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#820 by MP99 on 05 Jan, 2018 07:47
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If they inject directly into heliocentric orbit the apoapsis will be significantly out of the ecliptic and very far away from Mars. This might not be a bad thing, but it's not really a Mars-like orbit.
Yes expect direct injection - am very interested in how much C3 a kerolox cluster vehicle can impart.
This mission seems to be launching a very light payload to a C3 far short of what should be possible. (Unless there's a lot of ballast?)
Does this really demonstrate FH's capabilities?
Cheers, Martin
Sent from my GT-N5120 using Tapatalk
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#821 by hkultala on 05 Jan, 2018 07:58
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If they inject directly into heliocentric orbit the apoapsis will be significantly out of the ecliptic and very far away from Mars. This might not be a bad thing, but it's not really a Mars-like orbit.
Yes expect direct injection - am very interested in how much C3 a kerolox cluster vehicle can impart.
This mission seems to be launching a very light payload to a C3 far short of what should be possible. (Unless there's a lot of ballast?)
Does this really demonstrate FH's capabilities?
Yes, it demostrates capability to:
1) clear the pad and reach orbit with the 27 1st stage engines
2) two boosters to simultaneously RTLS
3) center core to do deorbit burn and land on barge from much higher velocity than what was has been done during F9 missions. Actually this light >C3 payload maximizes the velocity for the center core, making it harder than for average FH (military satellite) payload mission.
4) guidance system to push payloads into precise trajectories going very far away
The fact that is does not use all the fuel/lifting capacity available does not matter. If there would be something wrong decreasing the capacity, they would notice it because their tanks would ocntain less fuel than what they have calculated.
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#822 by MP99 on 05 Jan, 2018 08:28
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Yeah, I get that F9US is a mature system and they can gauge remaining prop loads from the acceleration profile, and therefore guesstimate the unused performance.If they inject directly into heliocentric orbit the apoapsis will be significantly out of the ecliptic and very far away from Mars. This might not be a bad thing, but it's not really a Mars-like orbit.
Yes expect direct injection - am very interested in how much C3 a kerolox cluster vehicle can impart.
This mission seems to be launching a very light payload to a C3 far short of what should be possible. (Unless there's a lot of ballast?)
Does this really demonstrate FH's capabilities?
Yes, it demostrates capacity to:
1) clear the pad and reach orbit with the 27 1st stage engines
2) two boosters to simultaneously RTLS
3) center core to do deorbit burn and land on barge from much higher velocity than what was has been done during F9 missions. Actually this light >C3 payload maximizes the velocity for the center core, making it harder than for average FH (military satellite) payload mission.
4) guidance system to push payloads into precise trajectories going very far away
The fact that is does not use all the fuel/lifting capacity available does not matter. If there would be something wrong decreasing the capacity, they would notice it because their tanks would ocntain less fuel than what they have calculated.
Nevertheless, it is disappointing that they aren't burning to depletion., with the Tesla doing a flyby of mars instead.
I suspect that the Tesla can't cope with the G levels. Don't want things breaking off and shifting the CoG.
But, still: boo!
Cheers, Martin
Sent from my GT-N5120 using Tapatalk
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#823 by Pete on 05 Jan, 2018 09:53
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This mission seems to be launching a very light payload to a C3 far short of what should be possible. (Unless there's a lot of ballast?)
Does this really demonstrate FH's capabilities?
Cheers, Martin
The purpose of this Falcon 9 Heavy launch is NOT to show the FH's improved lifting capabilities, but simply its ability to actually fly in a controlled and safe way.
IF all the bits work as planned, SpaceX knows quite accurately what the resultant performance will be. -
#824 by jee_c2 on 05 Jan, 2018 10:08
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Hi,
do we know about the FAA license for the flight?
It is not listed here: https://www.faa.gov/data_research/commercial_space_data/licenses/
Probably it is shown some other way, since a lot of planned flights are not listed here.
Thanks. -
#825 by john smith 19 on 05 Jan, 2018 10:13
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Yes, it demostrates capability to:
For future FH missions (and the ultimate goal of Mars) those are much more interesting capabilities that just sending an old car to Mars.
1) clear the pad and reach orbit with the 27 1st stage engines
2) two boosters to simultaneously RTLS
3) center core to do deorbit burn and land on barge from much higher velocity than what was has been done during F9 missions. Actually this light >C3 payload maximizes the velocity for the center core, making it harder than for average FH (military satellite) payload mission.
4) guidance system to push payloads into precise trajectories going very far away
The fact that is does not use all the fuel/lifting capacity available does not matter. If there would be something wrong decreasing the capacity, they would notice it because their tanks would ocntain less fuel than what they have calculated.
1) & 2) look to be in the "Shouldn't be an issue, but..." category. The shear number of things happening together, especially interaction effects. In principal a lot of this stuff should have been worked out well in advance, but of course there is always the possibility of those "Unknown unknowns" lurking.
3) Looks like the closest SX will come to being able to study the damage level of a returning US short of returning an upper stage. Expanding the knowledgebase prior to BFS heatshield design.
4) is good for any payloads. The tighter the repeatable orbital parameters the smaller the margins you can use, leaving either a bigger payload or more spare propellant for other purposes. But yes Mars is
potentially 500 000x further away than LEO and still close to 6000x further out than GEO. Unless you're JPL this is not something you're doing routinely, or planning to, unless you're SX.
Compared to acquiring these capabilities (and their effects on SX competitiveness for NSS launches) putting a Tesla in the vicinity of Mars is (literally) a side show. -
#826 by octavo on 05 Jan, 2018 10:38
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Is the roadster mounted at the angle it is to keep the 2nd stage thrust vector through the CoG?
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#827 by vanoord on 05 Jan, 2018 11:19
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Is the roadster mounted at the angle it is to keep the 2nd stage thrust vector through the CoG?
IIRC it's slightly too large to fit inside the fairing flat - the angle reduces the effective 'length' so it will fit. -
#828 by OneSpeed on 05 Jan, 2018 11:36
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My guess would be about 6:00 PM (18:00) local time. My reasoning is this: injection is most efficient when exactly the opposite of the intended target. To get maximum throw, they need the target to be tangent to the Earth's orbit. This puts the opposite point at about 18:00 local time. This is only true for direct injection - with a parking orbit, the earth departure burn needs to happen about 18:00 local time (in the probe's time zone) but the launch can be anytime.
Shouldn't that be midnight local time? For a Hohmann transfer, apoapsis and periapsis are on opposite sides of the sun, and the tangent to Earth is on the far side from the Sun.
You want to leave Earth at midnight local time, but the way to do that is to burn at 18:00 local time, then the Earth bends the trajectory around 1/4 turn. To see this, imagine a super long and skinny orbit around the Earth with a 40,000,000 km apogee. If the long axis of this orbit is tangent to the Earth's orbit, then the perigee is at the trailing edge of Earth, or about 18:00 local time. Since firing at perigee is the most efficient time, that's where the injection should be.
Yes, you do want to leave Earth orbit at midnight. But for direct injection, the burn only takes about 10 minutes, so launch should be at about 11:50pm local time. -
#829 by rpapo on 05 Jan, 2018 11:53
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Yes, you do want to leave Earth orbit at midnight. But for direct injection, the burn only takes about 10 minutes, so launch should be at about 11:50pm local time.
Rather than having a night launch as the first attempt, wouldn't it be better to launch in daylight (preferably just before sunset for the best lighting of the ascent), and then do the Hohmann transfer orbit injection burn once everything is (hopefully) safe in orbit? -
#830 by Heinrich on 05 Jan, 2018 12:15
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Yes, you do want to leave Earth orbit at midnight. But for direct injection, the burn only takes about 10 minutes, so launch should be at about 11:50pm local time.
You're forgetting the fact that the rocket travels downrange in those ten minutes. .. -
#831 by kevinof on 05 Jan, 2018 12:25
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Yes. I can't see them launching the first FH in the dark, just to make a preferred orbit timing. The launch is the primary goal, where the roadster ends up is very secondary.
Yes, you do want to leave Earth orbit at midnight. But for direct injection, the burn only takes about 10 minutes, so launch should be at about 11:50pm local time.
Rather than having a night launch as the first attempt, wouldn't it be better to launch in daylight (preferably just before sunset for the best lighting of the ascent), and then do the Hohmann transfer orbit injection burn once everything is (hopefully) safe in orbit? -
#832 by OneSpeed on 05 Jan, 2018 12:37
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Yes, you do want to leave Earth orbit at midnight. But for direct injection, the burn only takes about 10 minutes, so launch should be at about 11:50pm local time.
Rather than having a night launch as the first attempt, wouldn't it be better to launch in daylight (preferably just before sunset for the best lighting of the ascent), and then do the Hohmann transfer orbit injection burn once everything is (hopefully) safe in orbit?
It depends how lofted the demo mission profile will be. If the core stage ASDS landing is only 342 kms downrange as indicated here: https://apps.fcc.gov/oetcf/els/reports/STA_Print.cfm?mode=current&application_seq=80084&RequestTimeout=1000 , then my money is on direct injection. Conversely, an optimal FH burn to LEO would land the core stage much further downrange, and imply a separate TMI burn, as you suggest.
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#833 by Comga on 05 Jan, 2018 14:05
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If they inject directly into heliocentric orbit the apoapsis will be significantly out of the ecliptic and very far away from Mars. This might not be a bad thing, but it's not really a Mars-like orbit.
All GTO launches coast for about 1/4 orbit and relight over the equator. SpaceX obviously has the necessary tracking assets to do this, and even streams live video from the S2 over Africa, well out of range of the Florida stations. I see no reason why the TMI burn wouldn't also occur over Africa.
I imagine the equator is of little use for this launch, because the Earth is tilted a good 23ish degrees from the plane of the ecliptic.
As such, I assume SpaceX won't do much more than wait until the Earth rotates the rocket near the plane of the ecliptic and then just go for direct injection. Could probably zero out that 4 degree difference pretty easily if they really wanted to; but I too think that they'll deliberately send the stage slightly off-plane so they can guarantee that it will never hit Mars.
If they do need to go to an intermediate parking orbit, I think 23.5 degree is what they'd need to aim for.
As I understand it, to launch into the ecliptic in the direction of the Earth’s motion the escape burn has to happen over the intersection of the terminator and the ecliptic. At the Winter Solstice, with the South Pole tipped directly at the Sun, that would be the Equator. As time goes by it slides north, reaching 23.5 deg, the Tropic is Cancer, at the Vernal Equinox. The direction of travel to that antipode doesn't matter .
The point on the terminator is adjusted to change the inclination of the orbit with some small loss in velocity and aphelion.
Adjusting in that and longitude changes the location of the aphelion relative to the ecliptic and the orbit of Mars.
SpaceX can use this to prevent the Roadster intersecting Mars, or to get close if Musk wanted to start colonizing Mars on a microscopic scale. 😉
All of this can be found in any orbital mechanics textbook but they always supposed that the goal is to maximize payload and minimize the required velocity. This is not the case for Falcon Heavy throwing a little Roadster without a precise target.
Edit: That puts the launch near solar noon. Daytime to the extreme.
Edit 2: In the diagram above, assuming launch is soon, the Earth is viewed from forward along the Earth’s velocity and the antipode, the place for the burn, is opposite the shiny spot on the Equator. -
#834 by envy887 on 05 Jan, 2018 14:38
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If they inject directly into heliocentric orbit the apoapsis will be significantly out of the ecliptic and very far away from Mars. This might not be a bad thing, but it's not really a Mars-like orbit.
All GTO launches coast for about 1/4 orbit and relight over the equator. SpaceX obviously has the necessary tracking assets to do this, and even streams live video from the S2 over Africa, well out of range of the Florida stations. I see no reason why the TMI burn wouldn't also occur over Africa.
I imagine the equator is of little use for this launch, because the Earth is tilted a good 23ish degrees from the plane of the ecliptic.
As such, I assume SpaceX won't do much more than wait until the Earth rotates the rocket near the plane of the ecliptic and then just go for direct injection. Could probably zero out that 4 degree difference pretty easily if they really wanted to; but I too think that they'll deliberately send the stage slightly off-plane so they can guarantee that it will never hit Mars.
If they do need to go to an intermediate parking orbit, I think 23.5 degree is what they'd need to aim for.
As I understand it, to launch into the ecliptic in the direction of the Earth’s motion the escape burn has to happen over the intersection of the terminator and the ecliptic. At the Winter Solstice, with the South Pole tipped directly at the Sun, that would be the Equator. As time goes by it slides north, reaching 23.5 deg, the Tropic is Cancer, at the Vernal Equinox. The direction of travel to that antipode doesn't matter .
The point on the terminator is adjusted to change the inclination of the orbit with some small loss in velocity and aphelion.
Adjusting in that and longitude changes the location of the aphelion relative to the ecliptic and the orbit of Mars.
SpaceX can use this to prevent the Roadster intersecting Mars, or to get close if Musk wanted to start colonizing Mars on a microscopic scale. 😉
All of this can be found in any orbital mechanics textbook but they always supposed that the goal is to maximize payload and minimize the required velocity. This is not the case for Falcon Heavy throwing a little Roadster without a precise target.
Edit: That puts the launch near solar noon. Daytime to the extreme.
Edit 2: In the diagram above, assuming launch is soon, the Earth is viewed from forward along the Earth’s velocity and the antipode, the place for the burn, is opposite the shiny spot on the Equator.
I think that graphic is the view from the Sun at the Autumnal Equinox, with the "shiny spot" being a representation of the Sun's specular reflection. -
#835 by Comga on 05 Jan, 2018 14:57
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I think that graphic is the view from the Sun at the Autumnal Equinox, with the "shiny spot" being a representation of the Sun's specular reflection.
The image is generic and always true.
The viewpoint changes with the seasons.
It was the view back along the Eart’s orbit two weeks ago at the the Solstice.
Today it is swung around ~15 degrees. -
#836 by envy887 on 05 Jan, 2018 15:26
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I think that graphic is the view from the Sun at the Autumnal Equinox, with the "shiny spot" being a representation of the Sun's specular reflection.
The image is generic and always true.
The viewpoint changes with the seasons.
It was the view back along the Eart’s orbit two weeks ago at the the Solstice.
Today it is swung around ~15 degrees.
Indeed. But you will only see a fully-lit disk when looking from the direction of the Sun.
At any rate, why would the departure burn be at the terminator? At that point, the vehicle will be flying directly away from (or towards) the Sun.
Optimal Earth departure into a heliocentric Hohmann transfer towards an outer planet requires that the LEO velocity vector be aligned as closely as possible with the direction of the Hohmann orbit at periapsis. This Hohmann orbit is tangent with the the Earth's orbit, meaning the LEO orbit direction needs to be as close as possible to the Earth's heliocentric orbit direction which only happens at local midnight.
That is opposite the "shiny spot", as you mentioned, assuming the view is from the direction of the Sun. But if the view is from the direction of the Earth's travel then it would be on the left side of the disk and coming out of the page towards the viewer. Either way it's at local midnight. -
#837 by Nomadd on 05 Jan, 2018 15:55
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That view is obviously at the equinox.
The image is generic and always true.
The viewpoint changes with the seasons.
It was the view back along the Eart’s orbit two weeks ago at the the Solstice.
Today it is swung around ~15 degrees. -
#838 by LouScheffer on 05 Jan, 2018 16:03
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At any rate, why would the departure burn be at the terminator? At that point, the vehicle will be flying directly away from (or towards) the Sun.
Optimal Earth departure into a heliocentric Hohmann transfer towards an outer planet requires that the LEO velocity vector be aligned as closely as possible with the direction of the Hohmann orbit at periapsis. This Hohmann orbit is tangent with the the Earth's orbit, meaning the LEO orbit direction needs to be as close as possible to the Earth's heliocentric orbit direction which only happens at local midnight.
The problem is that the Earth's gravity turns the direction of the vector after the burn. If you add infinite velocity, then the Earth's gravity has no impact, and it's best to burn at midnight. But in practice, you are just above Earth escape, and you get a bend of almost 90 degrees. Imagine a really high apogee orbit as shown below, where the bend is exactly 90 degrees. The dot shows where you need to burn to go from the circular parking orbit to the high apogee orbit. This is the same place the direct injection needs to terminate.
Now as you add velocity above Earth escape, the optimum point rotates around, until with infinite added velocity it's at midnght. But a Mars apogee orbit is very much closer to earth escape than infinity, so the burn position is close to that shown. On the other hand, as another poster pointed out, the launch takes a finite amount of time, which rotates the launch point further clockwise. Overall you should launch much closer to 1800 than midnight.
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#839 by Space Ghost 1962 on 05 Jan, 2018 16:50
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Please also keep in mind two things:
The planets aren't always in the ecliptic (Mars and Jupiter are in Libra, both above the ecliptic) - see image.
The planets are non-coplanar, and if you are actually "shooting" for intersecting Mars orbit, one has to target in (at least) the Earth Sun N body case where it will be (for orbital entry or landing, you also need your entry vector into the Mars Sun N body case correct too). All this adds up to launch azimuth and inclination of ascent (gravity losses), and yes if you want to use props wisely absent the need to keep costs/margin/tracking requirement minimums, you'll want to handle this at the equator.
Oh and FH is a three stage LV, so optimum use of the core you'd also like to burn considerably downrange as well in the helioicentric frame, trading off the Earth/Sun (and often Moon) frame for the best choice of momentum transfer.
(Given the payload size, and the nature of this somewhat obtuse need to be "Mars-like" on a test launch, much of this is overkill (there's no kill like overkill).)