Solving Mars mission docking with Phobos

tahanson43206 · 2020-07-09 17:25:16

For GW Johnson re #99, and SpaceNut re #100

Thank you both for your thoughtful analysis of the situation which will be faced by a ship's navigator approaching Mars on a Hohmann transfer trajectory.

This post is reserved for notes after (what I hope will be) careful study.

***
In another topic, RobertDyck has undertaken analysis of and creative thinking about a large scale passenger ship for Earth/Mars travel.

It is too early to know if that initiative will go further or if it has reached its natural conclusion, but it it does continue, the navigation of a large vessel in the vicinity of Mars will be of concern to the crew, and to the mission planners who will be providing the equipment and supplies needed to insure a safe journey for up to 1000 people.

I will endeavor to show that Phobos is capable of imparting (on the order of) 2 km/s of velocity change (acceleration) to a vehicle that engages with the mass of the moon on the backswing, and then rides along as Phobos travels around Mars.

That 2 km/s of velocity change will (and does) represent a savings of fuel and oxidizer which would otherwise be needed to enter orbit around Mars.

(th)

GW Johnson · 2020-07-11 16:22:18

Sending 1000+ people to Mars is way to hell and gone beyond anything currently contemplated, even at Spacex. That is most definitely a very large-scale colonization ship effort.

My advice for that is to use nuclear pulse propulsion in a ship exceeding 10,000 tons, starting from the backside of the moon, to shield the Earth from the EMP of its departure "burn". It will need to use an arrival at Phobos, in order to shield those on the surface of Mars from the EMP of its arrival "burn". Such "burns" would likely be thousands of thermonuclear devices, not 1955-vintage fission devices.

Aerobraking for a ship of that capability will be entirely irrelevant. You are looking at something about like a WW2 pocket battleship, around 10,000 to 20,000 tons initial mass, and over half the ignition mass is dead-head payload. Because you are looking at 10,000 sec+ Isp at a vehicle acceleration in the 2-4 gee range. Made from 2-inch thick steel armor plate. Such a thing can reach Mars in 2-5 months single-stage, two-ways, and is big enough to easily spin for artificial gravity between "burns".

There are a number of missing support technologies necessary to make such a thing possible. Not the least of which is a nuclear propulsion development-and-test station on the moon, where there is no air or water to pollute, or neighbors to annoy, if you have a problem. You will need a way to transport that thousand people and all those thousands of tons of supplies to a location beyond the moon, in order to board a vessel like that. Plus a way to transport same from beyond Phobos to the surface of Mars.

You WON'T be launching things that large from Earth, or landing them upon Mars, because of the collateral-damage the EMP does. Compared to that, the radioactive fallout risk is trivial.

GW

Last edited by GW Johnson (2020-07-11 16:24:57)

tahanson43206 · 2020-07-11 17:24:33

For GW Johnson re #102

SearchTerm:1000PassengerNuclear
SearchTerm:NuclearFissionPassenger

(th)

Calliban · 2020-07-11 19:16:14

EMP is caused by gamma rays stripping electrons from air molecules in the upper atmosphere. These are subsequently deflected by the Earth's magnetic field, resulting in a huge induced current lasting for nanoseconds - the so called E1 pulse.

The effects scale with the square root of yield and are more severe in single phase fission devices, because there is no surrounding compressive charge to absorb gamma rays. Some mitigation options:

1. Smaller devices generate smaller pulse, all else being equal;
2. Gamma rays decline in intensity according to the inverse square law. A burst at 40,000km will have only 1% of the intensity at ground zero of a burst at 400km, but will effect the entire hemisphere. Unfortunately, launching at that height would pump the Van Allen belts with charged particles, which may damage satellites.
3. One could shield devices by encasing them in hydrogen rich materials to absorb gamma pulse. This will increase weight and reduce effective energy density of the pulse units.
4. Fire small devices within some kind of enclosing engine bell, and use the radiated energy to heat hydrogen propellant. That would do bad things to power to weight ratio and I expect the engine would need to be huge.
5. Launching from the poles would be advantageous, as there is no tangential magnetic field.

I doubt that the pulse rocket would need to launch from behind the moon. But it may be advisable to launch from above the inner Van Allen belt to avoid damage to geo stationary satellites.

Mars has a thin atmosphere and no magnetic field. I do not know how a EMP inducing event would play out under those conditions. It has never been tested.

Any decent performance space drive is a weapon of mass destruction. There is no way of releasing such huge amounts of energy without collateral damage. Interestingly, I am not sure that a pure fusion inertial confinement drive would solve these problems. The explosions are usually envisaged as being smaller, but there is no surrounding charge to absorb gamma rays. Not an easy problem to solve. Though there are solutions that would reduce the problem, maybe even making it tolerable.

If chemical propulsion can be used to ship people, propellant and freight to low earth orbit, then a large interplanetary transport ship using gas core nuclear thermal propulsion would be one way of avoiding EMP problems.

Last edited by Calliban (2020-07-11 19:31:31)

GW Johnson · 2020-07-12 12:32:34

Sending 1000 people to Mars at time has nothing to do with exploration, and everything to do with colonization or large base-building. I personally consider that day a very long ways off, especially for any government-led efforts.

But if it is a long ways off, then there is time to develop both gas core nuclear thermal and nuclear pulse propulsion, and there is time to build a safe place to do that development work, which in turn is a safe place from which to launch them. That safe place is the moon. Which should be obvious to the casual observer.

What you are looking at with gas core is either "nuclear light bulb" concepts (where the uranium is kept separated from the hydrogen) or open-cycle gas core, where an intimate contact flow scheme must be devised that effectively sets the uranium throughflow at 1/1000 of the hydrogen throughflow.

None of this has undergone any development at all, but the early exploratory design work says the light bulb concepts might have Isp ~ 1300 sec max, at engine thrust/weight greater than 1, maybe a lot greater than 1. They were looking at double-walled quartz window separation, cooled in between, so that thermal radiation transmitted through from the core fireball to the hydrogen propellant.

The open cycle gas core operates at much higher gas core temperatures and power levels, limited primarily by hydrogen transparency to thermal radiation when it gets hot enough. There was also a limit where regenerative cooling of the chamber and nozzle bell was no longer feasible, because the demand for coolant exceeded the supply (the engine throughflow of hydrogen).

Open cycle gas core was thought possible with regenerative cooling up to ~ 2000 maybe 2500 sec Isp, with engine thrust/weight exceeding 10, maybe far above 10. That could actually be used to launch from the surface of the Earth, if you can accept the radiation in the exhaust stream.

Such exhaust stream radiation is minimized but not eliminated, if you get to the target 1000:1 flow ratio. The fission products are there in the exhaust, at best. At worst, there are products plus unused uranium, all in very hot gas form, and moving really, really fast.

Operated at higher core power, with powered active cooling and a gigantic 3500 F waste heat radiator, open-cycle gas core could operate up to the transparency limit, thought to be no more than about 10,000 sec Isp. But nobody knew "for sure". Or how to actually build that radiator. Or what from.

The first design target for development set this at 6000 sec Isp, in order to stay well away from the transparency limit. This design necessarily includes the radiator and powered cooling circuit as part of the engine equipment, with an effective engine thrust/weight well under 0.1. Only suitable for in-space propulsion.

The other main attraction of open-cycle gas core over nuclear light bulb, or any of the solid core concepts, was safety after shutdown, which is abortability. Stop the flows, and you have an empty steel can, no reactor core at all. It "cools" quickly in a radiation sense to levels that can be dealt with, fairly easily. Nothing that retains a core, solid or gaseous, can do that.

Now as I said, this was all early design speculation, no real development testing was ever done with any of this. But the potential really is there.

As for pulse propulsion, it has not received any development at all, since it was shut down in 1965. The design was an armored hull with shock absorbers and a massive steel blast plate, using small kiloton-range shaped-charge fission devices based on our 1955 technology. It appeared mostly feasible at the time, excepting the lack of feasible fractional-kiloton device technology in 1955, needed to launch from the surface in the atmosphere. We have that now.

Many folks erroneously question the blast plate, but it was essentially a massive heat sink with radiational cooling, and pretty much survived in the same way as other massive steel items survived pretty much intact at Hiroshima, Nagasaki, and in the surface tests in Nevada.

This stuff actually works more efficiently in larger sizes. It was barely attractive at a ship initial mass of 5000 tons. But for ships in the 10,000 to 20,000 ton range, you are looking at effective Isp in the 10,000 to 20,000 sec range, and it is hard to hold the ship acceleration levels to the 2 to 4 gee range. In other words you are building vessels of steel armor plate that are the size of WW2 battleships. And totally unlike electric propulsion, the thrust levels are very high indeed. Even the Isp is higher than electric.

For really, really large vessels transporting 1000-to-10,000 people at a time, there is no other known choice but pulse propulsion. Nothing else we know scales up that big. It will scale up even larger. Pulse propulsion just won't scale down effectively, so it needs to be used only in such large applications, not the smaller ones. It just needs an update to 2010/2020 nuclear technology.

For transporting 100-1000 people (or 100-1000 tons) at a time, open-cycle gas core nuclear orbit-to-orbit ship designs might do well, with far fewer side effects. That's still early colonization or large base-building. Nothing to do with exploration. These would have to be combined with chemically-powered transports of one kind or another, at each end of the trip, to get people and cargo between the ship and the surfaces.

On a smaller-still scale (10-100 people or tons), the chemical and solid-core nuclear thermal schemes look practical. That addresses both explorations and early small base-building. These need not be orbit-to-orbit transports, if one can accept the conflicting design requirements of orbit transports versus landers. So, guess why the Spacex Starship/Superheavy design is what it is, and gets touted for this application.

Just putting some perspective to this.

GW

Calliban · 2020-07-12 14:51:25

Thank you very much for an interesting post GW.

I get the feeling that we won't be colonizing Mars until the world stops being scared of radiation. Both nuclear pulse and gas core nuclear rockets would bleed fission products into the atmosphere. If the world stopped caring about that, we could launch colonization ships like the one you described from Antarctica, where magnetic field lines are close to vertical and avoid most of the problems from EM pulse. We could have regular interplanetary transport. But the price would be higher background radiation levels for anyone living back on Earth.

What kind of a nation would do that? What kind of government? Certainly none that I know today. Maybe the Chinese are ruthless enough. But the US and Europeans would need governments that are nothing like the ones they have today. I just cannot imagine it.

Last edited by Calliban (2020-07-12 14:53:56)

SpaceNut · 2020-07-12 15:25:45

Radiation is solved on earth so why can we not do so for trips beyond LEO has to do with cost to lift the materials or sufficient mass which work. That is why we design with the water we must take, the food that is frozen, and with a variety of materials to make it so that less of that fear gets through.

GW Johnson · 2020-07-13 08:12:25

For exploration missions, which might or might not establish a small outpost, all you need is stuff like we already almost have, launched from the surface of the Earth or from low Earth orbit. That is exactly what the refilled/outside-LEO mission proposals for Starship / Superheavy are all about, and it is exactly what NASA claims SLS / Orion is for, even though it still cannot do such missions.

Spacex has serious issues to resolve with docking tail-to-tail, with transferring cryogenic propellants, with inert mass fractions in their vehicle designs, with landing pad bearing pressure on soft surfaces, and with landing leg pattern lateral dimension vs cg height static stability issues on rough ground. Resolving those is what the prototypes and testing is all about. That takes time, and failures are inherent in that.

NASA has serious issues to resolve, too. They have as yet no lander design for the moon, and nothing at all for Mars. They have no deep space habitat design ready to go, and what they have does not address microgravity disease or solar flare radiation protection. They do not have the throw weight capability in Block 1 SLS to do any of this, and it is doubtful we will ever see Block 1B or block 2B, because of the cost issue. The cost issue is that, pound for pound, flying on SLS is at least 10 times as expensive as anything else. Maybe 100 times as expensive, if one is honest with the book-keeping. They literally cannot afford to use their own rocket. And the flight rate is too low to do anything significant with it.

NASA's biggest problem is that it long ago devolved into a corporate welfare scheme for the big old-space contractors, micromanaged in terms of funded projects by congress. And that is exactly what congress wants from NASA, to serve its own porkbarrel politics interests.

So, you are looking at doing early exploratory-level stuff to the moon and Mars in the next several years. The transportation that does it will most likely be things like Starship / Superheavy, Falcon-9 / Falcon-Heavy, Atlas-5, Vulcan, and maybe New Glenn. I doubt that SLS will ever play much of a real role, but I do think there will be a few stunt flights. To this mix you need to add habitats resembling Skylab, equipped with radiation shelters, some scheme for at least partial spin gravity, and, phasing-in later, some updated nuclear thermal propulsion, most likely solid core, if you want to seriously send any "bigger things" to Mars.

Its going to take many, many years to build a mature transportation system between Earth and the moon, routinely shipping mass tonnages and big passenger loads back and forth. At that time is when we will need the advanced nuclear development station(s) on the moon, and it is from there that the really big colonization-class vehicles will depart, using that advanced nuclear propulsion. None of us will live to see that. All we can do is point the way for our descendants.

GW

Last edited by GW Johnson (2020-07-13 08:17:29)

tahanson43206 · 2020-07-13 08:58:09

For GW Johnson re #108

SearchTerm:ForecastSpaceDevelopment
SearchTerm:OutlookSpaceDevelopment
SearchTerm:PredictionSpaceDevelopment

SpaceNut · 2020-11-22 18:05:01

PHOBOS ON THE CHEAP proposes instead that we take a slow approach, incrementally building up towards an eventual manned landing.

tahanson43206 · 2021-01-25 09:04:18

This is for GW Johnson (hoping you're still keeping an eye on the forum) ....

Some time ago, you very kindly provided advice (and plenty of facts) to this topic.

I accepted the tyranny of the Hohmann Transfer orbit without question at the time.

However, recent discussion of an idea of Void led Calliban to make an observation about the velocity with which a delivery package would encounter Phobos.

I'm going to offer a question; Why are we obligated to only consider the Hohmann transfer orbit?

The Hohmann orbit has a lot going for it. It puts a payload/vehicle ahead of Mars at the peak of its elliptical trajectory.

Mars catches up with the vehicle and pulls on it. SpaceNut recently quoted SpaceX as expecting 7.5 kms for a Starship upon entry to the atmosphere of Mars.

Earlier in this topic, it was shown that the velocity of a vehicle approaching Phobos from a Hohmann orbit on the backside of Mars (ie, while Phobos is receding with respect to the arriving vessel) would be reduced by (about) 2 kms, as compared to the velocity relative to Mars.

I am now back asking the question: Why are we obligated to use the Hohmann orbit for package delivery?

A package delivery service is going to want to push the package off from Earth with a velocity calculated to exactly match the velocity of Phobos.

As I am imagining the trajectory right now (with the limited capability available to me) it would arrive at Phobos with exactly (or close to exactly) the velocity needed to dock gently with Phobos.

Is there such a trajectory?

This is a bulk material delivery vehicle, and time is not critical.

Energy invested in shipping the package is important, but it's safe arrival is even more important.

Thanks in advance for any assistance you may have time to provide.

Edit#1: The web page at the link below provides a discussion of the Hohmann transfer orbit at a high school level.

It makes the point that the Hohmann orbit is most efficient.

https://www.jpl.nasa.gov/edu/teach/acti … h-windows/

After thinking about the assertions made in the web site, I've come to wonder if the efficiency claimed includes actions needed at the destination to accelerate to match the velocity of Mars.

Phobos provides a variable velocity of (about) +/- 2 kms to work with, for a total buffer zone of 4 kms.

Edit#2: The web page at the link below appears to offer an open source program developed by NASA ? that can be used to compute spacecraft trajectories.

https://www.nasa.gov/feature/goddard/20 … aft-design

Apparently the software can be downloaded, but I did not see how to do that in the article.

(th)

GW Johnson · 2021-01-25 15:06:51

Tahanson43206:

Perhaps the best way to get you to the information you want is to refer you to my "exrocketman" article "Interplanetary Trajectories and Requirements", dated 11-24-19. The fast navigation tool is on the left of the page. Click on the year (2019), then the month (November). You won't need to click on the title, that article is the only one I posted that month. Click on a figure to see any of them enlarged.

The Hohmann transfer ellipse is the minimum-energy trajectory that can take you to Mars at all. What recommends it, is its lowest "cost" as measured by required mass ratio and propellant quantity. You will always arrive with that energy relative to Mars, at least. Higher energy trajectories merely have you arriving at higher velocities with respect to Mars. But you can NEVER get there slower than Hohmann!

The detailed numbers vary a little with exactly where Earth and Mars are in their elliptical orbits about the sun. What's in the article is for average distances and average orbital velocities. The effect is larger for Mars than Earth, because Mars's orbit is a bit more eccentric than that of the Earth. It's not a large effect, but it is significant when you are actually designing systems.

The difference between Mars's orbital velocity VECTOR about the sun, and your velocity VECTOR on your transfer orbit about the sun at Mars's distance from the sun, is the relative velocity VECTOR you have with respect to Mars, "far" from Mars. Only with Hohmann transfer are these vectors collinear, so that you can do an arithmetic difference of the velocity magnitudes. Higher energy trajectories require vector substraction, and things do NOT happen at aphelion. The article covers all that.

Your velocity magnitude "far" from Mars gets increased as you get "close" to Mars by the effect of Mars's gravity drawing you closer. There is a very simple kinetic energy equation by which you can convert speed far from Mars to speed close to Mars. The article covers that, too. It depends upon local Mars escape velocity at the "near" distance of interest to you.

It's a bigger increase in relative speed going into low Mars orbit, than it is going into an orbit at the distance of Phobos (which is what the corrections shown on the article are all about: I failed to do that correctly the first time).

At average planetary orbital conditions, your far-from-Mars speed off of Hohmann transfer is about 2.6 km/s relative to Mars. By the time you reach the distance of low Mars orbit, it has increased to about 5.7 km/s, give or take about 0.1 km/s for variation of planetary orbital positions.

If you use the higher-energy trajectory of a 2 year transfer orbital period (this is the free-return-abort orbit Zubrin favors), you get off the trajectory at Mars at a higher speed "far" from Mars. That is about 5.4 km/s. At low Mars orbit distance, the increase takes you to 7.4 km/s relative to Mars. Add about 0.1 km/s for worst-case orbital position effects, and you have the Spacex "worst case" free-entry figure of 7.5 km/s for its Starship to land directly on Mars!

Nice to know that orbital mechanics gives them the same answers as it gives me, right? These data are among the 4 cases shown in Figure 8, out of 16 figures in the article.

For Phobos, you arrive at the distance of Phobos, where the escape velocity from Mars is lower at 3 km/s instead of 5 km/s. That makes the kinetic energy increase "far" to "near" less. Fig 16 in the article has those data (as I corrected them) for the various transfer cases. From Hohmann, your 2.6 km/s becomes 4.0 km/s instead of 5.4 km/s, relative to Mars. From the faster 2-year-abort trajectory, your "far" 5.4 km/s becomes "near" 6.2 km/s, instead of 7.4 km/s, relative to Mars.

Now, if you have timed everything correctly, you arrive at Phobos's orbit about Mars, with Phobos right there when you arrive. You do this on the sunward side of Mars, so that Phobos is "running at you" slower than Mars is, so that you can subtract Phobos's orbit speed about Mars from your "near" velocity with respect to Mars.

That difference is your "near" velocity with respect to Phobos, essentially almost your delta-vee to land, uncorrected for any gravity or drag losses, and uncorrected for the escape velocity from Phobos (about 28 m/s). Airless and almost gravitationless, there are no effective corrections to make to the "near" velocity at Phobos, so you are needing only to add the Phobos escape velocity, for the actual delta-vee to land.

The numbers are 1.9 km/s to land on Phobos from Hohmann, and 4.1 km/s to land on Phobos from the faster 2-year-abort trajectory. That's what Figure 16 shows, in the top right block of data.

Now, add in the departure delta-vee from low Earth orbit, and a kitty for mid course and near-arrival corrections, and you have the total one-way trip delta-vee requirements shown in Fig 16, bottom block of data. The faster 2-year-abort trajectory puts you at 9.0 km/s delta-vee for the one-way trip, versus 6.1 km/s for the min-energy Hohmann trajectory. Give or take 0.1 to 0.2 km/s for non-average conditions.

Besides the mass ratio cost, there is the time spent traveling, which adds a lot of life support mass and energy if manned. At average planetary orbital conditions, this is 8.6 months one-way on min-energy Hohmann, and only about 4.3 months one-way on the faster 2-year abort trajectory.

These numbers are pretty real, and cannot be sidestepped with some "fancy trick" of orbital mechanics. These trajectories have been known for many decades, and in none of those decades has anything any better been found.

I'm no specialist in orbital mechanics, but I did have to learn the basic subject to get my first engineering degree. And I used it doing trajectory work on the "Scout" launcher as a graduate student, about half a century ago. Nothing has changed since.

Which is why I can still do this stuff using the basic equations and a spreadsheet, the way that I do in the article. It's even slower with a slide rule as I first did upon entering the workforce, but it is still the same pencil-and-paper-based process, the same one I used to set up the confirming mainframe trajectory computer runs for "Scout". Myself, I have no computer codes for this.

GW

tahanson43206 · 2021-01-25 18:31:30

For GW Johnson re #112

Thank you very much for your detailed reply, including the references to all the work you have done in your ExRocketman Blog!

The key information I am selecting is for support of the idea of Void, to deliver bulk materials to Mars using a ballistic system.

The numbers are 1.9 km/s to land on Phobos from Hohmann, and 4.1 km/s to land on Phobos from the faster 2-year-abort trajectory. That's what Figure 16 shows, in the top right block of data.

However, before I return to the Ballistic Delivery topic with the quote above, I'm going to set in place a few tags to try to help to find your post again when memory fades which is likely to happen (in my case, at any rate).

SearchTerm:Phobos docking from Hohmann transfer orbit by GW Johnson
SearchTerm:Hohmann Transfer orbit optimum performance arrival at Phobos

(th)

Mars_B4_Moon · 2021-05-30 17:52:15

Phobos and Deimos are Fragments of Larger Martian Moon, Study Suggests
http://www.sci-news.com/space/phobos-de … 09672.html

Another attempt at a PhobosGrunt mission?

https://en.wikipedia.org/wiki/Mars-Grunt

PADME was a NASA concept but it lost out to Psyche and Lucy missions.
https://www.nasa.gov/press-release/nasa … ar-system/

DePhine from Europe but this mission was rejected
https://www.esa.int/Our_Activities/Spac … _for_study

Last edited by Mars_B4_Moon (2021-05-30 17:53:54)

tahanson43206 · 2024-06-18 05:32:15

The purpose of this post is to bring the topic back into view.

In the opening series, GW Johnson provided an overview of the problem, but I came away from re-reading a bit perplexed, because GW's discussion includes many options, so that I ended up unsure of the final result.

The velocity of Mars coming up behind a space vessel on a Hohmann orbit appears to be well defined.

The velocity of Phobos as it revolves around Mars has a point at which it subtracts from the Hohmann Transfer velocity difference.

Whatever that velocity is, it needs to be subtracted from the Hohmann transfer velocity.

The net result is the dv that the vessel must apply to dock with Phobos.

If I can persuade GW Johnson to take another run at this, I'd like to see easy-to-understand diagrams, which I am happy to place on imgur.com.

The diagram I have in mind should (hopefully) capture the essential elements:

1) Mars is in it's orbit
2) A space vessel is on a Hohmann trajectory and it is ahead of Mars, so Mars is coming up behind at a dv on the order of 5 km/s
3) Phobos is in orbit around Mars, and apparently it's orbital plane is favorable for docking if the space vessel puts itself in the right place.
4) There is some combination of Mars, Vessel and Phobos that allows for the minimum dv needed for docking

I'm hoping a single diagram can show all of that.

I'd like to be in a position to propose a fuel depot on Phobos, and it would be helpful to have certainty about what is involved in matching orbits.

(th)

GW Johnson · 2024-06-18 18:17:46

This is true but incomplete:

"Whatever that velocity is, it needs to be subtracted from the Hohmann transfer velocity.

The net result is the dv that the vessel must apply to dock with Phobos."

The result is NOT what you need to dock with Phobos, but the velocity "far" from Phobos before 3-body effects of Phobos's gravity accelerate you further toward Phobos. With Phobos, that's not a large effect, but it is there! The accelerated speed "near" Phobos is your actual docking requirement with Phobos.

This is very well-covered complete with a simple estimate of the 3-body effect, in the "orbits+" course materials. The estimating equation is based on conservation of mechanical energy: Vfar^2 + Vesc^2 = Vnear^2. In the case of Phobos, its escape speed Vesc is quite small, which in turn is why the effect is quite small.

Going to orbit about Mars, the same sort of thing applies, except that the 3-body effect is due to Mars, not Phobos; so the Mars escape speed at orbit altitude is quite substantial. It is a large effect.

GW

Last edited by GW Johnson (2024-06-18 18:19:08)

tahanson43206 · 2024-06-18 20:09:03

For GW Johnson, re #116

The exercise of using the Orbits course to show the images and text for the Phobos dociking case would be a potentially valuable addition to this topic.

A complete navigation sequence would include:

1) Earth departure: Point at a star selected for the Hohmann transfer opportunity
2) Burn with the needed impulse for the required time while holding the point from #1
3) Report position periodically
4) Make adjustment mid-course (ie, point-burn)
5) Near Mars - confirm position wtth respect to Mars and Phobos
6) Make adjustment mid-course (ie, point-burn)
7) Note pull of Mars
8) Note pull of Phobos
9) Make docking burn at Phobs (ie, point-burn)

It is possible the existing course spreadsheet can generate images showing all these situations.

The navigator for each flight must possess the ability to work with software to solve the problems in real time:

a. Where to point for a given burn
b. What thrust to specify
c. How long to burn at that thrust
d. How to verify the desired result

(th)

GW Johnson · 2024-06-19 10:08:47

All of the discussions above aside, I'm curious about why we would want to go there? The min one-way dV to reach the surface of Mars with direct aerobraking entry off the interplanetary trajectory is the min possible value. It is larger than the departure dV plus the course correction budgets by only the retro-propulsion needed to touch down (crudely only 10% of what is needed to get there at all).

That's only one-way, and it's how most of the lander probes were sent. There's a bloody good reason for that, since the lander payloads were at the throw weight limits for the boosters that sent them there. Going home is actually worse than getting there, since it takes propulsion to escape from Mars, plus at least some course corrections to achieve a really high-velocity free aerobraking return at Earth. Plus whatever it takes to touchdown somehow.

The largest one-way dV requirement occurs when you decelerate into low Mars orbit. That's in the neighborhood of 2 to 2.5 km/s more than the sum of departure dV and the course correction budgets. If you want to visit more than one site on Mars in the one trip there, you stage out of orbit, and that's the least dV requirement for your landers. To go home, you will need that whole orbit trip budget again! The higher dV is just the price you pay for multi-site capability staged out of low orbit. There is no way around that. But the lander descent dV is a trivial de-orbit (50 m/s) plus a touchdown dV (around .7 to 1.4 km/s). Lander ascent dV is only bit more than low orbit speed (3.6 km/s).

Going to Phobos one-way has a dV requirement between those of the other two scenarios. The real question is why? If your goal is to reach the surface of Mars from Phobos, the landing requires a modest (but NOT trivial) departure dV, aerobraking, and a touchdown dV. But the return costs a lot: your one-way lander ascent dV is closer to Mars escape at 5 km/s one-way, not orbit speed of 3.6 km/s. You just made the lander ascent problem much worse, by about 40%.

Post 112 above gives some overall results and points to an "exrocketman" article, where I figured all these numbers about 4 years ago.

There are differences of opinion about this next item, here on the forums, but I rather think there are no volatiles in Phobos that could be made into propellants for refueling anything! More than anything else, it resembles the dry rubble-pile asteroids we see elsewhere in the inner solar system. Just loose chunks of rocky minerals. To find significant ices to mine, you have to go out past the giant planets. Closer in, the solar heating evaporated all the ices long ago, quicker in the smaller ones. Or so most astronomers say.

So, if the lander problem is much worse, and there are no suitable resources from which to make propellants, then why would we ever want to go to Phobos in the first place, other than just once or twice for scientific curiosity?

GW

tahanson43206 · 2024-06-19 10:39:37

For GW Johnson ...

Thanks for considering the challenge of navigating a cargo vessel to Phobos.

I am seeing Phobos as a supply depot and business hub. The mantra that I've heard for business success is "Location, Location, Location".

Some entrepreneur might decide to set up an orbiting depot in a different orbit, and after a few decades it might become well known and recognized as a destination.

That's quite a bit of risk to take, when Phobos is ** right there ** available for a business intending to operate for centuries in the Solar system import/export trade.

What I'm thinking about now is the same as in earlier times, when NewMars members helped me to create a topic about manufacture of methane and oxygen for Mars customers (inbound and outbound) using carbon and water imported from Earth. Carbon and water ship well and store well. Solar energy can perform the operations needed to turn those raw materials into product, but nuclear power will serve well when it becomes available.

The stage we are in now is comparable to the thousands of years when Humans explored the Earth on foot, and with small improvised water craft. It took a while for the massive movements of goods and services we see today to develop. Eventually Phobos will become a hub for economic activity simply because of location.

(th)

GW Johnson · 2024-06-19 13:33:44

My guess is that the total round trip dV from Mars to Phobos and back is less than the 1-way dV up from Earth's surface to Phobos. That means you mine the ice and ship it up to Phobos, using reusable vehicles. The carbon probably needs to come from Earth, as what's in "carbonaceous" asteroid materials is strongly chemically bound in those materials, requiring lots of energy input to recover. Charcoal or graphite from Earth can be utilized with little or no energy input to free the carbon. It'll cost you more dV to send those vehicles back to Earth, if they are to be reusable.

You have a big enough energy problem turning carbon and water into methane and oxygen. You don't need to try to "free" your carbon from local rocky minerals on Phobos. Solar power per square meter of collector is about half the power density we see here at Earth. And you will need to include batteries to get through the intervals when view of the sun is blocked by Mars. Plus these panels have to track the sun; it is not in a constant position overhead, even when the view is not blocked by Mars. Nuclear might be better. Something to think about.

GW

tahanson43206 · 2024-06-19 20:29:27

For GW Johnson re #120

Thank you for your encouraging reply about the use of Phobos as a psychological device for Business Investment, as distinguished from a purely mathematical treatment of the concept.

In another topic, Void has just set up an opportunity for (a few) NewMars members to take an introductory course on design of inlets for operation at supersonic and hypersonic speeds in an atmosphere. I hope Void follows through, and I'm alerting you so you can begin thinking about how you would structure a learning experience.

In the case of this topic, I would like to invite you to work with me (and hopefully others) to set up a sequence of exchanges that will (if all goes well) result in a step by step series that anyone can read to achieve the ability to launch a cargo to land on Phobos. The challenges are many, and the forces at work on the vessel are many as well. You have often reminded your readers of the importance of planning for the attraction of the destination body as an important factor for planning a flight. This topic was set up with a singular focus to try to help participants to acquire the knowledge and skills that would allow success were an opportunity to come along.

We have a Hohmann Transfer Window coming up, and at least one Nation is planning a flight to Mars. It might be possible for this forum to follow along as the mission planners deal with the challenges they will face.

(th)

GW Johnson · 2024-06-20 08:38:04

On the desirability of this vs. that mission plan: it isn't always about the numbers. It's fundamentally about what you really want to do.

On supersonic/hypersonic inlets: this topic occupies two full chapters of my ramjet book -- one on pitot/normal shock inlets, the other on supersonic inlets. It includes real-world band-aids for various real-world problems seen in these devices. The topics are important because inlets are what actually make ramjets work at all. I also have a good "exrocketman" article about supersonic inlets: "Fundamentals of Inlets", posted 9 November 2020 to http://exrocketman.blogspot.com.

The focus of all the ramjet and gas turbine applications is delivery of subsonic-decelerated air to the compressor face or ramjet combustor entry. That requires a diverging subsonic diffuser passage after the air is captured at the cowl, whether there is an external compression spike or ramp, or there is not.

For supersonic inlets operating supercritically, the shock system is swallowed, and the final normal shock-down from supersonic to subsonic internal flow occurs in this diverging passage. Supercritical operation is the lower drag scenario, and it is what must happen with ramjets, which need and will use all the airflow the inlet can scoop up.

For subcritical operation, the shock system is not swallowed, instead normal-shocking to subsonic flow just ahead of the cowl lip. That allows for variable air spillage around the cowl lip, required for matching turbine engine airflow demand with inlet scoop capability. It is a higher-drag scenario.

Supersonic combustion situations in scramjet are quite different. Those can use exactly the same cowl capture and external compression spike or ramp features, but what is done with the air after capture into internal flow is quite different. Once the air speed has been reduced to the desired supersonic internal Mach number, there is no divergent diffuser. There is a fairly significant-length very nearly constant area "isolator" duct that delivers the still-supersonic flow to the scramjet combustor. This geometry is entirely unlike, and extremely incompatible with, the geometry required for either gas turbine or ramjet air delivery.

The real-world problems of the supersonic-delivery "hypersonic" inlet are also quite different. That isolator duct is required for stable scramjet operation. Without it, scramjet combustors tend to explode, violently I might add. It is not exactly constant area, but very slightly divergent, just enough to offset the constricting effect of boundary layer growth and thickening.

The biggest bugaboo of all is injecting the fuel into a scramjet without provoking a shockwave upon each of the fuel jets, which invariably forces an unintended shock-down of the entire combustor flow to subsonic (defeating the entire purpose of the design). And yet, unless you risk those fuel jet shocks, or include a draggy separated-flow step (or multiple such steps), you cannot get the extremely-fast mixing rates that you simply have to have. This is still an extremely-difficult technology to implement, even today!

GW

Last edited by GW Johnson (2024-06-20 08:40:51)

SpaceNut · 2024-06-20 16:59:14

Remember that a ship as a payload delivery system must also account for its type of cargo one wishes to set up in you second project "Fuel Depot on Phobos - Ship Carbon and Water - Make methane and LOX by tahanson43206".

GW Johnson · 2024-06-22 15:46:16

I don't see much point to going to Phobos other than for scientific curiosity. Tom does think it will prove attractive, mostly seemingly "because it's there".

I don't know myself, but if Tom is right, the "because it's there" psychology would apply equally well to anything else built anywhere in some Mars orbit. Like some sort of space station.

I would suggest a station in a far lower orbit than Phobos, in order to reduce the dV for routine travel between it and the surface. There will have to be a lot more ships flying that route, than ships reaching the station from Earth, in order to transfer goods and people to and from the surface.

Just trying to look ahead, and trying to keep an open mind.

GW

SpaceNut · 2024-06-22 16:05:30

Mars atmosphere and mass of the moon over time as we make use of the moon will increase and begin to orbit lower and lower.

Simultaneous estimation of the masses of Mars, Phobos, and Deimos using spacecraft distant encounters

Impact of Utilizing Phobos and Deimos as Waypoints for Mars Human Surface Missions

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