction so far. Not even the physicists agree on what might be real and what is not real, as far as theories go.

Propulsion things tried experimentally but abandoned as “not practical” ---

Anything-containing-fluorine liquid oxidizers

Liquid fluorine and various fluorine compounds such as OF2 received experimental attention as rocket propellant oxidizers in the early 1960’s. The extremely toxic and corrosive behavior of the combusted stream (which contains hydrofluoric acid in high concentrations) are practical difficulties that simply preclude their application as rocket oxidizers.

Anything-containing-chlorine liquid oxidizers

Liquid chlorine received experimental attention as a rocket propellant oxidizer in the early 1960’s. The extremely toxic and corrosive behavior of the combusted stream (which contains hydrochloric acid in high concentrations) are practical difficulties that simply preclude its application as a rocket oxidizer.

Ozone as a liquid oxidizer

Liquid ozone is a more powerful oxidizer than ordinary liquid oxygen, but it is also extremely unstable. The gas spontaneously decomposes to ordinary oxygen on a timescale between an hour and a day. It can be liquified, but the cryogenic liquid will detonate (an unacceptable and violent hazard indeed) if it is allowed to approach its boiling point. These difficulties preclude its practical application except in dilute aqueous solution as a cleaning agent.

GW

Oldfart1939 · 2017-04-21 10:25:09

GW-

Just posting to say thanks for the effort expended to clarify the mystifying world of space vehicle propulsion to others participating on this website. As a one-time Aerospace Engineering undergraduate before my "military experience," and as a professional Ph.D. chemist by trade: you did a damned fine job!

You may note from some of my proposals, that I tend to emphasize current state of the art chemical propulsion, and not any SF schemes.

kbd512 · 2017-04-21 13:58:55

GW,

Magnetic confinement fusion has been unsuccessful at producing a plasma that is dense and hot enough to sustain the reaction for any substantial length of time suitable for generating electrical power without putting more energy in than what can be extracted, as a function of the extreme power requirements of the electromagnets to adequately confine and heat the plasma.

The fusion driven rocket doesn't attempt to confine the plasma so much as direct it. The actual confinement time is exceptionally short, so there's little in the way of thermal management issues. The plasma jet is expelled using an electromagnetic field, basically "shaped" like a rocket engine nozzle. The plasma itself is produced by very rapidly collapsing a thin Lithium foil liner around a D-T target. The foil absorbs most of the heat and radiation produced. It's basically a fusion pulse-jet engine.

Energy is stored in a bank of super capacitors charged by a solar array, the capacitors discharge into a coil to produce a rapidly collapsing electromagnetic field of increasing intensity that confines the D-T pellet in an enormous EM field between the liner and pellet, the pellet fuses, the Lithium absorbs the energy produced from fusion and is turned into a plasma, and the plasma is electromagnetically expelled. The burn times are measured in days, although the entire fusion process happens in a fraction of a second. It requires a shock dampening system to contend with the thrust pulse in order to prevent damage to radiators and solar arrays.

Oldfart1939 · 2017-04-21 17:08:49

One of my friends is a physicist at the NIF in Livermore, CA. They've yet to initiate a self sustaining Thermonuclear reaction. Yes, it CAN be done, but probably not soon enough to affect the outcome of Mars Colonization.

elderflower · 2017-04-22 05:42:55

I believe it can be done with addition of tritium but that gives all sorts of problems due to the excess radiation.

Tom Kalbfus · 2017-04-22 06:23:55

Oldfart1939 wrote:

One of my friends is a physicist at the NIF in Livermore, CA. They've yet to initiate a self sustaining Thermonuclear reaction. Yes, it CAN be done, but probably not soon enough to affect the outcome of Mars Colonization.

It all depends on when we colonize Mars. I think we may achieve sustained controlled fusion before I die, which I think will be sometime in the 2050s. I think we may need someone like Elon Musk to do it, rather than a huge government program.

elderflower · 2017-04-22 06:39:17

It is going to be available in 20 years time. It's been like that for at least 40 years.

Oldfart1939 · 2017-04-22 07:12:59

Nuclear fusion research has been hampered by paltry funding for a very long time. I wonder how much of the miserly funding is due to the energy giants and their lobbyists?

Oldfart1939 · 2017-04-22 07:28:30

GW-

I again reviewed my tables for the various fuels available for chemical propulsion, and it appears that the combination of hydrazine (N2H4) would have the best performance when combined with LOX, of all the possibilities available. At one time, Hydrazine was dirt cheap, but the introduction of many Federal Regulations during the early 1990s seem to have pushed the costs upward. It would appear to me that a real performance gain of ~ 6% would obtain over and above RP-1/LOX. The only downside is the relatively high melting point (freezing point for everyone else) that could make longer missions in deep space problematic without recourse to some tank heaters. The fuel oxidizer ratio is also attractive, making the hydrazine/LOX couple appear extremely promising. Your comments, please?

P.S. MMH and UDMH both have better freezing behavior, but at a sacrifice of some performance. In my mission model earlier, I resorted to one of the Hydrazine blends such as Aerozine 50.

Last edited by Oldfart1939 (2017-04-22 07:58:49)

elderflower · 2017-04-22 07:54:31

hydrazine freezing point can be depressed by mixing it with methyl hydrazine, as I understand.

GW Johnson · 2017-04-22 10:47:43

Hi Oldfart1939 and Elderflower:

My ancient Pratt handbook lists some fuel properties for the various hydrazines you both are looking at. Vapor pressures listed by temperature are measured in psia. Aerozine-50 is half hydrazine and half UDMH. I like MMH from a storability standpoint because it has the widest spread between FP and BP, and lower vapor pressures than UDMH or Aerozine-50. It is NTO that presents the worst storability problems relative to any of the hydrazines (so I put it in the table as well). The more-storable “storable” oxidizer is IRFNA, so I included it in the table as well. But it leads to lower rocket performance. IRFNA is 83% nitric acid, 2% water, and 15% NTO.

Mat’l…………formula…..FP,F….BP,F…VP 77 F…VP 160 F
Hydrazine … N2H4………34.7…236.4..0.28……..2.85
MMH…………CN2H6…….-62.3..189.5..1.0……….9.0
UDMH……….C2N2H8….-71.0…146……3.03…….18.5
Aerozine-50..mixture….18.8….158……2.75……15.1
NTO…………..N2O4………11.8…..70.1….17.2……111
IRFNA………..mixture……-63.4…150……2.57……19

My ancient Pratt handbook lists performance indices for some (but not all) of these hydrazines with liquid oxygen. Sea level Isp, sec is for expansion from 1000 psia to 14.7 psia at 100% nozzle kinetic energy efficiency (which should be closer to 98.3% for good designs). Vacuum Isp is for expanding from 100 psia (not 1000 !!!) through a nozzle area expansion ratio of 40, again for 100% nozzle efficiency. I’ve put the r values and chamber c* (ft/sec) in the table, too.

There’s LOX-two hydrazines, with LOX-methane, LOX-kerosene, and LOX-hydrogen for comparison. I also included for comparison all 4 hydrazines used with NTO. There’s not a lot of variation in delivered Isp performance for them. So, as a guess, there might not be much variation among the 4 hydrazines with LOX. But I do not know that outcome for certain. The two listed equal or exceed LOX-methane performance.

I also put IRFNA-UDMH in the table so you can see the performance decrement versus NTO-UDMH, and I included IRFNA-kerosene, so you can see the performance decrement switching out UDMH for kerosene when using IRFNA. It’s similar to the decrement switching out UDMH for kerosene when using LOX. They’re both about 10 sec of Isp.

Combination…..ISL…..rSL…..c*SL……Ivac…..rvac…….c*vac
LOX-hydrazine..313...0.90…6220…..367…..0.95…….6060
LOX-UDMH…….310….1.67..6120……364….1.67……..5960
LOX-methane…310….3.15..6120……365….3.25……..5960
LOX-kerosene…299….2.55..5900……351….2.6……….5730
LOX-LH2………….388…4.0…..7950…...454….4.5………7840
NTO-hydrazine..292..1.33…5860…….342…1.36…….5740
NTO-MMH………288…2.17…5730…….338…2.26…….5570
NTO-UDMH…….287…2.6…..5680…….336…2.7……….5520
NTO-Aeroz-50…287…2.0…..5560…….339…2.0……….5570
IRFNA-UDMH….272…3.1…..5410…….320…3.2……….5290
IRFNA-keros……263….5.0….5180…….309….5.1………5070

Some might quibble with the posted numbers, saying my vacuum numbers look low. But look carefully at the conditions for which they were calculated. Vacuum performance is at 100, not 1000 psia! As chamber pressure increases, c* increases, and so does pressure ratio across your nozzle. Those drive up Isp. That is why vacuum LOX-LH2 performance in the shuttle engines at 3000 psia (and I don’t know what expansion ratio) was reported to be 467 sec Isp.

Same thing applies to LOX-kerosene, or any other combination. That’s why Spacex is getting higher Isp numbers out of its Merlin engines than the table above indicates for LOX-kerosene. I’m not sure, but I think I saw chamber pressure near 5000 psia listed for the Merlin 1-D.

To really determine Isp performance, you have to do the ballistics from scratch, and that requires knowledge of the actual final chamber pressure, and the bled-off massflow(s) that run the turbopumps. You need real experimental test data to define both optimal r and delivered chamber c* as functions of chamber pressure, and you need a value for gas specific heat ratio. The nozzle geometry, gas specific heat ratio, and operating nozzle pressure ratio define your thrust coefficient by very well-accepted calculation methods. But be sure you are applying the geometry-dependent kinetic energy efficiency to the momentum term but none of the pressure-area terms in thrust. That is the most common mistake made.

Nozzle geometry and chamber pressure, plus that thrust coefficient, multiply together to define your delivered thrust. The “theoretical” Isp would be thrust divided by nozzle massflow, but what you get in practice is lower due to turbopump bleed masses dumped overboard. You need to divide thrust by the total massflow. That lower effective value is what relates Isp to stage mass ratio and delta-vee potential.

GW

Last edited by GW Johnson (2017-04-22 10:49:19)

GW Johnson · 2017-04-22 10:55:42

kbd512:

The fusion rocket you describe is a paper concept. I know of no tests or hardware anywhere that embody this concept. I actually think it could work, but we won't know until it is tested thoroughly. Like so many other things, this is a promising approach, but nobody is seriously funding any experiments.

I don't know what the potential is for mishap in such tests, but this is a nuclear thing with an exiting high speed jet driven by effective high pressure. There has to be some explosion hazard. I think this, the nuclear thermal concepts, and nuclear pulse propulsion ought to all be seriously researched and tested.

The safest place to do this without gigantic facilities to contain plumes is the moon. I think that might be the most important reason to return to the moon: a safe place to experiment with hazardous nuclear stuff. You need a stable bed for a thrust stand. This cannot be done hanging weightless in space where every test is a flight test.

GW

Last edited by GW Johnson (2017-04-22 10:59:32)

Oldfart1939 · 2017-04-22 12:32:36

GW-

I believe the new SpaceX methylox engine funnels the turbopump bleed back into the reaction chamber. The numbers are pretty phenomenal, and Musk stated in one of his talks an Isp of 383 sec (vac).
I wish SpaceX would consider a performance upgrade over and above RP-1 for their Falcon 2nd stage by using Hydrazine/LOX. Yes, this is just a "tweak," but it could provide an additional 5% burn time as a result. Now that Elon is talking about trying to recover the second stage, that could be significant.

elderflower · 2017-04-22 14:03:20

I'm just kicking around possible ISRU propellants, I think it is fairly clear that the earliest ones would be LOX/LCO as it has the simplest system to generate them. I like simple. This would power a hopper with range of a few hundred kms and keep its systems going for a few days, then return it to base for a refill and crew change. Maybe it also forms the second stage of the mars ascent vehicle. Each mission base will need at least two of these hoppers so that a crew can be collected from a failed unit.
There is little point in sending a crew of six people to any particular spot on Mars for two years if you don't give them a good degree of mobility. They would come up with a lot of detail about the surrounding twenty kms or so, but nothing about the fascinating and potentially useful stuff over the mountains. I don't think surface transport is likely to be satisfactory, its a trackless desert and there are no camels.

kbd512 · 2017-04-22 21:24:38

GW Johnson wrote:

The fusion rocket you describe is a paper concept. I know of no tests or hardware anywhere that embody this concept. I actually think it could work, but we won't know until it is tested thoroughly. Like so many other things, this is a promising approach, but nobody is seriously funding any experiments.

This is from the University of Washington's Web Site:

Rocket powered by nuclear fusion could send humans to Mars

MSNW LLC has various pictures of the foil liner formation and compression cavity (combustion chamber equivalent) and the electromagnetic nozzle (rocket nozzle equivalent) devices in their labs, so there is that.

Is it a fully integrated system? No. It's in pieces. The super capacitor banks and solar panels are COTS hardware, so Maxwell super capacitor modules and ATK MegaFlex solar arrays. The Lithium metal storage tanks are not COTS hardware, but there are storage tanks for Lithium metal. The foil liner formation unit basically "squirts" a thin "rubber band" of molten Lithium into the cavity, the electromagnets compress the foil around the D-T target, and it fuses. Whether or not that works hasn't been a question since 2012 or early 2013. I can't remember the exact date that they made that work.

The foil liner captures most of the neutron radiation, but the gamma rays, which are hard to shield against, are only attenuated by a shadow shield consisting of the device itself, the Lithium metal storage tanks, and distance.

GW Johnson wrote:

I don't know what the potential is for mishap in such tests, but this is a nuclear thing with an exiting high speed jet driven by effective high pressure. There has to be some explosion hazard. I think this, the nuclear thermal concepts, and nuclear pulse propulsion ought to all be seriously researched and tested.

Could anything possibly go wrong? Seriously?

It uses molten Lithium metal from a storage container loaded with multiple metric tons of highly reactive pure Lithium metal, contains bottles of radioactive Tritium, sends a 1 mega amp pulse from a bank of super capacitors into an electromagnet, and delivers 26,552,237lbf/s (even if only for a split second), producing a respectable gamma ray output in the process. Nope, nothing dangerous going on there. Even though there are very few moving parts, the potential for something to go wrong is ever-present. Want 5100s Isp and incredibly high thrust? If so, then accept the fact that whatever produces that kind of power will be inherently dangerous.

GW Johnson wrote:

The safest place to do this without gigantic facilities to contain plumes is the moon. I think that might be the most important reason to return to the moon: a safe place to experiment with hazardous nuclear stuff. You need a stable bed for a thrust stand. This cannot be done hanging weightless in space where every test is a flight test.
GW

We're doing these tests in a lab on a university campus. There are safety procedures in place. Nobody is near the device when they're crunching foil. The foil used in the crunchatizing (yes, I just made that word up) tests is Aluminum, which works just as well as Lithium. Lithium is lighter than Aluminum, so actual rocket engines will use Lithium.

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