RCgothic
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Posts posted by RCgothic
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Wow. No 2nd landing until Artemis V in 2028:
Show of hands: who thinks SpaceX won't land *any* commercial lunar astronauts in the 3 years following its first crewed landing? Anybody?
The Artemis program beyond Artemis III is making itself totally irrelevant. Who cares if they onboard a 2nd lander if this is the timeline?
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I don't think spacecraft in Star Trek generally "orbit" as such. They effectively use anti-grav tech to hover in place. Shuttle craft between the mothership and the surface therefore don't have to scrub off the excess velocity, they just have to ascend.
My reasoning for this is that ships are usually depicted as loitering over a surface position despite being shown as relatively close to the planet, and shuttlecraft aren't generally shown as scrubbing off realistic re-entry energies.
That said, Star Trek ships at sublight are also generally described as acheiving significant fractions of the speed of light with their non-neutonian impulse drives and what would be 10s of thousands of Gs of acceleration. Even if they did orbit at appropriate velocities, it would be a matter of seconds to shed that velocity and enter the atmosphere with a relatively benign propulsive descent.
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One thing about ending production of Dragon is that there won't be enough Dragons to service multiple commercial stations as well as ISS, as well as free flights, plus downtime and refurbishment.
They say they can always build more, but in reality their preference will be to use Starship.
And the thought of Starship serving commercial stations that are generously half its habitable volume is a bit ridiculous. The alternative at this point is the vastly more expensive option of Starliner.
So I think one effect will be that commercial stations will need to get a bit more ambitious.
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Figure this probably goes here:
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4 hours ago, Booots said:
To clarify up front, I'm looking for math help - not 'rules of thumb' or best practices.
As a spaceplane enters the atmosphere, it's got three forces acting on it: gravity, lift, and drag. For the purposes of this discussion, I've got a well-developed model that describes drag as a function of lift, and lift as a function of angle of attack (AoA) (accounting for mach number in both cases). This theoretically means that the spaceplane's downrange distance is a function solely of AoA (assuming AoA is constant throughout the entry if need be). It's not going to be a clean function, but it is a function of one variable. I'm only interested in the regime where the flight path is pseudo-ballistic (i.e. the vessel is not 'flying' per se). This question centers around the effects before the atmosphere is significant enough to treat the vessel as a simple glider (say, h > atm_height / 2).
Increasing AoA increases both lift and drag. More lift means the vessel's descent angle shallows and it postpones hitting thicker atmosphere, increasing downrange distance. More lift means more drag, though, which slows the vessel and decreases downrange distance. The question is, what is/are the critical AoA(s) where these two effects cancel? I believe there will be two roots, one where drag is minimized (low AoA) and one where lift is maximized (higher AoA). From my experiments, L/D(max) is not ideal for best range in early stages of entry - and this makes sense since it doesn't account for the delayed effect of prolonging the rarified atmosphere.
I've tried doing the calculus of looking at a snapshot of a purely ballistic trajectory and treating lift and drag as changing the launch angle and speed, but this seems to be a dead end (at least to my sleepy brain). Before I start brute forcing this through a differential equation solver, are there any suggestions to how to find the optimum AoA(s) for downrange distance in a rarified atmosphere?
Potential reading list:
https://ntrs.nasa.gov/api/citations/19720022195/downloads/19720022195.pdf
The answer is "it depends".
Even for a regular plane in conventional flight, the optimum AoA for maximum L/D depends heavily on the design of the plane. It also varies with speed because drag is highly nonlinear with speed. Re-entering spaceplanes necessarily have dramatically varying speeds.
Spaceplanes also then have to contend with heating. They don't generally solve for "max downrange" as a longer flight time results in a greater total absorbed energy at a lower temperature, which both gives the heat more time to penetrate through insulation and minimises the proportion that is re-radiated.
They also need to contend with structural loads and keeping vital elements out of the airstream.
If you want an answer along the lines of "the best AoA is 43°" then I'm sorry to disappoint.
This is a highly complex computational challenge.
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Point to point starship can do a full half-circumference by using lift to skip off the atmosphere. Normally ballistic suborbital trajectories can't do a full half-circumference.
The earth's circumference is 40000 km.
Also the OLP has the thrust simulator installed in 33-engine config:
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Psyche will be a 2x RTLS mission.
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I'm not yet willing to ascribe to malice that which incompetence adequately explains.
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Annoying but not unexpected.
"This is the fault of all those letters people wrote!"
That excuse only worked once. Consultation period is closed. It's not an excuse for month on month slips.
Also:
"Don't worry, SpaceX aren't ready anyway."
Putting on a rush is expensive. There's no point rushing if the end date isn't secure. But no doubt they could have made ready with a solid end date! 20/4 was basically ready months ago. 21/5 also could be in launch flow right now . 24/7 are ready for proof testing. Mechazilla is ready.
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Serious shake-up underway?
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This second lander had better go big or there's no point in it whatsoever:
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It'd be 15 F1 engines, although it could probably be less with vacuum nozzles!
I've run some numbers on Superheavy though. Assuming 33 engines at full thrust, similar burn time to F9 (162s), and similar CECO or downthrottle to Saturn V at 2.1km/s, Superheavy will develop 156GW of power.
Assuming a triangular power profile (starts at 0, increases linearly to max), that's ~80GW over the entire flight.
With 3 flights an hour (hourly turnaround), Superheavy will operate for 468s (3x162s)of every 3600s long hour. That's an aspiration to an average developed power of ~10GW.
Is it a problem that SpaceX plans to increase global power consumption by ~55%?Or are these just fantasy numbers that aren't apples to apples because rockets are extremely efficient at converting thrust into power and most of civilization isn't? I don't actually know.Edit: numbers were off by a factor of 1000.
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Also, interestingly, Saturn V produces 38.85MN of vacuum thrust. Placed on the moon facing retrograde at lunar orbital velocity of 1017m/s, that develops about 40GW of power.
So it'd take just 3 Saturn S-ICs placed on the moon and thrusting continuously to keep it in its place.
Crazy how a single Saturn V in flight eclipses total global power consumption! That sounds like a fact that can't possibly be true, but apparently is.
Could actually be a real problem if Superheavy starts flying anything even close to hourly.
Edit: MN multiplied by 1000m/s is GW, not TW. I'm off my a factor of 1000. >.<
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Orbital energy is the sum of kinetic and potential energy.
Potential energy= m*g*Dh
Mass of the moon is 7.34767E22kg.
Earth's gravity at lunar radius is given by GM/r^2
Moon's radius is 385000km. Earth's GM is 3.986E14m3/s2. So g at lunar radius is 2.689mm/s/s.
And dh is 38mm/y.
Potential energy is therefore increasing at 7.5E18 J/year.
Kinetic energy is 0.5mv^2.
Orbital v is given by SQRT(GM/r) = 1017.5m/s at 385000km, or 1017.5m/s at 385000km and 38mm. In fact the difference in orbital velocity is just 0.05nm/s.
But in fact this amounts to a kinetic energy difference of -3.75E18 J/year.
So by adding the two (one is negative) you'd need to add 3.75E18 J/year (119 TeraWatts) to halt the moon's recession.
And probably the same again to earth to replenish lost rotational energy.
Currently earthly civilization consumes a total of 18TW from all sources.
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Doesn't bode well TBH.
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Confirmation B4 and S20 aren't flying. Could still launch before SLS though.
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I really wonder if they should start naming these cores. B1051.13 has half as many flights as the space shuttle Endeavour.
Speaking of space shuttle Endeavour, it has 7179h in flight.
Dragon Endeavour has 6328h in flight. Gaining rapidly!
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The overalls the cosmonauts were wearing as they came aboard today were exceedingly stylish, IMO.
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The current plan is 1 Depot, 4 refilling flights, and 1 HLS.
Subsequent missions would require more refilling flights as the depot will launch with fuel that will be expended in the first mission, assuming the depot is reused.
More refillings may be needed for heavier payloads and more exotic missions.
Also a reasonable depot upgrade would be to include a sunshade/earthshade. In that instance it could easily be a case that keeping the fuel *warm* would be the issue, not boil off. Nice problem to have.
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2 hours ago, JoeSchmuckatelli said:
This is kind of what I was afraid of. I'm guessing that with vehicles of the masses the depot and the SS FuelTruck will have that minor velocity can have a big impact. Being able to soft-dock or have an "Expanse"-style docking tube between vessels seemed like a good solution; but apparently orbital mechanics hasn't gotten the memo from Hollywood!
Soft docking becomes more reasonable in orbits with longer periods or in deep space. Around Earth, Mars or Lunar low orbits it's probably not advisable.
SpaceX Discussion Thread
in Science & Spaceflight
Posted
Apparently the pace of F9 launches will continue to accelerate this year: