Space X News - lunar tourism and delayed BFR hop

louis · 2018-09-14 02:38:13

Space X ready to kickstart lunar tourism...great news.

The BFR hop flights look they have already slipped into late 2019, so the 2022 Mars mission is looking more precarious but lunar tourism will certainly put money in the Space X coffers.

"While Musk said earlier this year those tests could begin in the first half of 2019, Shotwell suggested recently that date had slipped. “I think we’ll be hopping that second stage next year, late next year,” she said during a panel at DARPA’s D60 conference Sept. 6 near Washington."

Also of interest to the rocketeers here:

"An illustration posted with the tweet about the lunar mission announcement appeared to show minor changes to the BFR spaceship design since Musk’s last major speech about it last September at the International Astronautical Congress (IAC) in Australia. Those changes include larger tail fins and seven engines in its base, versus six. Musk, ased on Twitter if the illustration represented the current design of the BFR, responded simply with “Yes.” "

https://spacenews.com/spacex-to-announc … ion-plans/

SpaceNut · 2018-09-14 17:19:23

This is going to be an Apollo 8 lunar flyby mission with a BFR.

Now we will not need a full fuel tank refill for this mission even with the 100 mt plus payload capability.

In fact if its only a situation of roughly 2 weeks of supplies say 20 mt it should be long enough to test out most of the systems required for longer missions.

louis · 2018-09-14 17:46:01

Yes. I think Space X are good at synergy - maximising impact by meshing different processes. I am sure a lunar tourism project would also be in effect a long mission testing programme.

SpaceNut wrote:

This is going to be an Apollo 8 lunar flyby mission with a BFR.
Now we will not need a full fuel tank refill for this mission even with the 100 mt plus payload capability.
In fact if its only a situation of roughly 2 weeks of supplies say 20 mt it should be long enough to test out most of the systems required for longer missions.

SpaceNut · 2018-09-14 18:06:20

The earth test hops would be a good start at testing what it will take to land on the moon....

Even apollo did test deorbits and returns before it went for the landing.

The names should be coming out soon....

Void · 2018-09-15 13:31:11

Entertaining Louis and Spacenut. I took a little time to try to puzzle this. Sometimes I get rather close on a puzzle like this, sometimes I really mess it up.

Anyway, here is the link Ala Louis:
https://spacenews.com/spacex-to-announc … ion-plans/

The things I notice:
1) The engine configuration in the picture does indicate 7 engines, but I expected that two of them would be for landing. One as a backup to assure air liner reliability against the probability of crashes. The artistic rendering does not support that notion of two landing engines at the center. Instead there is one in the center and 6 other engines around it.
2) Why the 7th engine? The engines cannot preform as expected? I don't think so. They want to add more lift in the initial launch path? I am inclined to think so. Why? Are the expanded wings and tail both extra weight and drag? Perhaps.
3) I consider this the third revision. On the second revision, small wings were added reluctantly to help balance the loads on flight. However now, they expand the wings and add a tail.

Some of the why might be indicated here:
Quote:

Shotwell, speaking at the World Satellite Business Week conference here Sept. 11, didn’t discuss lunar missions but did emphasize the importance of human spaceflight to the company’s long-term ambitions. “I do think ultimately — and I’m not going to talk about timelines — but I do think that will probably be the majority of our business in the future, flying people,” she said.

So, at least one version of BFS more like an airliner or space shuttle.

And that brings up more puzzling. I can see how the wings and tail might allow this new version to do some of the maneuvers of the space shuttle. I believe the shuttle did some maneuvers that would shed momentum to atmosphere. Circles?

For Earth and Mars, I might suppose that the wings and tail might help navigation at high speeds in very thin atmosphere???? Or am I wrong? Really interested as to why they now want a tail fin. Navigation I presume.

Bigger wings will be dead weight to lift going up and drag as well. However coming down, would they possibly help with aeroburn? The drawings don't all that much give strong clues on how big the wings will be. They are still rather small I think.

So, the things the space shuttle did with its form was re-enter the atmosphere really hot, probably to the margins of permission.

Thrusters to orient the craft at very high altitude, and then transitioning to atmospheric navigation methods as it descended.

Doing some curves and loops to shed speed I believe when it got lower into the atmosphere.

Doing a controlled stall as it intercepted the space above the runway.

Landing horizontally on wheels.

…
Reverse order speculative answers to the above:

I am expecting that they will not be adding wheels and horizontal landing on either the Moon or Mars, and at this time not for Earth. Too much complications added and also too much weight added I presume. But think 50 years from now, maybe grandchild of BFS-Earth might have wheels and horizontal landing. I would not know really. Is it better to use landing engines? On the Moon and Mars now certainly it is.

The controlled stall? Could you do it somewhat higher above the ground and use your landing engines to do a tail landing say from just a necessary safe stall height? I don't know if the glide characteristics of BFS Edition #3 could do such a stall. If it could might it conserve fuel? Could you do any kind of a high speed and higher altitude stall on Mars? I certainly don't know.

Curves and loops in the Earth atmosphere to shed speed? I would guess. Could it work to some useful degree on entry to Mars? I don't know. It would be nice if it could.

Navigation in the Earth atmosphere by both thrusters way on high, and wing and tail flaps lower, I am presuming as true for Earth. I wonder how much value for Mars.

Are the wings worth while for aeroburns for Earth and/or Mars? I am guessing for Earth yes, and maybe for Mars also. It would be a bit like having a ballute. The amount of aeroburn from the wings would be rather large I think relative to the contribution to dead weight they would add.

Perhaps some kind hearted rocket people could say better.

Done.

Last edited by Void (2018-09-15 14:03:14)

SpaceNut · 2018-09-15 17:33:50

You are correct in that adding second stage mass requires more thrust at lift off but it also reduces the payload to orbit or places it into a lower orbit in order to keep the larger mass. The winglets will add surface area for a landing on mars as the heashield will include the wings to slow the vehicle before doing its tail down ending.

Void · 2018-09-15 18:19:50

That was a nice reply. I have a further question:

While it is magnificent to put a BFS on top of a BFR, and move people/things to other locations planetwide or to the Moon, or around the Moon, or to Mars even, what about BFS on Earth the Moon or Mars?

Start with the Earth. Can you move people in continental (Usually) pathways with a suborbital BFS?

Consider the Americas. I should think that you could go from the northern Canada/Alaska to south Argentina/Chile, maybe even Antarctica with several sub orbital hops. And then to get to the less old world (Europe), cross Canada, Greenland, Iceland, Atlantic Europe (Trying to lift Greenland and Iceland into our domain ).

Siberia is another such potential pathway from the Americas to the not so old world to the rather more old world.

Many hops. I just wonder. Maybe.

Done.

Last edited by Void (2018-09-15 18:21:07)

SpaceNut · 2018-09-15 19:14:45

That would be problematic in that the trajectory could be confused with a ballistic missle launch.

I did however see in some of the early articles that it was an intent to do so as you have described with the BFR but at only 100 tickets for a launch, I think that it is a cost problem...
Its a modified first stage only for the hops.

Near the bottom of the page is the time schedules for such flights
https://www.spacex.com/mars

Void · 2018-09-15 22:14:41

A bit annoyed Spacenut.

I speculated on the technological capability of using the BFS without the BFR 1st stage, and then you instead of helping me with the needed understanding of capability of the device and if it could be done, tried to make me both fix international nuclear weapons relationships and economics.

I do understand that international assurance of safety from others both us and those who might fear us is going to be important. Still how does removing the BRR and using the BFS alone make the world more dangerous?

And then economics. I would think we can start small. How about if BFS will only be used in the Americas first. There is just one declared nuclear power in the Americas, and it likes money. We also like more than money, we like prosperity. That means in simplified reality having as many potatoes as you need and might want. So then we are not likely to use BFS to nuke other countries in the Americas as to do so would reduce profits, both by making extraction of materials we need more complicated, and to kill our potential customers. Not so business wise I think.

So then three apparent issues:
1) What is the technological capability of the BFS to transport people by hops?
and the two you raised:
2) How do we assure nervous international entities/ourselves that they/we, are safe from deception? I don't understand why using BFS without BFR makes this a bigger problem.
3) Economics. I think I posted a quote from a person from SpaceX who indicated that transporting people was likely to be their biggest business enterprise in the future stretching out. I guess I don't understand why using the BFS without the BFR would be less profitable, if there was a business case for short hops.

I don't know if there is a business case for transporting people on relatively short hops. I guess that was the major question.

So here we go again per Louis's link:
https://spacenews.com/spacex-to-announc … ion-plans/

Quote:

Shotwell, speaking at the World Satellite Business Week conference here Sept. 11, didn’t discuss lunar missions but did emphasize the importance of human spaceflight to the company’s long-term ambitions. “I do think ultimately — and I’m not going to talk about timelines — but I do think that will probably be the majority of our business in the future, flying people,” she said.

Why is it I cannot ask a question, without being bullied in an unreasonable fashion?

Perhaps we are so alien to each other that we completely misunderstand each other?

Done.

Last edited by Void (2018-09-15 22:32:02)

kbd512 · 2018-09-15 22:55:16

ICBM's typically don't have trajectories that carry them between major cities within a country or to territory not under the control of adversarial nations. I don't believe LA to Tokyo or LA to Dubai would be mistaken for LA to Vladivostok or LA to Moscow. This ICBM "freak out" scenario over a suborbital BFS launch seems implausible at best. The OTH radar technology available to the Russians and Chinese is at least as good as ours is, which means they can tell if something like a warhead is on a trajectory that carries it over their countries. I doubt a civilian BFS launch that's video taped and broadcast around the world would cause them to fear a sneak attack. A live broadcast has to be the worst possible way to start a sneak attack as well.

NASA routinely launches sounding rockets that more closely resemble potential ICBM launches with payloads that more closely resemble reentry vehicles, and some of their payloads are actually reentry vehicles, yet we don't hear about the Russians or Chinese confusing those technology demonstrator test flights with ICBM launches. Maybe there'd be more clutter with daily hops between major cities, but I don't see that being more of a potential problem than the space rocks that routinely reenter over their countries and our country.

SpaceNut · 2018-09-16 08:42:09

Thanks kbd512, so command structure and location to where we are going with a BFS is covered by the FAA and would get the same radar support to show where any flight is going from large metropolatan launch sites. If BFS does get built it will be no problem then with this monitoring system already in place.

louis · 2018-09-16 10:57:49

Video focussing on the latest BFR design:

https://www.youtube.com/watch?v=Yy82MwITw34

GW Johnson · 2018-09-16 11:39:10

I already told participants on these forums that BFR/BFS was a paper design early in the design process. The 2016 original was too big to afford and too unstable in pitch during ascent and re-entry without any fins (Spacex itself said so). The 2017 version had the stabilizing pair of fins, and the more affordable small size, and 4 landing legs with a small footprint size relative to the length of the vehicle.

Now, it appears more design work has been done, leading to a vertical fin to add yaw stability during ascent and reentry. Plus the other two fins seem to have been enlarged. Paying for this extra structure is probably the dominant effect of the 2018 revision from 150-ton payloads to "over-100 ton payloads". But, if the fins replace the landing legs with landing pads of some sort at the rear of their tips, the footprint is now wider relative to vehicle length, affording more reliability landing on rough ground.

It would appear someone has been addressing aerodynamics and flying qualities. The nose shape is different, more shuttle-like. And there seems to be some sort of aerodynamic surfaces added alongside the nose. My guess is that the undefined feature near the root of the two fins is a hinge line. Flatter like a pair of aft wings does a better job for pitch stability during entry. But to use them in lieu of separate landing legs, they need tips spaced 120 degrees apart.

I'm just guessing that the added surfaces alongside the nose have something to do with the strong pitch-up into the tail-slide maneuver that puts it descending vertically tail-first for the retropropulsive touchdown. On Earth, this does not need to be as nail-bitingly low-altitude a maneuver as on Mars, but it still needs to be done at rather modest altitudes, perhaps only a few km above the surface. It is high ballistic coefficient entry on Mars that drives the pitch-up into the tail slide to an altitude at or under 1 km.

Extra drag and weight with these changes is what probably drives the switch from 6 to 7 engines. It is nowhere near as difficult as some believe to build a high-performing engine that works from sea level to vacuum with one fixed bell design. You design for perfect expansion at some middle altitude, and operate overexpanded (but not separated !!!) at sea level, and underexpanded in vacuum, but with more expansion ratio than a sea level design can afford. The recurved shape of a curved expansion bell has nothing to do with separation, and everything to do with collimating exit streamlines more axially, in as short a shape as possible.

Having one engine design, not two, is more efficient to build, and offers more thrust tailoring and engine-out flexibility in operation. This is a positive step forward for a design that must land retropropulsively in vacuum as well as at sea level.

As for when short hop testing begins, Shotwell has pushed that back from late 2019 into 2020. There is quite a lot of testing to do as a single stage item, and separate testing of the BFR first stage, before the two can be combined and tested together. I think you can forget about first cargo flights to Mars in 2022, or first flights around the moon in 2022. 2026 looks more reasonable, provided they can fund all the enormous amount of work that has to be done quite fast, actually, to meet that date.

I know the P-51 went from concept to first test flight in 120 days, but this BFR/BFS thing is not a small propeller-powered single-seat airplane embodying a new combination of all-existing technologies, with two dozen steam gauges for the pilot to use while hand-flying it. There are many aspects to BFR/BFS that are completely unprecendented, except in science fiction.

GW

Last edited by GW Johnson (2018-09-16 12:00:53)

SpaceNut · 2018-09-16 15:09:15

The glide path of the BFS-com (commercial Flight) is not anything like that of a plain or a glider even with the winglets and is more like a lifting body with these little stubs. This version will not need the normal planetary heat shield but will be more like the payload shroud for what it needs to be made of but will need something for power source beyond batteries and its not solar power panels on the side of the craft.

kbd512 · 2018-09-16 16:22:52

SpaceNut,

That's why I came up with all those specialist variants that were optimized for a specific type of mission. Suborbital hops, interplanetary transit, landing in a thin atmosphere or a complete vacuum with varying gravitation, orbital propellant transfer, and robotic satellite delivery all have very different design requirements. It's too much for a single design because the design requirements are pulling in opposite directions. Maybe the variants can all use the same propellant tanks and engines, but all other similarity ends there.

Set the basic design parameters, optimize it to do one thing really well, and then produce as many as the market will support. That's what SpaceX delivered in spades with Falcon 9 and Falcon Heavy. There are no other rockets available today that have more general utility or lower operating costs. I fail to see how this new BFR/BFS design will fare substantially better without heavily optimizing it to do one thing exceptionally well.

There's no requirement for GCR radiation shielding for suborbital flights or robotic tankers and satellite delivery, for example. The tanker and cargo variants don't even need humans aboard, so that alone dramatically alters how the vehicle will be equipped. The tanker needs refueling probes for better standoff from the vehicle it's refueling and larger propellant tanks, but no cargo bay for satellites. Landing on Earth is nothing like landing on the moon or Mars, even if you can accomplish all three with retro-propulsion.

SpaceNut · 2018-09-16 17:09:57

The designs of boeing and Lockheed do have that same flexibilty to some extent but not the reuse or costs but then again they are from a different era where cost was no concern....The new platform are not changing either only just newer....

louis · 2018-09-16 18:02:50

Nice summary with added insight GW - but please can you give me a quote for your contention that "Shotwell has pushed that back from late 2019 into 2020." She said "next year" and next year is 2019. She has put the hop tests later than Musk did (first half of 2019) but it's still 2019. I think the confusion arose because she was referencing Musk's original outline, so you and perhaps some others thought next year meant the year after Musk's initial prediction.

So come on: give us the Shotwell quote where you think she is saying 2020.

GW Johnson wrote:

I already told participants on these forums that BFR/BFS was a paper design early in the design process. The 2016 original was too big to afford and too unstable in pitch during ascent and re-entry without any fins (Spacex itself said so). The 2017 version had the stabilizing pair of fins, and the more affordable small size, and 4 landing legs with a small footprint size relative to the length of the vehicle.
Now, it appears more design work has been done, leading to a vertical fin to add yaw stability during ascent and reentry. Plus the other two fins seem to have been enlarged. Paying for this extra structure is probably the dominant effect of the 2018 revision from 150-ton payloads to "over-100 ton payloads". But, if the fins replace the landing legs with landing pads of some sort at the rear of their tips, the footprint is now wider relative to vehicle length, affording more reliability landing on rough ground.
It would appear someone has been addressing aerodynamics and flying qualities. The nose shape is different, more shuttle-like. And there seems to be some sort of aerodynamic surfaces added alongside the nose. My guess is that the undefined feature near the root of the two fins is a hinge line. Flatter like a pair of aft wings does a better job for pitch stability during entry. But to use them in lieu of separate landing legs, they need tips spaced 120 degrees apart.
I'm just guessing that the added surfaces alongside the nose have something to do with the strong pitch-up into the tail-slide maneuver that puts it descending vertically tail-first for the retropropulsive touchdown. On Earth, this does not need to be as nail-bitingly low-altitude a maneuver as on Mars, but it still needs to be done at rather modest altitudes, perhaps only a few km above the surface. It is high ballistic coefficient entry on Mars that drives the pitch-up into the tail slide to an altitude at or under 1 km.
Extra drag and weight with these changes is what probably drives the switch from 6 to 7 engines. It is nowhere near as difficult as some believe to build a high-performing engine that works from sea level to vacuum with one fixed bell design. You design for perfect expansion at some middle altitude, and operate overexpanded (but not separated !!!) at sea level, and underexpanded in vacuum, but with more expansion ratio than a sea level design can afford. The recurved shape of a curved expansion bell has nothing to do with separation, and everything to do with collimating exit streamlines more axially, in as short a shape as possible.
Having one engine design, not two, is more efficient to build, and offers more thrust tailoring and engine-out flexibility in operation. This is a positive step forward for a design that must land retropropulsively in vacuum as well as at sea level.
As for when short hop testing begins, Shotwell has pushed that back from late 2019 into 2020. There is quite a lot of testing to do as a single stage item, and separate testing of the BFR first stage, before the two can be combined and tested together. I think you can forget about first cargo flights to Mars in 2022, or first flights around the moon in 2022. 2026 looks more reasonable, provided they can fund all the enormous amount of work that has to be done quite fast, actually, to meet that date.
I know the P-51 went from concept to first test flight in 120 days, but this BFR/BFS thing is not a small propeller-powered single-seat airplane embodying a new combination of all-existing technologies, with two dozen steam gauges for the pilot to use while hand-flying it. There are many aspects to BFR/BFS that are completely unprecendented, except in science fiction.
GW

kbd512 · 2018-09-16 19:08:17

SpaceNut,

I'm not even sure that Lockheed or Boeing have all the answers or even good / reasonable answers to all the known problems. In fact, I'm pretty sure they don't. SpaceX definitely brings important capabilities to the table, but critical thought needs to be applied to the overall process of getting people and cargo to and from various destinations. My personal opinion is that chemical propulsion is the wrong way to do that if mass and cost are constrained. I think SpaceX's lander concept is the best general concept for landing, but even this latest redesign still ignores the fundamental physics of landing on something that doesn't remotely resemble a dinner plate. However, it's the only realistic lander concept we have at the moment, so that's what we're going with until physics takes over and dictates a fatter lander with a lower CG and wider landing gear.

The choice of fuel for other planets is also a point of contention. Water is available pretty much anywhere that humans could potentially live. If there's no water there, then it's improbable that we'd live there. CO2 is not available in abundance in all the places we could potentially live within our solar system, but water sure is. As energy intensive as LOX/LH2 production is, does anyone here believe that a more sophisticated process with far less research behind it will fare better in the short term? As GW would say, there's a world of difference between a lab experiment and an industrialized technology that scales as required.

Anyway, I'm just happy BFR/BFS is moving forward. The devil, as they say, will be in the details.

SpaceNut · 2018-09-16 20:38:39

Well this image looks like the BFS-comm commercial flight unit tail fin and all...

SpaceX to give BFR update and announce a private Moon mission on Monday

“What we decided internally is to focus our future efforts on BFR. If that ends up taking longer than expected, then we’ll return to the idea of sending a Crew Dragon on Falcon Heavy around the Moon. But right now it looks like BFR development is moving quickly and it will not be necessary to qualify Falcon Heavy for crewed spaceflight.” – Elon Musk, 5 February 2018
Discussed last week, the render SpaceX published alongside this fresh announcement featured a new variant of BFR, suggesting that the company is still iterating on the spaceship’s design. This helps to explain a roughly 6-12 months schedule delay for prototype spaceship hop tests and a full BFR’s first orbital mission, slipping slightly from NET H1 2019 (hops) and 2020 (orbit) to late-2019 and 2021, respectively.

Louis answer is in bold....

louis · 2018-09-17 04:36:19

I am amused that the BFR is more and more resembling a sci-fi rocket from the 1950s. If only we'd gone with that concept back then!

I don't think that for either Space X or Blue Origin there is really much constraint by cost. There are only so many talented rocket engineers living in the States. There are only so many suitable materials, manufacturing techniques, ports, transport routes, test sites,launch sites etc.

Longer term I am sure we will find better alternatives to chemical propulsion.

kbd512 wrote:

SpaceNut,
I'm not even sure that Lockheed or Boeing have all the answers or even good / reasonable answers to all the known problems. In fact, I'm pretty sure they don't. SpaceX definitely brings important capabilities to the table, but critical thought needs to be applied to the overall process of getting people and cargo to and from various destinations. My personal opinion is that chemical propulsion is the wrong way to do that if mass and cost are constrained. I think SpaceX's lander concept is the best general concept for landing, but even this latest redesign still ignores the fundamental physics of landing on something that doesn't remotely resemble a dinner plate. However, it's the only realistic lander concept we have at the moment, so that's what we're going with until physics takes over and dictates a fatter lander with a lower CG and wider landing gear.
The choice of fuel for other planets is also a point of contention. Water is available pretty much anywhere that humans could potentially live. If there's no water there, then it's improbable that we'd live there. CO2 is not available in abundance in all the places we could potentially live within our solar system, but water sure is. As energy intensive as LOX/LH2 production is, does anyone here believe that a more sophisticated process with far less research behind it will fare better in the short term? As GW would say, there's a world of difference between a lab experiment and an industrialized technology that scales as required.
Anyway, I'm just happy BFR/BFS is moving forward. The devil, as they say, will be in the details.

Last edited by louis (2018-09-17 04:37:00)

GW Johnson · 2018-09-17 09:22:05

I see tip landing pads on the three fins in the illustration Spacenut provided. They show as cylindrical pods running the length of the fin tip. I don't see any indications of how a large pad surface area is to be obtained. But there are probably fold-out ways to do this, necessary to reduce the bearing pressure upon the soil.

There is some kind of linear feature near the root of the two more-or-less horizontal fins. There is no way to determine what these are yet. I speculate it might have to do with a hinge line. Aerodynamically, during entry, the bottom surface should be flat for more efficient local production of lift, and for reducing adverse reactions between the shock waves shed by the fins and the fuselage. In subsonic flight, and critically at landing, the three fins should be 120 degrees apart. That suggests variable mounting angle, with a hinge line.

The changes to the nose shape are a more rounded tip, and a flatter profile line between tip and main body on the top/leeward side than on the bottom/windward side. I see a bunch of windows on the top/leeward side. Re-entry must take place at significant pitch angle relative to the wind vector, enough so that the slipstream does not impact this surface. This is true whether inverted for downlift, or normal attitude for uplift. Just eyeballing the illustration, about 30 degrees off the wind vector is the minimum survivable pitch angle off the wind vector.

As proven on the shuttle, there is a max limit to this pitch-up angle relative to the wind vector, beyond which the cross-flow component creates twin vortices on either side of the nose. Because of the direction they spin, these vortices "suck" a part of the slipstream back down onto the surface. That surface-scrubbing flow would be really hard on those windows. (It was fatal ramming into the windscreen of the shuttle.) For the shuttle, the max survivable pitch angle off the wind vector was 40 degrees. I would hazard the guess that a similar max angle of 40-45 degrees applies here.

Reentry attitude control limits are very tight with any lifting body or winged entry design like this. Violate them, and you lose the vehicle. The windows disappear in a second or so at peak heating, creating a gaping hole and killing anybody inside. (That's what initially happened to shuttle Columbia when its wing failed and started the vehicle tumbling. Then it just broke into pieces as it tumbled broadside to the wind.)

GW

Last edited by GW Johnson (2018-09-17 09:38:13)

GW Johnson · 2018-09-17 09:48:38

What I said in post 13 above about designing an efficient engine bell applies to a mostly-unthrottled engine operating anywhere from sea level to vacuum. You design to perfect expansion at some intermediate backpressure, so as to optimize both sea level and vacuum performance as best you can.

If you throttle down deeply, you bump into backpressure-induced flow separation limits very, very quickly, which means you design your expansion ratio such that separation does not quite occur at min throttle and max ambient backpressure. There is only one design to build, and that's the one with the highest feasible expansion ratio, for best Isp.

Designing a near-vacuum-only engine is different. Your expansion ratio is only limited by how many engine bell exit plane diameters will fit behind your stage or vehicle. You won't run into backpressure-induced separation effects. Again, there is essentially only one feasible design: the biggest exit diameter you can stand.

GW

kbd512 · 2018-09-17 10:16:00

GW,

If you use a truncated / plug conical aerospike with multiple injectors arranged in sections, like pie slices, do you run into any such operating limits when creating an engine that has to go from sea level to vacuum and throttle deeply for landing?

GW Johnson · 2018-09-17 10:35:38

There's no separation limits, because of the way the free expansion surfaces adapt. But there is another problem not widely recognized: non-collimation of exiting streamlines directly axial.

All nozzles suffer from this, some more than others. I believe that this non-axial streamline effect reduces the nozzle kinetic energy efficiency of a free-expansion nozzle by some significant amount, that varies with the expansion conditions. Conventional nozzles typically have constant efficiencies in the vicinity of 98%.

That nozzle efficiency is a part of thrust coefficient, affecting the momentum term in thrust. That affects Isp.

GW

Last edited by GW Johnson (2018-09-17 10:37:01)

Void · 2018-09-17 10:55:54

Mr. Johnson, I am enjoying your communications. Hopefully you won't think I am rude if I bring in something else I hope.

Other members have been talking elsewhere, about the inclusion of Venus flyby's to expand the opportunities to go to Mars.
Spacenut and O.F. are currently at it.
http://newmars.com/forums/viewtopic.php?id=5557
By co-incidence, I also have been thinking similar, but with much less knowledge.

Looking at the new BFS, I am very interested as to how it might interact with the atmosphere of Venus for various purposes.
I currently rule out a full aeroburn landing into the atmosphere of Venus. It would perhaps then be able to float in the atmospheric acid bath, but that I suspect would be very unwise. Maybe a long time from now if humans have created floating cities, and chambers with floating lids that a BFS could land into and then be protected by shutting it up. But that is a long time from now, and I do believe that a BFR would be required to reach Venus orbit again. Way too complex at this time.

But where you might like to do a pass by Venus, and get a gravity assist at the situations where it is possible, you might I suppose be able to modify your path by a glancing aeroskip on your way to Mars. I don't know if that would be useful or not, but it is an idea.

What I really want though is to access the orbital gravity well of Venus with the BFS. This then implies aerocapture as an option.

There are some reasons to want to include the orbital gravity well of Venus in the near human experience. Of course we could better explore Venus in certain ways from there. And then there is the possibility that if you are headed to Mars, via Venus, if something goes wrong with the ship on the way to the Venus pass you may be able to abort to Venus orbit to save the crew, or salvage the cargo. This presumes that you would have significant orbital assets in the orbital gravity well of Venus.

Other than exploring Venus from that vantage point, and sending down probes, you might best test the Skyhook there I think, to mine the atmosphere of Venus for Carbon, Nitrogen, Oxygen, Argon, ect.

Of the three terrestrial planets where we could do skyhook atmospheric mining, I think that Venus qualifies as the best option.

The Earth has lots of space junk now for collisions, and Mars likely picks up stuff from the asteroid belt.

Further, a skyhook needs a propulsion method to make up for the drag of the atmosphere and lifting loads to orbit.
I believe the solar wind and magnetic bubble drives could supply that. And of course Venus will have the biggest solar energy output from solar panels.

So, where we already anticipate using the Earth and Moon and Mars, I want to explore this slice of the Venus situation that I suspect BFS could access.

And of course I intend to see the Moon involved where within a gravity well, animation could construct enclosures which could be partially filled with water, and rocketed off of the lunar surface to go to Venus for aerocapture. Then they would be filled with Nitrogen and Oxygen, and re-enforced with Carbon based materials.

Handling the radiation would involve magnetic fields, and mass. Some of it Carbon perhaps.

I just wanted to put this somewhere and this seems an appropriate spot, but I regret that I interrupt.

Have a good day.

Done.

Last edited by Void (2018-09-17 11:24:10)

New Mars Forums

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#1 2018-09-14 02:38:13

Space X News - lunar tourism and delayed BFR hop

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