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sevenperforce

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Posts posted by sevenperforce

  1. 9 minutes ago, Spacescifi said:

    If Earth's rotation suddenly sped up, what is the max rate of spin it could have and still support life and how would that effect life?

    A more-rapidly rotating planet will tend to become progressively more oblate. The increased distance to the center of the Earth and the increased centrifugal force at the equator combine to have progressively lower and lower effective gravity at the equator. That's about the only change.

    9 minutes ago, Spacescifi said:

    My guess is that at the maximum you may get what seems to be a stiff breeze, purely from the Earth spinning faster than the air overhead.

    At unlivable rotation speeds it would be like a global hurricane that would wreck all but some sea creatures that do not depend so much on what happens on the surface.

    My dude, the atmosphere rotates with the Earth.

  2. 1 minute ago, Spacescifi said:

    Thank you for more or less killing off the idea of The Expanse TV series... at least it would not look the same at all.

    The Epstein Drive in that series is a torch drive that employs Brachistochrone trajectories.

    4 minutes ago, Spacescifi said:

    Still there is some hope for gas core that could improve performance by combining it with MHD tech.

    An airbreathing gas core MHD engine could get very high specific impulse and reasonably high thrust, yes, but thrust drops off once you're out of the atmosphere. Of course, that's perfectly fine. You don't need high constant acceleration for some specified period of time. I don't know why you keep insisting otherwise. Your total dV is your total dV, whether you dump it in 70 minutes or 20 minutes or fifteen hours.

    10 minutes ago, Spacescifi said:

    About Impulse Warp: Your speed and velocity and inertia will be the same it was when you engaged the impulse warp drive.

    Saying "speed and velocity" is like saying "temperature and hotness" -- one includes the other.

    If your velocity vector is the same, then it needs to be your velocity vector relative to something. Is it your velocity vector relative to the Sun?

     

  3. On 10/29/2022 at 7:03 PM, Spacescifi said:

    Let's say you have a rocket drive that can expend it's fuel of 90 tons in 70 minutes at 3g max acceleration or longer at lower accelerations.

    Oh dear, this again.

    Acceleration is NOT a feature of a "rocket drive" at all. A rocket engine produces thrust. Acceleration is the result of dividing thrust by mass. Let me say it again. Rocket engines DO NOT produce acceleration; they produce thrust. Period.

    And because the mass of a vehicle changes over time, the acceleration of a vehicle changes over time, assuming constant thrust. Specifically, it goes up.

    Since you said "3g max acceleration" I'm going to assume (probably wrongly) that you're thinking of something sane. For a vehicle to have a 70-minute burn time with 90 tonnes of propellant and achieve 3 gees at burnout, then (clearly) it must be burning 21.4 kg per second and it must be yeeting those 21.4 kg/s out the back end at a sufficient exhaust velocity that the impulse to the dry mass of the vehicle is 29.43 m/s2.

    This gives a range of values depending on the dry mass of the vehicle and thus the corresponding mass ratio. Here are some comparisons:

    Ratio of Fuel to Dry Mass Thrust (kN) Isp (sec)

    Initial Acceleration
    (gees)

    Total Δv
    (km/s)
    24:1 (typical for some chemical upper stages) 110 465 0.12 14.7
    6:1 (typical for a laden first stage rocket) 441 2,100 0.43 40.1
    3:1 (enough thrust to get off the ground) 1,177 5,606 1.00 76.2
    1:1 (typical for aircraft like a 747 or a B-2) 2,649 12,618 1.50 85.8

    Having a specific impulse in the range of 2,000-5,000 seconds isn't completely out of the question; that's the theoretical specific impulse range of your typical open-cycle gas-core nuclear rocket. But of course a gas-core nuclear rocket sprays out rather unpleasantly radioactive exhaust, and it's doubtful that you'll be able to get the amount of thrust you need. The NASA studies of gas-core designs found they wouldn't produce enough thrust to even lift half the weight of their pressure vessel alone, and that's before factoring in the weight of a moderator/reflector and radiators.

    If you want to get up above 12,000 seconds of specific impulse, you're going to need a better energy source. Antimatter could do the trick. If you had perfect conversion of potential energy to kinetic energy, then you'd be accelerating your 21.4 kg/s of propellant to 124 km/s, requiring 164.5 GW of power. That's going to require you to burn about one milligram of antimatter per second, or a total of 4.2 kilograms over your entire journey. Of course, you won't, because the maximum efficiency of any heat engine (the Carnot cycle) is around 70%. That means 30% of your energy is lost to heat, so you're actually going to need 6 kilograms of antimatter. But that means we're going to have to have a way to reject 70.5 GJ of thermal energy per second.

    There are two ways you can do this. The first is by using an open-cycle cooling loop where you're just dumping part of your propellant overboard. That propellant will of course achieve very high temperature (that's the point) so it will basically be an entirely separate rocket engine similar to a regular NTR, except the heat it's receiving is waste heat from the antimatter rocket rather than from a nuclear reactor. Assuming liquid hydrogen propellant and a coolant exhaust specific impulse on the order of 1000 seconds (but now benefiting from the 70% limit of the Carnot cycle), each kilogram of coolant is carrying away 69 MJ of heat and producing an impulse of 9.8 kN. But at this rate, you'd need to be dumping over 1000 kilograms per second just to deal with the waste heat of the engine, and our propellant budget is only 21.4 kg/s.

    So we turn to the second approach: a closed-loop cooling cycle where the coolant runs through a series of radiators and is then injected into the reaction chamber as the main source of propellant. That will work, but it will approximately double the weight of your engine.

    But let's suppose we ignore all of the waste heat issues entirely.

    Some antimatter engines have T/W ratios on the order of 4:1, and so if we're just gonna handwave and say this is achievable here, we'll end up with an engine that weighs around 67 tonnes. This leaves 23 tonnes for payload, structure, and propellant tanks.

    On 10/29/2022 at 7:03 PM, Spacescifi said:

    If you want to last longer than 70 minutes at 3g you just carry 180 tons of fuel instead and either increase the amount of engine nozzles for higher thrust or otherwise lighten the ship's mass so that it will be similar to what you started with so you do not have to raise thrust to accomodate extra weight.

    If you double the amount of propellant then you cut your acceleration in half. If you double your thrust to accommodate this, you use up your propellant twice as fast, and you're right back where you started.

    You can't exactly "lighten the ships mass" when your engine and propellant alone are 87% of the ship's laden mass.

    And if you're trying to accelerate at 3 gees the entire time...that makes no sense. Engines don't produce acceleration; they produce thrust. As your mass goes down, your acceleration will go up.

    On 10/29/2022 at 7:03 PM, Spacescifi said:

    Impulse Warp Drive: Warps space ahead of your vessels that it moves in the opposite direction your vessel is currently accelerating with main engines... at the same rate of acceleration.

    So what you do is accelerate at max acceleration for a moment and engage impulse warp, then shut off main rocket engines. Impulse warp will continually warp space past you at 3g until you shut it down... allowing you to traverse vast distances without needing a torch drive.

    Why is it not overpowered? Well.. the ship's ACTUAL momentum/velocity never changed. If it hit something  while at warp it will be just the same as if it was not at warp.. since it's actual velocity and momentum vecotor would all reveal itself on impact.

    So space is accelerating past you, but you're not gaining kinetic energy? Ok, let's imagine that.

    If you have this, you don't need 70 minutes of whatever wacky acceleration you're imagining.

    I'm assuming you can't activate this impulse warp in atmosphere?

    So you just need about 2 km/s to get out of the atmosphere and point in the desired direction, and then engage impulse warp.

    At three gees of acceleration, that's going to be a burn time of just about 68 seconds, and you're going to only need a fuel to dry mass ratio of about 1:1. 

    The problem is this: when you drop out of warp, you said you have the "actual velocity and momentum vecotor [sic]" from the beginning. But actual velocity relative to what? Relative to your home planet? Relative to your destination? Earth is moving at 30 km/s around the sun. If I point my vehicle toward Jupiter and warp toward it at 3 gees, then drop out of warp near Jupiter, I'm going to be outpacing Jupiter by a whopping 7 km/s. And that's if Jupiter is perfectly lined up with Earth so that our velocity vectors are pointing in the same direction. If our velocity vectors are pointing in different directions, it gets even worse.

    On 10/29/2022 at 7:03 PM, Spacescifi said:

    With a single nozzle you need all the thrust and heat to come out a single nozzle, which may be too much for a single engine to pull off

    Why?

    Why would multiple nozzles be any different from a single nozzle?

    In terms of thrust and heat, this makes no sense at all.

    On 10/29/2022 at 7:03 PM, Spacescifi said:

    increase the number of engine nozzles so you can burn more fuel in less time making it less efficient but more thrusty.

    Increasing the number of engines doesn't make it less efficient. It increases your dry mass but the specific impulse isn't going to change.

    Please just learn the rocket equation.

  4. The RS-25 is very close to being a FFSC engine. After all, it has two different preburners and two different turbopumps.

    The only difference is that both preburners are fuel-rich, because hydrogen's heat capacity and specific energy are both just so much greater than oxygen that running both preburners fuel-rich makes more sense.

    But sure -- if you replaced the RS-25's fuel-rich preburner attached to the oxidizer turbopump with an oxygen-rich preburner, you could do it. It just wouldn't be quite as efficient because hydrogen just works better than oxygen.

     

  5. 1 hour ago, Beccab said:

    There's another part of the article I missed previously: SpaceX intends to keep the depot continously filled as far as possible instead of just filling it before a mission. This means that instead of a stream of tankers starting a few weeks before HLS launches, we'll likely see it happen right after it launches (same for every Starship mission going decently beyond LEO, excluding probably things like lunar flybys)

    They talk about missions other than Artemis but I'm unsure what other missions they'd need depots for, unless they're talking about Mars.

    In theory, such a depot could make @tater's dream of a fully reusable stretched Lunar Starship possible by coming up to meet it in MEO.

  6. 2 minutes ago, StrandedonEarth said:

    The venting from the recipient tank could easily provide the thrust needed for settling the propellants. Exquisitely simple, like the Soyuz boosters venting oxygen to provide separation thrust…

    Yes, but that would cause rotation around the pitch axis because you'd only be venting from one of the vehicles and so you'd have off-axis thrust.

    OTOH, I wonder if an alternative way to settle the propellants would be to get them rotating together in a penguin roll. Would the centrifugal force then assist in settling the propellant? Hard to know, I think...lots of CoM shifting going on.

  7. On 11/2/2022 at 2:09 PM, magnemoe said:

    You forget that the tanks are pressurized, if pressure is larger in the tanker than target fuel will flow, you want some trust for settling but you are not afraid of bubbles other than it reduce the pressure difference. Benefit of [Thing 1] is that you could have two docking ports for added rigidity, it will be some torque as the center of mass is shifting. 

    Ah, yes, you're absolutely right. A little thrust to provide consistent settling, and then the recipient Starship can vent to lower its tank pressure and thus accept flow from the donor Starship.

    Depending on the viscosity of liquid methane and liquid oxygen, the constant settling might not even be necessary; once it gets flowing it should stay settled and the tank pressure differential will do the rest.

  8. On 11/2/2022 at 11:52 AM, Beccab said:

    I assume that having both ships translate towards the same direction at the same time would work even side by side, though throttled to take into account the different mass of the two spacecrafts.

    Either that or they dock side by side and only one of them translates laterally, but going forward or backwards seems less complex

    I think it depends to some degree on the internal plumbing, no?

    If I recall correctly, this is the propellant fill adapter that the Quick Disconnect arm interfaces with, right?

    fill.png

    According to the renders I've seen, the fill lines run up the inside of the skirt and go directly into the methane downcomer and the LOX tank, respectively:

    fill.png

    So if you were to connect two starships back-to-back (which I will call "Thing 1" for no reason whatsoever), open all the valves between these fill lines, and start your settling/prop-transfer burn, the tanker can only fill the depot (or other recipient ship) until the levels are equal. On the other hand, if you connect the ships with one inverted (we'll call this "Thing 2" for absolutely no reason again) then you can transfer as much as you want:

    fill.png

    The other possibility would be to have some sort of positive displacement pump involved so that the "Thing 1" position could work. This would require an entire new system. However, it might be necessary if the "spray it into the empty portion of the tank" approach in the "Thing 2" position would create too much evaporation.

  9. 4 minutes ago, Beccab said:

    the second Starship launch NASA is tracking after the maiden one features a tank-to-tank propellant transfer inside a single starship

    Oh, that's smart. They'll still be able to utilize a low-level continuous settling burn to effect the transfer.

    Unclear, however, how a settling burn will operate if they are docking side-to-side now instead of tail-to-tail.

  10. 23 hours ago, steve9728 said:

    CMS's official Weibo: The orbital parameters of the wreckage of the CZ-5B Y4 rocket are: perigee 170.8km, apogee 301.4km, inclination 41.6°.

    Interesting that this is a significantly lower perigee than in prior launches. With the last one, the initial orbital parameters were a perigee of 182.5 km and an apogee of 299.3 km. By the time the perigee had dropped to ~170 km, the apogee was already down to ~232 km and it was very close to re-entry.

  11. 24 minutes ago, JoeSchmuckatelli said:
    28 minutes ago, sevenperforce said:

    I wonder if the entire 3,750 kg payload is being placed into GEO or if parts of it broke off in transit (in LEO or GTO).

    On 10/31/2022 at 6:30 PM, tater said:

     

     

     

    From that article it looks like everything went to GEO together.

  12. 1 hour ago, Piscator said:

    The external footage of the two boosters coming down didn't seem much different from earlier flights.

    It seemed very different to me. The ground tracking cam showed the entry burn on one booster starting at approximately the same time the other started, and the landing burn of one was visible from the other.

  13. People assume that nuclear rocket engines must be wildly, wildly overpowered, but they really aren't.

    The specific impulse of a rocket engine is primarily a function of three things: chamber temperature, expansion ratio, and propellant molecular weight. Believe it or not, the peak chamber temperature of the RS-25 (3,300°C) is actually 29% higher than the chamber temperature of a NERVA nuclear thermal engine (2,500°C). The reason NERVA gets higher specific impulse is not because it imparts greater energy to its propellant, but because its propellant is pure hydrogen, which has a much lower molecular weight than the mixture of hydrogen and water that comes spewing out the back end of an RS-25.

     

    If you're planning a mission and you want as much Δv as possible for a small payload, you're actually going to opt for a high-thrust engine on your upper stage with less efficient propellants, because your mass fraction is going to be the biggest factor. On the other hand, if you have a very large payload and a relatively low Δv requirement, you're going to want to maximize specific impulse, since your mass fraction is limited by the size of your payload. So a nuclear thermal engine is good for sending big payloads, not for sending small payloads fast.

    An NTR would be great for sending very large monolithic payloads to the moon. It would not get you to the moon in a matter of hours.

  14. On 9/21/2022 at 12:27 PM, darthgently said:

    I think the aero forces once the abort engine starts dropping out are what is inducing that wobble.  Big forces in thick air at high speed; very quick deceleration

    Passengers would be redding-out rather than blacking out if they went out at all.  Elderly people would be at higher risk for stroke, aneurism maybe with that much neg g

    Ooooooh yes that makes sense.

    On 9/21/2022 at 1:20 PM, Ultimate Steve said:

    Alright that's it I'm going frame by frame and plotting acceleration and velocity based on the official telemetry. There's been too much speculation on this. I might have a little bit of free time tomorrow morning to do it, but in general this week is very busy for me so it will take a while.

    Did you ever manage this?

  15. FH.png

    You can see the grey paint on the upper stage for solar warming of the kerosene; you can also see the lack of grid fins and landing legs on the center core.

    Interesting that boostback burn shutdown and MECO happened almost exactly simulatenously.

    MECO took place at 3.97 km/s and an altitude of 114 km, which gives the upper stage a NICE kick to what it needs to reach LEO, GTO, and GEO.

    FH.png

    Unfortunately both side booster cameras are fogged over. I wonder if that was soot from the center core?

    Fortunately it cleared up:

    FH.png

    Go SpaceX!

  16. On 10/27/2022 at 10:43 PM, tater said:
     

     

    In this photo you can see a thick grey band around the kerosene tank on the second stage. That grey band is intended to increase solar heating to that tank in order to prevent the kerosene from freezing during that longer persistence period, since the stage is going to be doing a direct GEO insertion.

    Really fascinating and a cool solution.

  17. On 10/28/2022 at 3:01 PM, magnemoe said:

    You save dV doing an direct burn over getting into low earth orbit like Apollo. And as I read most missions do this an Apollo was an exception.

    There is a small difference in efficiency between a direct burn and a parking orbit, but it's rarely going to make or break anything. The degree of efficiency improvement is also often a factor of the T/W ratios of the each stage. If the stage which provides the bulk of the Δv for orbit (whether that's a second/sustainer stage which doesn't make orbit or the same stage that performs the BLEO burn) has a low T/W ratio, then it will need to have a lofted trajectory, which results in Oberth losses because you're performing your burn from a higher altitude. This effect is further multiplied because a stage with a higher T/W ratio can perform the BLEO burn more quickly, which means it can take more of an advantage of the Oberth effect.

    Other changes in efficiency are associated with any Δv lost to drag in the parking orbit. This is small but not always insignificant; Centaur continually vents hydrogen press gas to make up for drag losses in its parking orbit. If you have an upper stage with a low T/W ratio and so you have an initially lofted trajectory, then there will be a balance to how much of a perigee raise you perform. Too much, and you're losing Oberth efficiency; too little, and you incur higher drag losses during your dip through the very upper reaches of the atmosphere.

    On 10/28/2022 at 3:01 PM, magnemoe said:

    Or if you want an inclination who don't fit with your launch window? 

    Not just inclination; it's also an issue of phasing.

    On 10/29/2022 at 10:10 AM, farmerben said:

    Everybody should try this in KSP you might be surprised how well it works.   This is anecdotal, I find that a rocket powerful enough to orbit on the first stage can easily reach apogee millions of km above Kerbin.  The second payload stage from there can easily go on it's interplanetary mission, without wasting much second stage fuel.

    Sure, it works. Anyone who plays around with KSP will probably have experienced this. But just because you CAN do something doesn't mean it's the most efficient way of doing it. For any rocket design, you will find that you can deliver greater payload to an interplanetary orbit by entering a parking orbit first, rather than launching straight up, if you perform a reasonably efficient gravity turn.

    On 10/30/2022 at 12:25 PM, magnemoe said:

    Falcon 9 upper stage has much higher TWR so they don't have this issue.  Note that real world rocket don't do circulation burns in LEO like we do in KSP, they just burn second stage until orbit is circular. 

    Because Earth is physically much larger than Kerbin, orbital velocity is much higher, and upper stage T/W ratios tend to be lower, your orbital insertion burn is MUCH longer than it would be on Kerbin. Accordingly (at least generally speaking), you're much less likely to end up with an ascent profile where your time to apogee is very long at all.

    If you've practiced launching from the Mun, you really get good at aiming for an optimal ascent where your time-to-apogee is less than 10-20 seconds all the way until you've completed circularization.

    The Space Shuttle and SLS are good examples of a common exception. Because they are sustainer architectures, the T/W ratio is quite high at burnout and so the time to apogee increases rapidly at the end of the burn, leading to the need for suborbital tank staging and a long circularization burn.

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