Orbital Mechanics

GW Johnson · 2021-02-05 17:21:23

Tahanson43206:

I guess the best way to understand the things that I have done, is to look at the fundamental equations for something, and try to figure out how to use that to calculate what real systems do, point by point, as if it were a set of equations, a calculation sequence, a pencil, a really big sheet of paper, and a slide rule or pocket calculator. Using the spreadsheet just automates it, but I was doing spreadsheet format with slide rules and calculators long before there was any such thing as a desk top computer. Much less the "Office" software we all use today.

That kind of modeling is only an approximation, it takes real computer software to do the job "right". But these automated hand calculations are "in the ballpark" if you know what you are doing. The real utility of that is two-fold:

(1) it helps you determine exactly what model you want to run in the computer, especially when the choices are many and the informed guidance is slim or nonexistent. It takes effort to set up a proper computer model. There is no point expending that effort multiple times trying to firm up your choice of the model you really want. Do the and calculations "right", then use them to set up the computer model to find the proper answer. Once.

(2) it helps you detect when you are having the "garbage-in/garbage-out" problem. This can occur when your inputs are just inappropriate, or when your computer software just cannot model the physics that you really need (and you may not know that initially). CFD software still suffers from that second issue, even today. To be successful, YOU have to be able to recognize garbage-out when you see it! Being able to do the engineering calculations the manual way that I do, is the best way to detect computer outputs that are garbage, because then you know bad numbers when you see them. Failing that, you might build hardware based on computer predictions that are unknowingly worthless, and end up with an expensive, or even fatal, disaster. Happens all the time.

That is the sort of thing I did for 20 years of an aerospace-defense engineering career focused on new product development. Nobody was ever injured or killed on my watch. But I and my team-mates went where no one has ever been before, many times, and were quite successful at it. It doesn't have to be formal, it just has to tell you what you need to know. I went to the shop countless times with dimensioned sketches on a quad pad, supported by that sort of "manual spreadsheet" analysis, and had hardware built that I fired successfully.

GW

PS -- my defense engineering career was cut short by a plant closure. After that I did some civil engineering, and a lot of teaching, at levels from 7th grade to college graduate school. I also did most of my own automobile overhauls. Which got me my first job after the rocket plant closed: teaching in an automotive and manufacturing engineering technology program at Minnesota State. Where I got my ASE cert as a job requirement, and ended up teaching all their design analysis courses, as well as auto shop classes. You really can pee "yellow ice cubes" if it is -10 F or colder. I have done that.

Last edited by GW Johnson (2021-02-05 17:31:55)

tahanson43206 · 2021-02-05 20:31:39

For GW Johnson re recent posts and this entire topic ...

Thank you for additional guidance as we try to find our way (as a forum).

Here is a draft of a proposal I wrote to SpaceNut recently.

http://newmars.com/forums/viewtopic.php … 99#p176499

I'd appreciate your giving the idea some thought.

We don't have any students (that I know of), who would want to follow a step-by-step construction of your spreadsheet, but there are surely folks "out there" in the world who would appreciate an opportunity to study under your leadership.

***
Regarding the spreadsheet for analysis of interaction with atmosphere ...

This sounds like another valuable collection of experience, insight and knowledge, that it would be appropriate for this forum to develop in tutorial form.

SearchTerm:ballistic entry analysis GW Johnson http://newmars.com/forums/viewtopic.php … 12#p176512
SearchTerm:understand GW Johnson in post above ... how to understand what I've done

This might be something Calliban would be interested in?

I'm definitely interested, but my estimate is that full development of the orbit calculation spreadsheet as a tutorial will take about an Earth-year.

(th)

GW Johnson · 2021-02-06 19:26:03

Tahanson43206:

I'm not at all sure what to tell you about teaching anybody how to use the spreadsheets, or by extension, the equations that underlie them. I thought the user manuals I wrote would do a pretty good job explaining what things were, and where to input them.

The orbital mechanics spreadsheet is just the organized use of about 3 fundamental-equation results of orbital mechanics for elliptical orbits. The one worksheet in it that evaluates Starship performance to Mars is just the organized use of the rocket equation. That last could be updated to include actual thrust sizing and vehicle thrust/weight constraints. Something like that can be done for pretty much any ballistic vehicle and any mission.

The entry mechanics spreadsheet is just the application of what H. Julian Allen did in the early 1950's for steep warhead entry. It's only an approximation, but it was a very good approximation for that application. The main results from it are the peak deceleration and peak heating, and for Mars at modest to high ballistic coefficient, the low altitudes at which the hypersonics are over.

Using it for shallow entries runs smack into the limits of 2-D Cartesian modeling, but if you wrap the range around the curved Earth (or other planet), and consider the constant trajectory angle as locally relative to a curving surface, it's still in the ballpark. After the peak deceleration pulse is when the trajectory noticeably bends downward in the real world, something unmodeled in the analysis.

It's a whole lot less straightforward doing entry heat transfer, but I do have some spreadsheets that supported the entry analysis articles on "exrocketman". Those spreadsheets have no user manuals. But I hoped the "exrocketman" articles were informative.

I'm getting old enough to worry about dying before I can pass this stuff on. That is the reason for the "exrocketman" blog, and for my contributions on this forum. If anything I have done or posted looks interesting, I would be more than happy to share it. But, as you can see, this stuff is not something I can pass on in a simple NewMars posting.

GW

tahanson43206 · 2021-02-06 20:05:16

For GW Johnson re topic and #103 in particular ...

While I would not have spoken as you did about the need to pass along hard earned expertise to the next generation, I've definitely been thinking along those lines.

I am hoping that the idea of creating a tutorial based upon your work will begin to collect some momentum in the members who are currently active in the forum. We also have the potential to bring new (hopefully younger) members into the group who would be at just the point in their studies when your work would fit into their schedule, if we can package it appropriately.

If there is a member of the group who would like to look over the materials GW Johnson has created, just drop a note to NewMarsMember * gmail.com. All the files are saved there.

To create the kind of tutorial I have in mind will take an extended period of time, because my concept is to tackle each cell of the spreadsheet one at a time. Some will contain equations that are at the heart of the science of orbital mechanics, and those will require more than a day for a thorough discussion.

Ultimately I expect to see computers on board space craft in flight with a human crew that can perform all the calculations needed to set up a burn to a particular destination while juggling the positions of the planets and moons that are close enough to have an effect against the all pervading gravity of the Sun, as well as the position, vector and mass of the craft itself.

Whatever that computer looks like, it will be running on the equations GW Johnson put into his spreadsheets, and the operator would benefit from having gained at least a rudimentary understanding of those equations and their use.

Edit: It has crossed my mind that a book organized around Orbital Mechanics by Spreadsheet might have an audience.

It would be something like the radio kits the kids of my generation soldered together. We had to have the ability to perform simple mechanical actions, and to carefully follow directions, and the result was a working radio.

Most young folks today are able to enter values into a spreadsheet. This would be a step beyond balancing a checkbook, but still within the capability of the average person who can follow directions carefully.

(th)

SpaceNut · 2021-02-06 20:39:13

Use a snipit tool basically screen shot to make an image of the spread sheets to post with equations.
Next show how the numbers in the equations effect the look of information on the spread sheet.
Mix with some words and you are teaching the how and why things happen.

tahanson43206 · 2021-02-06 22:01:15

For SpaceNut ... thanks for thinking about how the complex material Dr. Johnson has assembled might be presented.

Some thought needs to be given to the nature of the audience.

Formal education programs seem to try to introduce concepts in small increments, and students are generally given an opportunity to master each increment before going on to the next. In this case, a student would be able to confidently compute a mission plan from Earth to Leo, from Leo to LMO (or Phobos) and from LMO to the surface.

In addition, on behalf of the Ballistic Cargo Delivery topic, I am interested in trajectories that arrive at Phobos (in particular) with velocity so well matched that only the pull of Phobos itself is acting to bring the cargo pod to rest on the regolith.

It certainly seems possible that views of parts of the various spreadsheets would be part of a well designed lesson plan.

I'd like to refer back now to the series of posts collected by GW Johnson with the search string "educationdoneright"

A key element I retain from that very thorough presentation is the mantra to have students demonstrate newly absorbed material as soon as possible after the theoretical session. In this case, it would be appropriate for the student to demonstrate understanding and mastery of the elements of the lesson plan as they are defined.

(th)

tahanson43206 · 2021-02-07 07:39:28

For SpaceNut and for GW Johnson ...

It seems to me (after thinking about SpaceNut's ideas overnight) that the work of GW Johnson can be used as a practical framework around which to build a course of study whose objective is to turn out a fully qualified celestial navigator.

A spreadsheet is a form of automation that is approachable by a great number of people, yet it can perform complex operations under the covers. A comparable phenomenon in the automotive field is the modern automobile. I read a short commentary recently, about how the steering wheel is an interface that performs the same simple, understandable function in a vehicle from the crudest wooden racing cart to the latest Tesla.

GW Johnson has packed knowledge into a dense package that looks approachable to anyone who has seen a spreadsheet.

It is the knowledge that CREATED the spread sheet that an education program would seek to impart to a student.

The only way I know to solve complex multi-year problems like this one is to decide what size steps to take, and then to start the journey.

Today is the Last Sol of Year 35 on Mars. In the Today on Mars topic, there is cause for celebration. After 668 Sols of following the progress of Mars around the Sun, we are about to pass from New Year's Eve to New Year's Day.

Thanks to the guidance of RobertDyck, we are about to demonstrate the power of the Business Calendar designed with his key suggestions.

The achievement was accomplished one Sol at a time, for 668 Sols.

So it is that we should be able to develop a comprehensive educational program to deliver qualify Celestial Navigators, one step at a time.

We can start by thinking about the library of supplemental readings that would form the detail constructed on the Spreadsheet Scaffolding in the mind of the student.

(th)

SpaceNut · 2021-02-07 09:27:03

So thinking about going to mars starts with after you have planned the rocket and manifest, landing site plus science to do, we have set out on the planned trip launch date to make it a reality.

Next comes solving to when to launch to go to mars.

What is a Mars launch window?

Mars and Earth are at their closest to each other when they are at opposition. However, we don't actually want to launch at this point. We want to launch before this point.
We want to use a minimum energy transfer orbit in order to use the least amount of fuel. A Hohmann transfer orbit does this. Our spacecraft starts at Earth's orbit. A Hohmann transfer orbit uses a burn at the starting point (periapsis) that increases the aphelion of the orbit such that it occurs at the orbit of Mars. This will be 180 degrees later in the orbit.
We calculate the period of the orbit that our spacecraft will be in. That turns out to be about 520 days. Our spacecraft is traveling half of an orbit, so our trip will be about 260 days. Mars has an orbital period of 687 days. In 260 days, Mars will travel an angular distance of 136 degrees. That means the optimal time to launch the spacecraft is when Mars is 44 degrees (180-136) ahead of Earth in its orbit, as shown below.

Look we do have a page depository started
Earth-Mars Transfer Trajectory

Of course we are seeing the reality of a planned trip coming up soon for 3 explorers Mars. First the Emirates Mars Mission, then China’s Huoxing-1, and finally NASA’s Mars 2020..

Mars Alert: Why Three Spacecraft Must Leave For The Red Planet Within Weeks Or Miss Their Chance

So we are to late for the one which we are seeing the results of which started last year in July of 2020.

That said the launch windows occur regularly.

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

looks like the next launch window comes in early 2022 with Rosalid Franklin to be launched...

GW Johnson · 2021-02-07 16:15:37

To be accurate, you can't just assume a constant angle rate around an orbit. The most accurate measure is about 3 centuries old. The radius vector from primary to satellite (not center to satellite!!) sweeps out equal areas in equal times. The more the orbit is eccentric, the more the satellite velocity varies around the orbit.

In the old days, we'd plot half the ellipse with its long axis the horizontal axis. We'd measure the areas with a planimeter: the half-ellipse and the area swept out by the radius vector, compute their ratio, and multiply half the orbital period by that ratio. Today, you have to integrate the area under the curve and subtract a triangle or two.

I'm guessing most of the readership is too young to know what a planimeter is, and what it does.

Same problem as "what is a codpiece"? One of my favorite sayings about doing things multiply-redundant against failure is "suspenders, belt, and armored codpiece". I've had to explain what a codpiece is to most of my classes in which I said that.

GW

Last edited by GW Johnson (2021-02-07 16:16:23)

SpaceNut · 2021-02-07 17:50:42

tahanson43206 captured that mars velocity in the calendar.

SpaceNut · 2021-02-08 17:30:02

GW Johnson wrote:

TH:
Of course you can use the rocket vehicle performance spreadsheet for any stage doing any sequence of burns, up to 3 burns! If you have less than 3, input the dV into the burn or burns that you do make, as dV values for burns 1 (and 2 as appropriate). Input a zero dV and any dummy engine values for burn 3 (and for 2 if that dV is also 0).
For vertical launch from Earth with a clean slender vehicle, I usually factor the first stage burn by 1 plus .05 for drag loss plus .05 for gravity loss. The second stage of a two-stage vehicle is usually exoatmospheric and very nearly horizontal, so its portion need not be factored, or at most only about 1.02 for gravity loss.
For those analyses that don't suffer from being a tad cruder, I usually apply the 1+.05+.05 factors to the surface circular orbit velocity, for launch from Earth, and unfactored (factor = 1.000) for departure from orbit. The surface circular velocity is higher than circular at orbit altitude, by precisely the potential energy of going to that altitude.
For Mars, I reduce the gravity factor with Mars surface gee: .05*.384, and the drag factor with Mars's relative surface density: .05*.007. Similarly, for launch from the moon, I reduce the gravity factor by the moon's surface gee: .05*.165, and zero-out the drag factor.
For propulsive landing burns, the ideal dV is just the speed at which you finally approach the surface. It needs a whopping factor applied to it, to reflect any hover or divert or other maneuvering you need to do in order to avoid landing obstacles, or to correct being off-course. I have been using factor 1.5, but factor 2 is not unreasonable.
I took a closer look at the two demo Starship examples in the spreadsheet, and increased them to four examples. I "upped" the required vertical gee capability at landing from 0.5 to 0.7 gees net, just because Spacex has been having landing problems with SN-8 and SN-9.
For the average Hohmann transfer, I'm showing 353 metric tons of payload deliverable to Mars, with nearly-dry tanks at landing, and a full-capacity 1200 metric ton propellant load aboard at departure from low Earth orbit. That's an 8.6 month trip, one-way.
If you go to the faster 2-year abort trajectory, you get there in 4.3 months one way. Under the same dry-tanks landing and full-capacity propellant load conditions, you can deliver 248 metric tons of payload to Mars.
Those require something like 6-8, maybe 9 tankers to refill to full capacity in LEO. Spacex has never shown that many tankers on its website presentations, so I looked at a nominal 150 metric ton payload to Mars, and the min propellant loadout in LEO to deliver it.
For the 8.6 month average Hohmann trip, 150 tons payload required only 686 tons of propellant out of a 1200 ton capacity. For the 4.3 month trip on the fast trajectory, that same 150 ton payload delivery required 879 metric tons of propellant be aboard at LEO departure. Bear in mind that the ship is supposed to arrive in LEO with the allowance for deorbit and landing still aboard. Something on the order of 50 tons. Maybe less.
I was surprised and pleased by those greatly-reduced propellant loadout numbers. Instead of 6-8, maybe 9 tankers, we are looking at 3-4, maybe 5 tankers. That is remarkable, and I believe it is why Spacex has been showing the on-orbit refilling representations that it has.
This kind of reverse-engineering is not new to me. Decades ago, I did some pretty similar analyses reverse-engineering the Russian SA-6 surface-to-air missile. I and 3 propellant chemists also reverse-engineered their ramjet fuel propellant, right down to the level of their quality control on the process.
If Moshe Gill is still alive in Israel, he and I are the only two westerners left alive who know how to make that stuff at production readiness. The other 2 chemists died many years ago.
GW

tahanson43206 · 2021-02-08 20:07:51

For GW Johnson re a point that keeps coming up ...

For some reason, every discussion I have ever seen about flights to Mars (or anywhere in the Solar System) seems to be limited to a space craft carrying the fuel it needs to accelerate away from Earth.

Many decades ago, humans solved that problem for flights from Earth to orbit. It is now generally accepted that a vehicle benefits by receiving a push from Earth. Elon Musk has achieved a break through by demonstrating re-usability of the first stage.

For some reason that I can't fathom, the idea of using a first stage for a push to Mars just doesn't seem to occur to anyone.

I suppose the fact that no one has demonstrated refueling in orbit is a reasonable explanation. But I think this is a Chicken-Egg problem.

No one has mastered refueling in orbit because no one ** wanted ** it enough to justify the investment needed to make it happen.

Once Elon and his team ** do ** demonstrate refueling in orbit, ** suddenly ** it will occur to someone that it is ridiculous (or whatever the scholarly equivalent might be) to send a rocket to Mars without a full load of fuel on board.

The Earth-based shipping industry long ago figured out how to use tug boats of all shapes and sizes to assist large ocean-going vessels to maneuver in port.

Some large ocean-going tugs even haul unpowered full sized ships across the ocean.

There is no practical reason that I can think of why a space pusher/tug vehicle can't make a good living for the crew and stock holders by giving Elon's Starships a vigorous sendoff, so they arrive at Mars with more than enough fuel to perform multiple maneuvers.

(th)

SpaceNut · 2021-02-08 20:48:59

Space x sacrifices fuel rather than rocket stages but that comes at a penalty of a stage that is on orbit with next to no basic abilities to do anything for going anywhere as its got to get fully refueled to achieve that next step.

The space tug is not a new concept and it relies on the refueling while in orbit to achieve movement from fuel depots.

Space tug

ATV as a tug

On Orbit fuel depot

There are many topics which mention both of these and other options for what exists that could be converted to the task.

kbd512 · 2021-02-08 22:09:49

tahanson43206,

Your thinking about delivering a fully fueled rocket booster to Mars runs afoul of the rocket equation. It takes fuel mass to push any other mass. The goal is to deliver maximum tonnage of food / water / air / medical supplies / life support equipment / construction materials / power plants / water extraction equipment / scientific instruments / machinery to make new machines / and people to run all of that stuff to the surface of Mars. Once all that stuff gets there, it's never coming back. Burning even more fuel, merely to ship fuel, to a place chock full of the chemical constituents to produce whatever additional fuel is required, is not a worthy goal unto itself. If we do decide to ship fuel to Mars, then it's a means to an end.

Calliban · 2021-02-09 05:44:45

kbd512 wrote:

tahanson43206,
Your thinking about delivering a fully fueled rocket booster to Mars runs afoul of the rocket equation. It takes fuel mass to push any other mass. The goal is to deliver maximum tonnage of food / water / air / medical supplies / life support equipment / construction materials / power plants / water extraction equipment / scientific instruments / machinery to make new machines / and people to run all of that stuff to the surface of Mars. Once all that stuff gets there, it's never coming back. Burning even more fuel, merely to ship fuel, to a place chock full of the chemical constituents to produce whatever additional fuel is required, is not a worthy goal unto itself. If we do decide to ship fuel to Mars, then it's a means to an end.

I think what tahanson is getting at, if I understand what he is saying, is using a low thrust ionic propulsion 'tug' to drag a fully fuelled upper stage from LEO into high Earth orbit. This would mean that most of the energy needed to get from low earth orbit to Earth escape would be provided by the solar electric propelled tug. It would increase the overall cargo carrying capacity of a Starship upper stage significantly. No doubt it will eventually be done, if Elon is able to get his Mars colonisation plans off the ground.

One of the first things that a Mars colony can sensibly export, is the propellant needed to to travel between Mars orbit and low Earth orbit. Lifting materials from Mars surface to LMO is much easier and would have much superior mass ratio than lifting from Earth surface to LEO. It could be done using the sort of SSTOs that have high enough structural mass to be reusable in the way that jet aeroplanes are here on Earth. If high ISP electric propulsion can be used to transport goods between LMO and LEO, then it will eventually make financial sense to manufacture propellant on Mars and transport it to LEO to refuel Starship upper stages. In fact, anything heavy but technologically simple that is needed in LEO, would be cheaper to make on Mars and ship to LEO, than it would be to launch from Earth surface. We could ship argon from Mars to LEO and use it to propel outgoing or transorbital electric transportation.

We could have facilities in Earth orbit that manufacture satellites and manned spacecraft. The people, design services and low mass, high tech components would come from Earth. High mass components and propellant would be delivered from Mars. The only place that a rocket engine would ultimately get used is in transportation from Earth surface to LEO or from Mars surface to LMO. I think that ultimately, Mars will have a significant cost advantage in manufacturing anything that human beings intend to use in space. Earth will have an advantage in providing design and information services and certain high precision components. All of this is some way down the road of course. It will be a while before any colony on Mars is able to perform any sort of heavy manufacturing activity.

Last edited by Calliban (2021-02-09 06:17:32)

tahanson43206 · 2021-02-09 07:56:53

For kbd512 re #114

Thanks for beginning to think about how a fully fueled Starship might be launched to Mars without using any of its onboard fuel.

You are not alone in having difficulty thinking this through. It is clear (from available writings that I have seen) that NO ONE has written about using a pusher vehicle to shove a fully loaded Starship on it's way to Mars.

For Calliban .... thank you for picking up on the idea ... The idea of using ion engines is certainly a good one, and I expect it will become common if not standard practice.

However, I will try again ...

To launch mass from the surface of the Earth, humans have learned to use multiple stages ...

The number of stages varies from two to four, but at NO time in human history has a vehicle (that I've ever heard of) made it's way to LEO with only one stage.

However, for some reason that I can only assume is due to inability to refuel vehicles, no one has ever proposed to top off a vehicle in orbit, and then use another vehicle to provide the impulse needed to push it to the elliptical trajectory it needs.

The pusher vehicle would be (and I'm confident, ** will ** be) as ubiquitous as Elon's first stage re-usable rockets are now.

There is no reason to consume ANY of the Starship's propellant to leave Earth.

Thanks to both of you for engaging with me on this ...

For SpaceNut ... thanks for helping as well, with links to earlier posts in the archive, and other resources as well.

There is a secondary business opportunity waiting for development. As far as I can see, it is readers of the NewMars forum who are first to begin thinking about it.

(th)

GW Johnson · 2021-02-09 10:54:41

Each vehicle design requires a development effort that is very significant. We saw that with Falcon-9, then Falcon-Heavy, and now currently with Starship, and soon its Superheavy booster stage. Doing one of these at a time is enough even for the largest outfits.

When Starship/Superheavy begins flying to orbit, assuming all this effort is successful, there won't be any other vehicles to act as tugs or pushers or anything else, precisely because they haven't yet been developed. What that means is if you are going to use Starship for anything other than coasting in LEO, you MUST do on-orbit propellant refilling at one level or another. Spacex is already working on how to do that, but it's still only on paper right now.

Bear in mind that Superheavy will take a fully-loaded Starship to around 30-50 miles up at a speed in the neighborhood of 2 km/s with respect to Earth, and a path angle under 10 degrees. From there, Starship alone has to speed on up into orbit at around 200 miles up and 8 km/s speed. It takes most of the propellant aboard the vehicle to do that, if loaded with something like 100-150 tons of payload. There will be around 30-50 tons of propellant left, which is required for the deorbit burn, and for the flip and touchdown burn (which is larger).

I had the blinders on for the last 2-3 years, thinking that the only feasible way for Starship to leave Earth orbit and go anywhere else, was to fully refill its propellant to the max capacity (currently said to be 1200 tons). When I ran numbers like that for going to Mars, I got max payloads bigger than it could carry up to low Earth orbit.

When I finally looked at carrying to Mars only what it could lift to orbit, I found I did not need a full refill to propellant capacity. I had never looked at that before! That's the "blinders" I mentioned. It's less payload to Mars, but it cuts the tankers to around half to two thirds of what it required for a full refill, and that cuts ticket price dramatically, because that is proportional to the total number of launches.

I was late coming to that realization. My guess is that Spacex came to that realization a few years ago, because in none of their public presentation materials do I see more than about 4 tankers represented. That's a partial propellant load. It goes with the reduced payload, down to what the vehicle can carry to low Earth orbit. Now it all makes sense.

A vehicle sent to Mars like that arrives with only around 50+ tons of propellant with which to make the landing. The escape from LEO onto the transfer trajectory consumes the great bulk of the propellant. Then there are required course correction burns, midcourse and approaching Mars. Entry is direct aerobrake from interplanetary speeds in the 5.5-7.5 km/s range with respect to Mars, which is why the heat shield must be robust. Sticking the landing on Mars needs more propellant than on Earth, because the atmosphere is thinner, and the trajectory is necessarily different. You'll be near dry tanks upon arrival, which is what Spacex has always said about this design concept.

You have to refill fully on Mars in order to come home. The escape from the surface of Mars is substantially more demanding than the escape from low Earth orbit, which is why return payload is always lower, despite the full refill with locally-made propellant on Mars. You still have the course corrections to make, and you still need around 20-30 tons of propellant to stick the landing. Your heat shield needs to be more robust than an Earth orbit entry-capable heat shield (7.9-8 km/s), because the free entry speed returning to Earth falls in the 12-14 km/s range. It has to survive a Mars entry, and then an Earth entry, without refurbishment.

That last makes me think that reradiating refractory ceramic tiles will prove infeasible, because they won't be able to re-radiate efficiently once the plasma radiation heating makes the plasma more opaque, at speeds above 10 km/s. Ablatives may be the only feasible way to go. I don't know, and I doubt Spacex does, yet. That will come with time. But re-radiating ceramics are a viable option for entry speeds 10 km/s and under. That I also know.

GW

Last edited by GW Johnson (2021-02-09 11:07:17)

SpaceNut · 2021-02-09 18:06:22

Right now adding ceramic tiles is a performance that will need to be achieve when the super-heavy is finished and leads to the two staged beast which depending on if that vehicle doubles as the landing return vehicle will need or not any heat shielding for staying in orbit to catch a ride home with another vehicle.
Leaving a non heat shielded starship unit in orbit would use less fuels to depart and depending on destination may still not need a heat shield to perform its duties for lunar missions.
For mars we need to know if we are going to make use of a seperate transport or the same vehicle for the Mars landing as that changes mission profiles.

SpaceNut · 2021-02-15 10:38:28

Rocket Vehicle Performance Spreadsheet

http://marspedia.org/Marspedia_Tutorials

tahanson43206 · 2021-02-25 07:31:20

For GW Johnson re Sun Synchronous orbits for Personal SPS

Here is a link to a Wikipedia article on sun synchronous orbits, for those in the readership who may wish to refresh ...

https://en.wikipedia.org/wiki/Sun-synchronous_orbit

For GW Johnson ... The US Defense Department just announced a test of a solar panel in the X37b. The announcement implied that the researchers had accomplished something, but I now understand that anything they achieved was of low impact.

What the announcement ** did ** accomplish was to awaken (in my mind at least) awareness that SPS need not necessarily ride out at GEO. Calliban then reminded me (in the Technology Updates topic) that SPS located in GEO would require a large rectenna on the ground, due to dispersion of the microwave beam over the distance involved.

An SPS operating in LEO could deliver substantial power to a smaller receiving antenna, but (as Calliban reminded me) the satellite would be in view for only part of the day, and the Sun would be available for less than half of a day.

In scanning the Wikipedia article cited above, I noted that spy and observation satellites have been placed into sun synchronous orbits for years. There is one category of these orbits (apparently) that allows the satellite to enjoy continuous illumination by the Sun.

Such a satellite, in such an orbit could (presumably) provide 30 minutes or so of high density power to a customer on the ground once a day, but always at the same time each day, although I'm less certain about that detail.

Do you have a suggestion for how to think about this potential business opportunity?

(th)

GW Johnson · 2021-02-26 13:01:58

TH:

Re sun-synchronous orbits: I'm no orbital mechanics expert, as I have said before. I can do the basic two-body textbook model, and I know the necessary "trick" to combine 2-body results to estimate delta-vee almost as accurately as a real 3-body computer code. I can support things on these forums with that capability, which is related to vehicle sizing, not to actual navigation. But I know next-to-nothing about the nonuniform mass distribution stuff that is behind the precession of these sun-synchronous orbits.

Re SPS applications: assuming SPS means "solar power satellite", what I know about that matches up with what you heard from Calliban - that geosynch distances suffer from excessive beam spreading, requiring a very large receiving antenna array.

I do know that this beam-spreading problem is compensated by a steady aiming of the antenna array at the satellite, and by a steady aiming of the beam at the receiving array. The danger, if that aim fails, is the beam irradiating unintended people or hardware: consequences are potentially deadly.

If you put one of these SPS things in a low orbit, and make it sun-synchronous, you solve the problem of always steadily receiving sunlight, but you greatly aggravate both of the aiming problems. The array has to rapidly track the satellite while it is in view, and the satellite has to "hit" (only) the array with its beam, which looks like a fast-moving target to the satellite. That greatly raises the risks of having the beam hit unintended targets.

It's one thing to aim a camera lens at a moving target (especially when rigged to an already-folded optics system in a spy satellite). It is quite another to steer a microwave beam at a fast-moving target very, very accurately, and with utter reliability.

Those are some very serious things to think about. Geosynchronous might well be the easier set of problems to solve.

GW

Last edited by GW Johnson (2021-02-26 13:04:40)

tahanson43206 · 2021-02-26 16:55:34

For Gw Johnson re #121

Thanks for taking up the question! Your post is worth re-reading and reflection, so here is a pointer:

SearchTerm:SunSynchronousOrbit SPS in LEO proposal issues
SearchTerm:SPSinLEO
SearchTerm:SPS

The intensity of a microwave beam is a valid concern.

The effect of a large number of such beams on the atmosphere (due to agitation of water vapor) is a concern.

SPS in GEO might well be hastened (as you've suggested) by serious study of the LEO option.

The US Military experiments are (presumably) directed at meeting power supply needs of deployed troops, so they will be having to deal with the issues you've identified!

(th)

SpaceNut · 2021-03-28 17:03:55

Gee have we really forgotten how to pilot man to the moon's surface and back?

https://www.spacedaily.com/m/reports/NA … s_999.html

tahanson43206 · 2021-03-28 17:18:04

For SpaceNut re #123

The people who achieved success with the Apollo missions are either deceased or long out of circulation.

In any case, the celestial objects have moved into new positions, so each new mission has to be recalculated.

In glancing at the article, I see signs that the primitive techniques used by the Apollo crews are still available, but greatly supplemented by "modern" technologies. If need be, the astronauts could still take optical star sightings as the crew had to do in the 1960's, but I get the impression modern location reporting systems (GPS, et al) will assist the computers in making the fine adjustments needed.

(th)

RobertDyck · 2021-03-28 20:11:17

A friend was president of the local chapter of the Royal Astronomical Society of Canada. He took me to several meetings while he was president. At one meeting, the presenter detailed the computer of Apollo LM. He said it had 75 kilobytes (kB) of memory. The LM had 2 panes of glass for windows. The commander's side had tick marks etched. The commander would move his head up and down until tick marks on the inner and outer panes both lined up with a landmark he wanted to head toward, then he would call out the number of that tick mark. The lunar module pilot would then enter that number into the keypad of the LM computer. The computer had a single line numeric read-out, like a calculator. That number told the commander how to navigate. I forget what the read-out number meant exactly, but that's how they navigated.

Alien movie premiered in 1979. Here's a clip from the movie, showing entry into an atmospheric entry trajectory. This shows 3D visualization, today we can do even better. (Click image for YouTube. I set starting time to navigation display.)

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