Why we need fast flights to Mars.

RGClark · 2020-06-14 10:19:40

Astronaut who spent 197 days on the ISS shows how hard it is to walk on Earth again.
https://twitter.com/amazlngscience/stat … 23494?s=21

This is why we need fast flights to Mars. Counterintuitive fact: it’s actually EASIER than doing a long, ca. 6 month duration flight.

C.f.:
https://exoscientist.blogspot.com/2015/ … etary.html

Bob Clark

GW Johnson · 2020-06-14 11:07:41

Hi Bob:

What you say is true. But, it's a tradeoff. If you fly slow, you need spin gravity to stay healthy. There is no way around that, although whole bunches of folks still deny it. But you need propellant depots if you want to fly fast. No way around that, either. The trouble with both is that they are harder to do in actual practice than the concepts would suggest.

I didn't mention radiation protection in that tradeoff, because you have to have it either way you go (fast or slow). It's the solar flare event, not GCR, that demands this. And it's still true to a reduced bit, down on the surface of Mars, because it lacks a magnetic field.

If you have to pre-position propellants shipped from Earth at your depots, that pretty much makes them impractical to actually do. Which is why depot propellants need to be made from local materials. The "kicker" is that too many folks deny the accumulating result that small asteroid bodies are dry of volatiles. Only the big ones have volatiles. Asteroids showing tails does not necessarily mean the presence of volatiles; some of these seem to be thermal-shock generated dry dust.

What that means for Mars missions is that both Phobos and Deimos are very unlikely to contain any volatiles that you could use to make propellant. These seem to be rubble piles with high void fractions, surfaced with a space-weathered hard crust that is punctured and fractured. They and the small C-type asteroids, seem to be all about the same. The M-type would have no volatiles, and the S-types are intermediate between M- and C-types.

The icy stuff is way out in the outer solar system from Jupiter out into the Kuiper belt, not anywhere in the inner solar system. Even the comets we have seen so far seem to be more rocks and dust than any ices. They are very hard to reach, because of their orbits.

It's big main-belt asteroids like Ceres that may really have volatiles inside. But even the white splotches seen there have proven to be salt, not ice. So if the volatiles are really there, they are buried much deeper inside. That means drilling or tunnel-mining are required to get at them. You have to get there to start that mining to set up a depot, and you have to make that initial trip without that depot. Chicken-and-egg. Plus, the main belt is much further away from us than Mars. Doesn't really help us for a Mars mission. Unless you predicate that Mars mission upon setting up a propellant depot on Ceres, and shipping its output to Mars, before we go to Mars. It would seem easier to just go "slowboat" to Mars, using spin gravity to stay healthy.

There might be some ice at the lunar poles. There does seem to be at least some. But the rest of the moon has so far been looking rather dry of volatiles. You might be able to create a propellant depot with local propellant manufacture at the lunar poles. But I doubt very strongly that any of the near-Earth asteroids, the moons of Mars, or even the vast bulk of small main-belt asteroids have any volatiles you could use.

That leaves us just the moon, and any depot there is at the bottom of a significant gravity well. Not a pretty picture, but it seems to be the best we have.

GW

Last edited by GW Johnson (2020-06-14 11:22:16)

GW Johnson · 2020-06-16 12:45:29

BTW, I did look at repeat-pass aerobraking for entry into low Mars orbit. Those results got posted over at "exrocketman" today. It'll work, but you must combine it with some propulsion, to cover the uncertainties induced by highly-variable upper atmosphere density. If you don't, there's a more-than-fair chance of a fatal outcome.

GW

GW Johnson · 2020-07-26 11:09:20

Thinking about this topic, there's a second reason for flying fast, at least in the vicinity of the Earth. The Van Allen radiation belts. You have to cross them fast, or else suffer bad exposure, up to even fatal. The moon and Mars do not have such, but Earth does, because its magnetic field is relatively strong.

That issue rules out sending men, or cargo vulnerable to radiation damage, slowly spiralling out from Earth with electric propulsion. It also rules out spiralling in with electric propulsion when returning to Earth.

I think this same issue will impact substantially the notion of using repeat-pass aerobraking at Earth. That requires the use of rather elongated elliptic orbits with perigee in the atmosphere. Problem: the base of the radiation belts is only about 900 miles up. Elongated elliptic orbits put much of your trajectory out in the radiation belts. That's also the slow end of your trajectory, so you will spend a lot of time immersed in the danger on each pass.

Any ship traveling outside LEO is going to need a solar storm shelter, which entails some sort of radiation shielding. Solar storms are the lethal threat, not galactic cosmic rays. We on these forums have looked at that issue before. So far, it is the 1972 event between 2 Apollo moon missions that sets the shielding requirement at 15-20 g/sq.cm. I don't know if that's enough to protect a crew from the Van Allen belts, but my guess is that it is.

Be that as it may, you'll have to shield the whole manned ship to travel slowly through the Van Allen belts. The crew cannot stay cramped inside a tiny shelter space, for weeks or months on end. Gemini 7 proved the 2 week limit for cramped quarters.

We have high-Isp electric propulsion, but its thrust/equipment mass is the tiniest of whispers. That puts vehicle accelerations in the milli-gee to micro-gee range. Speeds builds up, just ever so slowly. Fine for unmanned stuff, but not so much for crewed vehicles.

Our chemical propulsion is high thrust but lower Isp (a couple of hundred to 400-something seconds). Solid core nuclear thermal is only factor 2 better on Isp, and hurts some on engine thrust to weight (by about a factor of 6 to 10).

I think you have to use a combination of the two for crewed missions. High thrust gets you moving quickly, and gets you quickly out of bad spots like the Van Allen belts. During what would otherwise be long coasts with Sir Isaac Newton in the driver's seat, that's when you speed things up a bit by using electric to accelerate to midpoint, then decelerate to arrival.

To do better than that, we're going to need some real breakthrough propulsion results, like gas core nuclear thermal, or an updated form of nuclear pulse propulsion. I wouldn't hold my breath waiting on things like "warp drive", though. That stuff is speculative at best. May not even be scientifically sound, or maybe it is; my real point is, we don't know. We do know with rocketry.

GW

Last edited by GW Johnson (2020-07-26 11:13:19)

kbd512 · 2020-07-26 16:16:54

GW,

You don't have to spiral into LEO. You could also spiral into various lunar orbits, for example. However, adequate braking thrust can also be provided by a combination of electric and chemical propulsion. The electric propulsion can bleed off the greatest portion of the entry velocity prior to entering the Van Allen belts, followed by an impulsive burn using chemical engines to establish a stable orbit.

SpaceNut · 2020-07-26 17:07:52

https://apollofusion.com/acemax.html

GW Johnson · 2020-07-27 08:38:47

Kbd512 and I are actually in agreement regarding use of combined chemical and electric propulsion. I would add solid core nuclear thermal to the mix as a feasible near-term item.

I reiterate that there are severe limits to the orbits achievable about Earth because of the Van Allen belts. Any manned craft flying into spaces above about 800-900-1000 miles will need effective radiation shielding for its crew. That has been known since the discovery of the existence of the Van Allen belts by Explorer 1 in 1958. The exception is the South Atlantic anomaly, where the radiation extends down to LEO altitudes. And the poles, where radiation particles hitting the atmosphere make the aurorae.

GW

tahanson43206 · 2020-07-27 09:39:22

For GW Johnson and kbd512 ...

Your conversation in this topic included (from memory) a suggestion that a hybrid design (chemical rockets for major impulse), and ion drives for sustained though (very) small acceleration might be worth evaluation.

I'd like to invite your combined consideration of a hybrid design that features a massive chemical acceleration at the launch from Mars, followed by sustained deceleration to put the vessel in orbit around Earth. Because of the warnings (from GW Johnson in particular) about the Van Allen belt, I'd like to invite consideration of GEO as the target orbit.

A vehicle returning from Mars could be reached by ferries from Earth, which would be designed for rapid movement through the Van Allen belts.

I'm hoping a well run spreadsheet program could compute the tradeoffs that would make such a hybrid travel plan practical.

In order for this idea to receive serious attention, defeating considerations should be set aside.

Defeating considerations would include cost.

Let those who specialize in dealing with finances worry about costs. The result I am hoping for here would be a scenario that would deserve serious consideration by future mission planners.

(th)

Last edited by tahanson43206 (2020-07-27 09:40:00)

SpaceNut · 2020-07-27 17:01:52

The problem is mass to duration of the escape burn for the delta that we need.

Calliban · 2020-07-27 17:49:25

One promising high-thrust, high-ISP propulsion system that gets around the EMP problem, is muon catalyzed fusion-fission micro explosions. A kind of mini-Orion.

Imagine a thin spherical shell of 235U, about 1cm in diameter, containing liquid tritium-deuterium. A high energy proton beam hits the outer uranium shell, generating a shower of pions, which rapidly decay into muons inside of the pellet. The muons catalyse fusion, yielding fast neutrons which cause fission in the outer shell and alpha particles, which heat the hydrogen. The temperature of the hydrogen rapidly rises and the pressure exerted by the uranium shell increases until the fusion burn within the pellet is self-sustaining. Note that the muons are only there to start the reaction, by creating sufficient heating.

Most likely, detonations will take place at a rate of several hundred per minute, but the low yield of each pulse will minimise EMP effects on the upper atmosphere. Each pulse will create a burst of high energy plasma, which will be directed along the axis of the ship by a restraining magnetic field, generated by a conducting circumferential loop around the engine.

Last edited by Calliban (2020-07-27 17:52:15)

tahanson43206 · 2020-07-27 18:58:35

For Calliban re #10

The system you've described seemed a good fit for the "impulse engines" of Starship Enterprise. Those were never described (that I've ever seen) except that they were for use when the Warp Drive was inappropriate (such as achieving an orbit with a planet). The Enterprise would not have been using chemical rockets, so something like what you described would seem about right.

Edit#1: I'm assuming your source has worked out the physics so I won't ask about that. However, the manufacturing process for the pellets would seem a bit advanced for humans right now. Did your source offer any suggestions for making those little pellets?

(th)

Last edited by tahanson43206 (2020-07-27 19:00:55)

kbd512 · 2020-07-28 10:56:31

I still think that future rockets will be fusion driven. An important distinction must be made between trying to generate any amount of electrical power from fusion and simply achieving fusion to begin with. Thus far, we've been wildly successful initiating fusion, just not in sustaining it, primarily because we're searching for a process so efficient that it can sustain fusion continuously and actually produce useful amounts of output electrical power. That last bit is a very tough nut to crack and results in a truly massive device. The first part is inordinately simpler and doesn't require fusion reactors the size of city blocks. Desktop devices have been initiating fusion since human-initiated fusion was "a thing". The entire drive behind fusion power has been to generate electrical power, but rocketry would benefit enormously from a small, lightweight device, comparatively speaking, that accepts input power and produces super heated plasma from fusion.

If that plasma is directed out the back of the rocket, then you get a ton of thrust at very high Isp (dozens of megawatts of jet power, with efficiency very comparable to a turbofan engine), ranging from 2,000s to 5,000s. It'll never be a replacement for a chemical engine since TWR isn't greater than 1, but it doesn't have to be. Propulsion in the void of space is all about the fuel economy and the Sun provides more than enough input electrical power to "pulse" a bank of super capacitors that cause a thin Aluminum or Lithium foil liner to evenly "crush" around a Deuterium-Tritium (D-T) pellet, creating an exponentially increasing magnetic field around the D-T pellet and initiating fusion as a result (it relies upon a supersonic implosion, much like a fission-initiated nuclear weapon).

That said, what has happened in all scenarios to date that attempt to generate electrical power output from fusion is that the input power required to achieve sustainment exceeds output power, given current thermal-to-electrical conversion efficiencies. Typically, we lose containment over the super-heated plasma because holding onto that plasma is likely trying to wrap your hand around greased jello. The harder you squeeze, the more the jello wants to come squirting out of the containment field. However, we've recently discovered through experimentation that much stronger electromagnetic fields seem to have a "stabilizing" effect on the otherwise unstable swirling plasma maelstrom (the tendency of that super heated plasma generated to fuse D-T molecules to fly out of the toroidal magnetic containment device and strike the walls of said tokamak), keeping it safely confined and away from the walls of the containment vessel. Lockheed-Martin's "high-beta" fusion reactor presents an exponentially increasing magnetic field to plasma trying to squirt out of the electromagnetic containment. The "only" problem, of course, is that those much stronger electromagnetic fields also require "that much more input power".

So, imagine for a second that we intentionally allow plasma to come squirting out of our containment device, except that we use the electromagnets to direct the plasma out the back of the rocket, producing lots of thrust in the process. That's pretty much what the fusion driven rocket concept that Dr. Slough and Dr. Kirtley came up with using NASA's seed money. It's a linear pulsed fusion electromagnetic accelerator, very similar to a railgun that intentionally vaporizes its armature (around the D-T pellet, obviously, which is where the incredible heat comes from) to generate thrust. You have tanks of Reynolds Wrap (standard Aluminum or exotic Lithium foil varieties) that shield the payload from any radiation nasties and the "rocket nozzle" is an electromagnet. They recently came up with an even more advanced version of the basic concept that doesn't put so much stress on components by pulsing huge amounts of electricity through them, which is what NASA is currently funding, with respect to this technology. Instead of pulsing super caps through a linear accelerator, the new "version" of the design supersonically accelerates the foil liner and D-T pellet into a constant force electromagnetic field, thus creating that exponentially increasing magnetic force instead of generating an exponentially increasing field with a burst of power that is also trying to twist the structure holding onto the electromagnets into a pretzel. It's a much better design from a long-term durability standpoint.

Why is this so important?

Well, you can turn the device on and off easily because there's no massive thermal load and very low residual radiation to deal with, it doesn't require gigantic radiators or any cryogenics to cool it because the vaporized foil carries the heat out the back of the rocket nozzle, and you can create devices of arbitrary size / thrust / specific impulse levels after the minimum size has been met. The minimum size device is roughly the weight of a truck (not a fully loaded semi-tractor trailer, just the truck itself). Only tiny quantities of the "fusion fuel" are required and the "thrust fuel" is a light alloy metal, so it's trivial to store it in space. The fuel economy is so good that you can have 50% payload and less than 50% fuel for your prototypical 6 month trip to Mars. A 60t payload is connected to a 8t to 13t fusion engine (fuel tanks, electromagnetic accelerator, solar panels, super capacitors, radiators, control electronics, absolutely everything required to light the candle) requires a bit less than 50% Aluminum or Lithium fuel (Lithium is preferred since it's much lighter than Aluminum, is still very soft and malleable, and deforms like plastic at supersonic speeds, which is what you want / need, meaning it doesn't tear apart when being "crushed" around the D-T pellet).

Your "thrusting periods" last a matter of hours, vs seconds with chemical or minutes with nuclear thermal or months with ion engines, but the end result is the same and the craft can achieve any velocity that the available fuel supply will permit. It's very possible to do "fast transits" with less payload, meaning 3 month trips. The 60t payload included a 23t Orion capsule since that's what NASA specified, but if you stage your landers and supplies at Mars the way Lockheed did in their plans, then you can do very fast transits with propulsive captures into low orbits on both ends, so no risky aero-capture maneuvers are required. The solar arrays also provide lots of juice for scientific instruments during the cruise phase of the transit.

We know how to store Aluminum and Lithium in space. We've been doing it in the form of the ISS for decades now. We know how to lose containment of a fusion plasma (we're batting a thousand in that department- it's the one operational aspect of nuclear fusion that we don't know how to screw up). We also know how to create over-glorified rail guns. In short, we can actually do this. Yes, further development will be required and yes that will require additional funding, but it's not like development of a nuclear thermal rocket engine at all. We don't need a standing army of nuclear scientists and a 50-mile exclusion zone established at Jackass Flats to test this engine. We can do it in the basement of properly equipped university plasma physics laboratory buildings rather than reactor halls. We can then conduct on-orbit tests without fear of exploding reactors producing radioactive debris, even if the device reenters Earth's atmosphere. No, you can't stand inside the device when it's operating, mostly because you'd be crushed and/or vaporized, but the minimal neutron radiation produced from D-T fusion in operation pales in comparison to any kind of fission reactor.

Let's get the best of nuclear thermal and ion engines wrapped into one small, easy-to-ship package. A pair of Falcon Heavy rockets can deliver the payload and propulsion system to LEO, whereupon a Dragon and Falcon 9 can deliver the crew, and then the vehicle can go to Mars orbit and then come back to Earth. The faster we "do this", the faster we can prove to everyone that humanity can venture into deep space, stop by another planet, explore a little bit, come home, and "live to tell the tale". Later on, colonists can go and stay for as long as we can keep people healthy on Mars. This is the only type of mission solution that can use commodity rockets and commodity mission hardware (less the specialized engine, of course). If Elon Musk can deliver tonnage to orbit 10 times cheaper with his mega rocket, then great, let's do that. However, 1t of light alloy per 1t of payload delivered will ultimately be cheaper / easier / faster than thousands of tons of cryogenic liquids.

Aluminum is $1,700/t
Lithium $13,000/t
LNG $231/t

The fuel contained within Starship to reach Mars costs at least $277K, but that doesn't include the cost of the fuel for the 5 tanker flights to deliver the fuel required to send Starship to Mars. Even if you were using Lithium as fuel, you still don't achieve the operational costs associated with refueling a single Starship to depart for Mars (unless someone here actually believes that 5 Saturn V class tanker flights will be had for less than $1.3M, plus the cost of fuel for a single rocket to leave Earth).

We will need a fleet of ships to send a million passengers or we will need ships that make SpaceX's ITS look like a toy. We simply can't colonize another planet with the efficiency of chemical rockets. It's time we accept that reality so we can move forward. This is a qualitatively better solution that will drastically reduce transportation costs. We will still use chemical rockets to leave Earth for some time, but once we're in space we need the fuel economy of an airliner for this dream to become real. Instead of being dumped into landfills, all those tens of millions of tons of empty soda cans can enjoy a more useful second life shipping cargo to Mars.

Calliban · 2020-07-28 14:29:28

Kbd, what you are describing sounds like z-pinch. I had no idea that this was so far along.

One problem, 80% of the energy from De-T fusion is carried away by neutrons. Presumably, these stream out of the engine?

One thought: there is aluminium on the moon, as a composite oxide (anorthite). Could this device produce sufficient thrust against lunar gravity to achieve takeoff from the surface and landing? A ship that could travel from LEO to the lunar surface and back on a single tank of fuel would be valuable. The moon is close enough, that the ship could achieve hundreds of round trips in a realistic operational lifetime. That is airliner economics.

Last edited by Calliban (2020-07-28 14:30:28)

GW Johnson · 2020-07-28 20:35:25

For Tahanson43206 re post 8:

I'm sorry, I have no idea how to compute a thrusted trajectory for electric propulsion, without a real orbital-type trajectory code. Without that, there is no estimating of reliable delta-vees, and thus no possibility of spreadsheet-amenable rocket equation work.

It has been nearly 50 years since I did that kind of trajectory work with a company-owned code named NEMAR for Vought, while working on "Scout". I never had a copy of that code for myself, and the default central body in it was Earth, and only Earth. It had a model for the Earth's atmosphere, and used standard compressible flow aerodynamic drag tables based on Mach number and angles-of-attack and -yaw.

There is nothing I know of, about that thrusted (and maybe draggy) trajectory analysis, that is amenable to spreadsheet work, and that would allow analysis of long-duration thrusted (and drag-susceptible) systems. We would need one that is usable at places other than Earth, as well as at Earth itself. NEMAR was very definitely NOT that generalized!

My orbital mechanics capabilities are the classic undergrad textook stuff of unthrusted vehicles in a two-body system. You combine that with "impulsive" delta-vees to change trajectories. Impulsive means essentially "zero" burn time. Really, just "short" times. Which electric propulsion is NOT.

That sort of 2-D unthrusted thing actually is amenable to spreadsheet work, but it is quite limited indeed, compared to what we want to investigate. Limited to the point of inapplicability.

Sorry. I wish I could help. But I cannot. I do not know how, and I have no appropriate tools, even if I did know how.

GW

SpaceNut · 2020-07-28 21:10:05

Minimal earth orbit escape followed by steady ion engine to achive better than hofhman transfer coasting with course corrections.
https://space.stackexchange.com/questio … n-transfer

https://en.wikipedia.org/wiki/Escape_velocity
http://www.phy6.org/stargaze/Smars2.htm
https://jatan.space/the-moon-as-a-rocket-platform/

delta-v-earth-moon-mars.jpg?fit=1024%2C576&ssl=1

Once in orbit, a further boost of about 3.2 km/s will break you free of Earth's gravity. If a rocket applies that boost, it will enter an orbit around the Sun very similar to the Earth's and slowly drift further and further from the Earth.

tahanson43206 · 2020-07-29 06:51:49

For SpaceNut re #15 ... Nice!

For GW Johnson ... "attitude" is a thing.

The word is sometimes used (or it can be used) as a synonym for "angle of attack"

In past posts, you have provided enough personal history so that I can be confident you've still got a number of Earth years left to make contributions.

It is true that as we collect candles, we lose capability.

The local paper recently featured a half page image of a local farmer, tossing (I mean ** tossing ** ) hay bails onto a stack above his head. A helper half or a third his age was in the image, clearly dodging to avoid the bail as it arced onto the stack. The reporter made much of the fact this gent is 74.

In the interview, the reporter expressed wonder at the performance, and the farmer disclaimed anything special, saying only that you had to keep at it.

Suppose that a longevity method were to suddenly appear, and you were offered a chance to live another thirty years at the level you find yourself in now.

You would have a choice of setting out to learn something new, or staying with 50 year old skills and contributing with those.

It is possible you could learn something new, such as computer codes to perform at the level needed for planning interplanetary flights with or without constant thrust systems.

It is also possible your ability to learn has faded along with other powers. I can only offer encouragement.

One thing I'm sure of, from having seen it happen many times, is that an older person with a major academic achievement and significant accomplishment in the real world can inspire younger people to work together to accomplish things not thought possible.

(th)

kbd512 · 2020-07-29 10:30:45

Calliban,

Imagine you have an electromagnetic funnel. You fire a small cylinder of Reynolds Wrap with a D-T pellet trapped inside of it down the funnel. As the foil liner decreases in size, the electromagnetic field between it and the D-T pellet inside it exponentially increases. If you do that at supersonic speeds, the D-T pellet fuses from the force generated by the electromagnetic field, the foil liner absorbs the pulse of neutron radiation causing the foil to vaporize, and the electromagnet carries the vaporized foil out the back, creating thrust. It's a pulsing cycle, so the repetition rate is limited by available power input and the heat rejection of the electromagnetic funnel structure. If the power is supplied externally, then heat rejection and the structural integrity of the electromagnetic funnel structure become the limiting factors.

I would say FDR is not suitable for takeoff from the lunar surface because TWR is nowhere near 1. For some reason, Dr. Slough and Dr. Kirtley measured thrust in megawatts. The first generation device is supposed to provide 36MW of jet power (for a tiny fraction of a second). They're talking about pulsing this device at a rate of roughly 10 to 12 times per second, so you really only get a tiny fraction of a second of that total "jet power", per second. A 180kW solar array is required to maintain that repetition rate and was sized so that it could provide adequate thrust to come home from Mars orbit. A "burn" to break orbit is supposed to take between 12 and 24 hours or so. If you can pulse faster, then you can accelerate faster.

1MW = 101,971.62 kgf per m/s

Each event may be 1/100th of a second in duration, so maybe 3,671 kgf/s for a device with 36MW of jet power (barely enough to lift itself off the ground, much less anything else). Sure, you have 1/6g on the moon, but add the fuel for that dV increment and payload and you end up with nothing left to accelerate. It could definitely be used as an upper stage combined with a reusable booster, but I would use a rail gun to shoot Aluminum "fuel" into orbit around the moon. That's probably the most efficient method. Here on Earth, that equates to ~28kWh worth of electricity per kg of payload sent into orbit, though obviously a lot less than that on the moon.

kbd512 · 2020-07-29 23:05:34

GW,

JPL apparently can at least demonstrate some of this stuff using spreadsheets. I'm sure it's not as accurate as proper mission planning software, but we tend to "ballpark" things here to show what's feasible:

Ion Propulsion: Using Spreadsheets to Model Additive Velocity

Math Encounters Blog - Ion Propulsion Math

"The Book":

Fundamentals of Electric Propulsion: Ion and Hall Thrusters by Dan M. Goebel and Ira Katz - Jet Propulsion Laboratory - California Institute of Technology

This was mildly entertaining and he even provided the Python program he wrote:

What’d Make a Better Rocket, Nuclear or Ion Engines? by Rhett Alain

So far as I know, integrating the result using a given number of time steps is the only way to do it accurately.

What I would like to know is, how does the gravity field affect the results as you gradually accelerate away from Earth. It must be gradually decreasing as you spiral out. The gravitational effect of the Sun must also have some effect on the craft, given the thrust levels and time periods involved.

Actually, reading this paper answered my questions and I have the equations now, so never mind:

Design and Performance Analysis Study of an Ion Thruster by Carlos Sánchez Lara

The salient bits are in "Chapter 3: Mission Analysis", for anyone else who is interested.

The short answer to reducing travel time using any kind of low-thrust, long-duration propulsive maneuver seems to be starting with an advantageous orbit from the chemical propulsion portion of the maneuver.

This is a good "quick and dirty" explanation of how electric propulsion design parameters affect trip time, delivered payload tonnage, etc:

ANALYSIS OF HALL-EFFECT THRUSTERS AND ION ENGINES FOR ORBIT TRANSFER MISSIONS by Frank S. Gulczinski III and Ronald A. Spores - Propulsion Directorate - OL-AC Phillips Laboratory - Edwards AFB, CA 93524

I really liked this one because it gets into issues with current state-of-the-art technologies:

Ion thrusters for electric propulsion: Scientific issues developing a niche technology into a game changer - AIP

Perspectives, frontiers, and new horizons for plasma-based space electric propulsion - AIP

Edit:

This is a good general trade space analysis for using different types of propulsion for transits to and from Mars:

Comparison of Four Space Propulsion Methods for Reducing Transfer Times of Manned Mars Mission

Last edited by kbd512 (2020-07-29 23:16:44)

kbd512 · 2020-07-30 03:00:00

All,

If anyone actually bothers to read that last document in Post #18, you'll finally understand why solid core NTR's provide very little tangible benefit until your total spacecraft mass is well above 1,000t, which is why NASA hasn't pursued it more vigorously. A DRA 5.0 class mission doesn't see much benefit from using a NERVA-derivative NTR. It's better than pure chemical or chemical plus electric, just not a lot better. The RL-10 CECE provides equivalent or better delivered payload mass fraction, presuming transit times are "a thing" (and they are) up to that point due to the weight of the nuclear engines. You don't get remarkably better results until your total spacecraft mass is 1,750t or greater, well above DRA 5.0 specifications. Until your total spacecraft mass reaches that point, solid core NTR doesn't make much sense.

It's funny how time-of-flight and mass restrictions work against us for a Mars mission no matter what technology we choose. Maybe GW was right about artificial gravity being mandatory for extended duration deep space missions so we can stay in space for as long as our provisions hold out. If the radiation shielding and consumables are present, then longer missions that are AG-enabled (real spaceships) likely pose less risk to the astronauts or colonists undertaking them and then we can mass-optimize that mission's propellant mass and sacrifice time-of-flight, which is still a good trade if your people are well protected by a heavier but slower ship.

From the article:

Our interpretation of these results is that continuous thrust is not the best choice of propulsion technology for the proposed goal of minimising the ToF for this architecture. Low intensity continuous thrust requires time to be effective, but to diminish the travel time is exactly the goal. As the technology improves, by improving the specific impulse or the thrust-to-weight ratio, the travel time diminishes, making the propulsion less effective and requiring more chemical fuel for the capture. This agrees with the existing literature: keeping the thrust-to-weight ratio constant and increasing the specific impulse increases the interplanetary transfer time while the interplanetary propellant mass decreases (i.e. changing the value of Isp has a positive effect on the mass, but a negative one on the transfer time) [34]. Whereas the opposite effect for the transfer time and propellant mass is seen when the Isp is fixed and the thrust-to-weight is increased [34]. The surprisingly small increase in performance suggests that large gains cannot be achieved even with future technological developments of the continuous propulsion.

Translation:
Only high-thrust impulsive engines / maneuvers materially improve ToF to Mars from Earth using current and expected future technology.

Comparing the classical chemical solution with the 25 engines electric solution, we observe that adding to a largely optimised CECE system, a low mass and high Isp system such as the electric propulsion, leads to improvements, in spite the limitations of the electric propulsion in increasing performance emphasised in section 4.3.
When using impulsive systems, given their low Isp, an average of about 80% of the spacecraft mass is fuel (here including propellants and respective tanks to transport them), for this mass range. When adding the electric system this drops to about 75%, as the electric system is introduced not only for the spacecraft to reach its destination faster, but also for saving fuel by braking before arrival. Of this 75% fuel mass, only ~1% corresponds to the electric system fuel.
Combining all EtM and MtE missions (as described in section 4.1), we determine the waiting time, total mission time and total mission mass. For the missions options presented in Fig. 6 the minimum waiting time achievable is ~520 days (which corresponds to total mission times higher than ~900 days). On the other extreme (total mission masses close to the 2,500 t limit and 850 days of mission time) the waiting time rises to ~570 days, for continuous thrust options, and to about 660 days, for the classical chemical and nuclear thermal options. The classical options show waiting times consistently higher.
The waiting time evolution exposes its relation with travel time (and consequently the heliocentric angle). Even if we travel faster to Mars and back, the mission time is about the same, as the waiting time increases. Moreover, the waiting time is not only related to the travel time, but also with θv, i.e. a different θv requires the target planet to be in a different rendezvous location, affecting the waiting time for the return. This reveals another advantage of using continuous thrust, the change in the velocity direction that can be imposed to the spacecraft. In many of these mission options the thrust is not aligned with the spacecraft velocity, but has a transverse component, which shortens the distance travelled, and affects the waiting time.
Comparing the mission outlined here (using 25 electric propulsion engines) with the Mars Direct mission [1], we show that it is possible to save about 50 days on the total mission time (and save ~60 days on total ToF), while keeping the manned spacecraft below the 1,000 t limit as advocated in Mars Direct.

Translation:
The trade space for existing rocket propulsion technology is very small if ToF is restricted to 180 days or less and the total spacecraft mass / total mission mass (NASA calls it IMLEO, or Initial Mass in Low Earth Orbit) is 1,000t or less. However, less of the mass will be fuel if pure electric propulsion is used. Chemical propulsion is best used for sending things into orbit. Solid core NTR only saves significantly on total spacecraft mass when the vehicle being propelled is quite large. Only pure EP allows for significant payload mass fractions and that mass fraction is what makes the interplanetary transport vehicle durable enough for multiple missions / protective from radiation and space debris / sufficiently redundant in critical capabilities like life support, which is precisely what you need to have in deep space. AG, combined with pure EP, is the way to go to enable more of the mass fraction to be useful payload.

My Takeaway:
Build a big-honkin spaceship that's constructed like a brick outhouse, use some form of EP to drastically reduce propellant mass fraction (so that it feasibly can be built like a brick outhouse), use AG to keep everyone healthy, and take the transit time hit. Who cares if it takes an extra month or two, so long as you're every bit as healthy as when you left Earth when you arrive? If the goal is to actually go to other planets to explore and colonize, then stop screwing around with these dinky little toy spacecraft and build REAL SHIPS! A real fast frigate is in the 2,500t to 5,000t range. That's how big our SMALLEST ships should be. Anything much smaller is quite likely to be an overly-delicate death trap waiting for some unfortunate event to kill everyone aboard.

The entire purpose behind having an affordable and reusable super heavy lift launch vehicle should be to build real ships that don't have stupid design limitations that prevent them from being durable / survivable deep space transports, like a paper thin hull dictated by the propellant mass fraction required by any stage of an orbital class launch vehicle.

ISS proves every single day that even a series of relatively thin-walled Aluminum cans are perfectly capable of surviving in orbit for decades at a time, so long as they never leave orbit, when a reasonably regular supply of repair parts, crew members, and consumables are provided. Since it's been continuously occupied for half of my life, I'll go way out on a limb and say that even with all of the complexity and failures associated with current chemical rocket technology, we have that process down pat. There's nothing fundamentally different about this proposed ship vs ISS, apart from its size and mass.

Reducing travel time is the wrong goal. Increasing the survivability of the ship through increased structural mass and an acceptance that humans require gravity to function properly, made possible by not insisting on combining launch vehicle performance requirements with requirements for protection from reentry or other extreme events, fortuitously enabled by high-Isp propulsion, is the correct goal for transporting lots of people and cargo to Mars with the expectation that they'll arrive in one piece.

Bob already correctly noted in his blog post that when you impart extra velocity to your spacecraft, then you inevitably have to back that out when you arrive at your destination. There's one small problem with doing that at Mars, though. There's no thick atmosphere to absorb all that added velocity upon arrival. Even if there was, the end result is that the vehicle has to be heavier or specially constructed to survive the aerodynamic heating involved. That's feasible to do, if not very practical, but it also contributes nothing to the goal of maximizing useful payload. Apart from consuming lots of additional fuel and ensuring that the spacecraft won't be as durable as it otherwise could be as a result, what does speed galore do for us? In this case I'd say, "well, not very much". Even with the benefits from using various orbital mechanics peculiarities and caching extra fuel in propellant depots that he also talked about, we're still far better off using any propellant mass savings to build more durable ships or to carry more cargo.

My Conclusion:
The simple physics of this time-of-flight (ToF) problem for Mars missions is not amenable to any particular propulsion technology, outside of non-existent Star Trek level propulsion technology. Going any faster than minimally necessary drastically increases fuel consumption for marginal, if not trivial, transit time reductions. If adding 90 days of travel time to a mission that lasts at least 26 months is a deal breaker, then it means we weren't ready to go for basic technological reasons. If you're going to spend the rest of your life on Mars as a colonist, then it's even less meaningful. In my opinion, it's not a very good trade. Since we seem to have misplaced the plans for a working impulse engine or warp drive, going slow and yet arriving in one piece while inside a very sturdy ship that supplies artificial gravity, and with far less fuel expenditure for pure high-Isp / low-thrust solutions, seems to be the correct way to create that inner solar system railroad that Dr. Zubrin spoke of.

An ocean-going freighter isn't the fastest transport solution available, yet real human civilization here on Earth requires this form of transport to thrive. Ships move incredible tonnage back and forth, yet they're only about 10% fuel and 90% structure and payload. Rockets are far faster than cargo ships, but they're also quite literally flying gas cans that are easily 90% fuel and only 10% everything else. If you have high-Isp propulsion capability, then use that advantage to offload fuel mass in favor of greater structural and payload mass unless the ultimate objective is to go really fast. Airliners are only feasible because ocean going cargo freighters and oil tankers supply the raw materials to make them and the copious quantities of fuel to use them. We don't yet have an equivalent system in operation in space to enable faster practical space travel and we never will until we understand and accept the logic behind that slow boat to Mars.

tahanson43206 · 2020-07-30 06:08:44

For kbd512 re #18 and #19

Thank you for these hefty posts! I had a chance to read them during the countdown to the launch of Perseverance this morning, and your writing is equally inspiring parallel to the performance of the ULA team.

Your argument for a large, slow, well appointed passenger vessel seems (to me at least) to match up pretty well with some of the concepts RobertDyck has introduced to his Large Ship topic.

The list of references is helpful.

SearchTerm:IonDriveReferenceList
SearchTerm:SlowConstantAcceleration

(th)

Calliban · 2020-07-30 11:57:21

Excellent post Kbd. It should be no problem for a ship equipped with artificial gravity to spend a year getting to Mars.

One thing that would help the development of artificial gravity for such a ship would be an understanding of human ability to adapt to living in short radius centrifuges. If humans can live in a compact centrifuge 10m in diameter say, it makes the shielding arrangement much easier.

One problem with long journey times to Mars and back: tritium has a half-life of 12 years, assuming we are still considering the magnetic implosion fusion propulsion system.

I wonder if the Martian moons could provide reaction mass for a low thrust, high-ISP propulsion system?

Last edited by Calliban (2020-07-30 11:58:29)

SpaceNut · 2020-07-30 18:12:44

https://en.wikipedia.org/wiki/Artificial_gravity

radius of 10 m, a period of just over 6 s would be required to produce standard gravity (at the hips; gravity would be 11% higher at the feet), while 4.5 s would produce 2g.

Not a very uniform level over height. Also sensitive to spin time.

A larger diameter reduces these conditions.

GW Johnson · 2020-08-01 08:52:08

4 rpm at 56 m radius is what produces 1 gee at that radius (two significant figure data).

There will inherently be a gravity gradient with radius, proportional to radius, varying from zero gee at the center, to full gee at the full radius.

The bigger the radius relative to the height of a human, the smaller will be the head-to-toe gradient over a human height standing at that full radius.

How much gradient can be tolerated can be estimated from 4 or 5 gees endured successfully "long term" by seated pilots. Stay under about half that figure.

If you don't have the dimension to spin like a rifle bullet, then you probably do have the dimension to spin end-over-end like a baton. Either is stable, and both avoid the need for parasitic truss masses or cables that you cannot push on (which REALLY complicates the dynamics of spin-up and spin-down).

GW

Last edited by GW Johnson (2020-08-01 08:53:51)

kbd512 · 2020-08-03 21:09:55

Calliban,

Since we'd only refuel the ship after it returns to Earth, its load of Deuterium and Tritium should be nearly depleted by that point. Our water-cooled nuclear reactors here on Earth are always making more of that stuff, so there should be a healthy supply available. My overall thought process about how this would work is to use the fusion drive for impulsive burns that supply just enough energy to complete the orbital transfers and insertions. It'll also have some reserve fuel for contingencies. I plan to supply ~600t of Aluminum or Lithium fuel, with a preference for Aluminum due to ease of handling and cost, providing ~6.2km/s of dV capability with the FDR's expected 5000s Isp. By my count, a fully loaded frigate needs 37 fusion engines to provide the same TWR as the version using the TransHab and Orion as a payload. The difference between my plan for how to use this new technology and their plan is that their plan calls for 49km/s worth of dV (yes, you read that correctly), whereas mine requires a little over ~4km/s using minimum energy transfers and elliptical orbits.

A 5,000t ship, with an airliner's structural mass fraction, should provide adequate radiation shielding for its crew and artificial gravity, so movement between orbits, however slow, isn't a major health concern. The habitable space will consist of a pair of contra-rotating decks entirely contained within the hull. Those two decks will be protected by thicker hull plates. A central solar storm shelter / water tank will protect the crew from the most severe solar storms. The habitable / rotating section will have a little more than double the pressurized volume of an AN-225's cargo bay, dictated by the minimum rotation diameter / height to clear heads / rpm combination for human comfort while providing 1g of artificial gravity. The ship will be built on Earth, more or less gutted internally, and launched like an upper stage of a rocket, albeit one time only. SpaceX's original ITS booster was about the correct size. Once the ship is in orbit, it's staying in orbit. Any repairs will be done using a space station. The ship will be shaped like a giant lifting body, sort of like a spear head. It will be capable of reentry and landing on a dry lake bed runway after installing a flexible single-use fabric heat shield, though this feature will never be used in normal operations, unless someone can think of a good reason to use it. I can't think of any, but just in case (there's always something I never thought of), the design will be minimally capable of gliding, just as our Space Shuttle was.

The best visualization I can provide is here (a more aerodynamic and smaller version of this):

Star Trek Stingray Starship Concept Artwork

Why not just build a giant metal tube and call it a day? Well, I suppose we could, but that would be more of a miniature O'Neill space station with no possibility of return to Earth. In much simpler terms, sleek starships inspire wonder and excitement, whereas giant metal gas tanks do not. If landing is ever required, then this lifting body design could feasibly do that.

It will feature a particle deflector shield, though perhaps not akin the giant glowing dish model from Star Trek, but it will be a cryogenically cooled electromagnet that uses the hull and an ionized gas to project / inflate a miniature magnetosphere. When it's not thrusting, it'll be pointed in the direction of the Sun to provide a protective ionized plasma bubble (largely created by protons from our Sun becoming trapped in the electromagnetic field) around the vehicle when it's in interplanetary space. From testing, the power required to do this is nominal and it deflected all protons with the energies that those given off by the Sun have and better than 95% of the protons with GCR energy levels. In short, highly effective, but because of the electrostatic repulsion from particles with like-charges, rather than magnetism. Charged particle repulsion force is something like 10^39 more powerful than magnetism or gravitation, thus why gravity is called the "weak force".

Since a ship of this size will be about as maneuverable as a battleship is here on Earth, it will feature a very powerful fiber-based laser array to deflect or vaporize chunks of space debris that might strike and penetrate the hull. I presume Lockheed-Martin or Raytheon will provide the laser. No "photon torpedoes", though. Sorry sci-fi fans, but this is an interplanetary transport, not a warship. However, it will contain small "ejector tubes" to dispense atmospheric or in-space exploration probes. A sophisticated suite of onboard optical sensors will monitor the space around the vessel to avoid having to use that laser. Radio waves will likely be attenuated by the deflector shield, hence the need to use optical sensors and communication. The other prominent Star Trek-type feature will be escape pods based upon our Cygnus cargo containers. In normal operations, these naval frigate-sized ships will travel in convoys of 2 to 5 ships to provide mutual protection / search and rescue. Total complement will include 250 crew and colonists, remarkably similar to a large naval frigate.

Here are a few of the big numbers associated with this ship:
2000t dry mass
600t of Aluminum foil fuel for the fusion engines
150t of food for 2 years of operations (when the colonists depart for the surface of Mars, then most of this will be offloaded there unless colonists are also brought back to Earth)
250t of potable water
260t of pure water to supply the PEMFC with O2/H2 with 480MWh worth of power for the impulsive burns
1MW of solar power for life support and ship's functions
20MW PEM fuel cells (10t total for a redundant array of 20 1MW fuel cells; only ~6.26MW required; extra power provided for shorter burn times and reduced gravity losses; burn times of no more than a day expected in normal operations; nominally, 6 burns per round trip- TMI, MCC, MOI, TEI, MCC, EOI)

So, why no nuclear reactor? This is a small ship that only requires a little over 6MW of electrical power for a handful of days per mission. The additional power output from the fuel cells is to shorten the burns and the fuel (what will become water at the end of the burn) contributes redundancy. A reactor capable of providing equivalent surge power would increase the deadweight tonnage due to shielding mass and it probably wouldn't be redundant. About 99% of the time, the power that a fission reactor could deliver is simply not required. There's plenty of power over time from the solar array to re-split the water in-transit to Mars or Earth and to provide life support. We must have reserves of water for life support at all times, no matter where we go. If the ship was 10,000t or more, then a small fission reactor would make more sense.

I figure we have enough mass budget for 1,000t of cargo per crewed flight. The remaining tonnage is devoted to various crew survivability features not present in other spacecraft. The stripped (of human habitation features) RPV may provide ~2,000t of cargo. However, even 1,000t of cargo is equal to 10 SpaceX Starships per flight. I think that's an absolute minimum delivered tonnage capability to create a second self-sustaining branch of human civilization on another planet. Even my plan presumes we source most of materials from Mars, but in order to do that, we need to bring in lots of heavy machinery. In short, we need big ships.

I'm not a fan of windows in spacecraft, but each vessel will include an ALON-enclosed observation deck for truly spectacular unobstructed views of space.

Certain variants of this ship will be remotely piloted and all of the available internal volume will be dedicated to provisions or cargo or fuels. I figure a convoy would have four crewed ships and one remotely piloted ship carrying provisions and/or very large pieces of equipment to robotically deliver to the surface of Mars. Examples of such equipment might include nuclear reactor cores, tanks for chemical storage pieces of construction or mining equipment, drilling rigs to drill for water or other subsurface volatiles like Methane, etc. The RPV would contain the propellants for landing the equipment and humans, rather than placing humans in unnecessarily close proximity to massive quantities of rocket propellants for months at a time.

A combination of thin film photovoltaics laminated onto the hull and batteries will provide life support power. A fuel cell stack will supply surge power to the super capacitor bank and fusion engines for the impulsive burn maneuvers. The burn times are essentially a function of how fast the power source can recharge the super capacitor banks since the entire rear end of the ship will be a gigantic integrated aluminum radiator to absorb the thermal load from the fuel cells and engines. A pair of redundant 10MW PEM fuel cells will recharge the super capacitors. Each fuel cell can supply 40% more power than the original FDR concept required, so burn times will typically be shorter in normal operations. Scaling to the power levels required for this ship, I determined that PEMFC supply more power at a faster rate than equivalent rigid PV panels could supply (dictated by the force of the impulsive events and the size of the PV array required), for no significant increase in mass over PV. The O2/H2 required will be split into reactant tanks in orbit, just prior to departure, turned back into water during the burn, split back into O2/H2 during the lengthy transit, and then turned into water again during the insertion burn. Since the hull is primarily constructed of Aluminum alloy, the radiator mass is essentially the ship's rather healthy structural mass.

The FDR reference mission is a 210 day total mission time has a payload mass fraction of 46% and a total mission dV of 49km/s. It's 134t IMLEO with a 61t payload and requires a 180kW PV array providing at least 200W/kg. My mission is a 180 day transit, each way, and I want to put a 5,000t ship in LEO (built and launched in pieces, obviously). To enable a similarly speedy departure on a minimum energy trajectory, I presume I would need a 6.7MW power source.

tahanson43206 · 2020-08-04 06:23:35

For kbd512 re #24

SearchTerm:CounterRotatingHabitatSections

First time I've seen this suggestion. Nice!

Edit#1: Are you interested in seeing drawings of your design for a large vessel?

RobertDyck has a parallel track going, where he's posted an illustration or two.

I went back over the entire topic, and found numerous illustrations but no drawings.

It might be possible to draw younger folks with the needed skills into the mix, if you were to undertake a process of sketching your design.

It's possible your thinking might intersect with that of RobertDyck at some point.

(th)

Last edited by tahanson43206 (2020-08-04 10:41:04)

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