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RCgothic

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Posts posted by RCgothic

  1. 21 minutes ago, tater said:

    What's so sad/annoying/enraging is that it's not like NASA doesn't know this.

    That number—the minimum TLI capability for SLS/Orion to be useful for any crew mission at all—is right there. That's the target.

    Maybe not for Block 1, Block 1B, etc, but if there is no evolution to "65t to TLI" SLS is a complete waste of time and treasure.

    The 38t to TLI that B1B can do is more than enough to get Orion into LLO with a stretched service module. They should have just come straight to this as the baseline version.

    Then send the lander on a second launch. 38t doesn't get you 4 people for an extended stay, but it might get you 2 for an extended stay. 38t to TLI is roughly 28t in LLO, nearly twice Apollo.

    But there's no plan to upgrade the ESM, and they couldn't build SLSs fast enough to support this anyway.

    "But even a long stay isn't a sustainable presence, that's why we need gateway!" they counter-argue despite gateway going to be mostly vacant and not actually anywhere interesting in itself. That's just a half-measures. Go the whole way and make a permanently inhabited surface base. Cheap frequent cargo is what enables sustainable presences. Does SLS do that? No.

  2. Working an Apollo lander through my spreadsheet (~15t) comes up with about 64t to TLI with Orion, so yeah I agree with your figure.

     

    That'd be 155t to LEO with SLS, which isn't on their upgrade path at all. 

    Ironically, uprated Saturn V with F1As could probably have done it, but TBH safety is one of the few things you can't really fault SLS Orion on. Saturn V was probably lucky not to lose a crew, nevermind the crew they almost lost on Apollo.

  3. 11 minutes ago, tater said:

    I think that they'd need to be throw at least 60-70 tons to TLI to be able to use the Orion capsule on a single stack lunar surface mission. That's not even on the table for SLS, ever. Not Block 2, not advanced Block 2.

    ^^^Note I am assuming a minimalist landing, Apollo style.

    The difference between what 2 people need for a brief excursion and what 4 people need for an extended stay plus a tonne of cargo retun gets quite large, aye.

  4. 5 minutes ago, RyanRising said:

    Wow. Thanks for doing the math! I was expecting a ridiculous result, and boy howdy I guess I got one. I think this could be used as a good preface to introducing modern moon missions: it explains why doing things like Apollo did doesn't scale up well at all, and why we need to take a different approach this time around. Tyranny of the rocket equation indeed.

     

    Sure, but there were other options between "do it Apollo style" and "do it SLS style", even without considering the launchers that have come along in the mean time.

    Capsule and lander LLO rendezvous with a cadence of 4 missions/8 launches per year should have been possible if SLS and Orion had actually be designed coherently. If SLS and the ESM were a little bigger (B1B as a base) and/or Orion was a little lighter.

    EOR was and remains viable, with/without Lunar rendezvous as well. Delta IV Heavy and Atlas V both could have been crew rated with a will. Then the Falcon family came along.

    You just look at the program and go "yes, we're going back and that's cool, but we could be doing this so much better."

    And despite NASAcs continuous use of the political Nov 2021 date for Artemis 1, multiple sources now hearing the realistic internal target is NET Feb 2022:

     

    Sure I've seen at least one other as well, but struggling to dig it up right now.

     

     

     

  5. On 2/23/2021 at 8:02 AM, RyanRising said:

    I know this absolutely, positively will not and cannot happen, for many, many reasons. But what sort of rocket would it take to send Orion, an appropriately upgraded service module, and a four-person lander on a lunar landing mission single-launch? Expense, debris, and politics aside.

    Ouch, basically.

    Let's assume a lunar module. Apollo Ascent was roughly 2.4t dry for 2 people, so let's assume double that for 4, plus 100% again for an extended stay and a bit more comfort. Roughly 7t. Plus return with 1t of samples. To get back to LLO (1730m/s) takes 7 more tons of propellant at 310 ISP to give a bit of margin for rendezvous (1910m/s) so the ascent stage will weigh 14t landed, 15t on launch, and 8t back in LLO.

    To land the ascent stage (14t) from LLO plus 1t of cargo might be 21t landed, assuming 6t for the stage dry (again, a little under 3x Apollo). To get there from LLO (1730m/s) including a decent landing margin (2110m/s) is 21t of propellant at 310 ISP, so 42t total for the lunar lander system.

    Then we need to know how much is needed to return to earth. Orion weighs 10.4t and the ESM weighs about 4.9t dry for its original 8.6t of propellant. It'll need a bit more propellant to shift Orion and 42t of LM into LLO, so it'll need to be stretched a bit. It'll work out at about 7t dry with larger tanks (see later). Plus we've taken 1t of samples from the surface, so that's 10.4+7+1= 18.4t dry. To send that back to earth (820m/s)with some margin (880m/s) requires 6t of residual  propellant in the tanks at 319s ISP.

    So! Now we know how much has to be braked into LLO by the ESM. 42t of lander (inc 1t landed cargo). 10.4t Orion. 7t ESM. And 6t propellant to get home. 65.4t "dry", although in this case dry just means mass at end of burn because there'll be residual propellant in the ESM. To drop that into LLO from TLI (820m/s) would take 24t of propellant, plus some margin for rendezvous/ orbital manoeuvres/station keeping/ (980m/s). So we need to send 65.4t payload plus 24t fuel is 89.4t to the moon from LEO not counting Earth Departure Stage.

    (Note that the total fuel for the ESM is therefore 30t, or 21.4t extra. At 10% tank fraction for hypergolic tanks that's an extra 2.1t on the original 4.9 gives the new dry weight of 7t I've been using.)

    To send 90t near enough to TLI from LEO (plus ~5t of payload attach fittings) takes 115t of hydrogen and 12t of earth departure stage. SLS would therefore have had to be designed to put around 220t into LEO. That's more than twice its block 1 configuration and even 90,t more than Block 2.

     

     

    But is there a rocket in development that can do it?

    At 370s ISP,  a Starship Derived Stage would need to put roughly 380t into LEO to send 95t of payload/PAF and 61t of stripped down Starship to TLI. 225t propellant required.  That sounds implausible, but I'll run some numbers anyway.

    Starship can put ~100t of payload into LEO in its reusable configuration. That's not counting ~30t of landing fuel, or the ~80t dry mass, 210t all inc. Extraneous weight like flaps, fairing and heat shield can be deleted. Include the landing fuel for departure instead and take more fuel instead of the unnecessary bits and an expendable version could be 61t for the stage (5% of 1220t propellant -assuming Stainless isn't quite as good as F9US's 3.5% despite Starship's square/cube advantage) and 95t for payload and PAF, leaving 54t of fuel (210t total in LEO). That's still 174t short of TLI.

    So we look at Superheavy. How much margin is wasted on an RTLS landing Vs expendable? It's hard to be sure. It's thought that F9 takes a roughly 40% payload penalty when going RTLS, so let's go with that. If 210t to LEO is 60% of Superheavy's expendable capability, it might be able to manage ~350t to LEO fully expendable.

    So no. Not even fully expendable Starship/Superheavy could do that in a single launch.

     

    However! That assumes the starship derived rocket is lugging an entire starship to TLI, which is carrying oversized tanks for 1200t of propellant because it's also doing the job of an upper stage. That isn't a disadvantage I put on the hydrogen powered equivalent, so let's look at a raptor derived 3rd stage, which would be much lighter!

    With 150t of propellant, a Raptor 3rd stage could weigh 7.5t. Plus 94.5t of payload  is 250t to LEO. That should be well within the capabilities of an expendable Starship/Superheavy!

  6. 40 minutes ago, RyanRising said:

    Definitely more him agreeing to six vacuum engines than coming out with it himself, but it's still some validation on the idea.

     

    6 hours ago, RCgothic said:

    Elon has previously agreed a pure tanker with stretched tanks and 9 raptors (6Vac, 3SL) might make sense.

    Yup. :P

  7. 6 hours ago, xebx said:

    Starship only need 3 Raptor Vac and 3 Raptor SL (SL can be shut down at around 4500m/s) to deliver approximately :

    120t (high DeltaV loss = SH come back to launch pad)

    or 180t (low-med DeltaV loss = SH land on a boat)

     or 240t (no recovery).

    (SH : 69MN, 215t dry weight, 3400t fuel, SLT=1.35; Starship : 105t dry weight, 1170t fuel, Twr=0.98 at staging, once in orbit 3 Raptor Vac can do all the job even fully refueled; DeltaV Starship 8700 m/s+11t fuel for landing)

    Btw, I'd expect a fully expendable starship to have a substantially lighter dry weight. No fins, no heatshield, no header tanks and disposable fairing.

    At 5% dry mass, disposable starship would be ~62.5t. F9US is 3.9%. Expendable Starship could even be better because of the way square cube works.

    Expendable Starship may be a fairly niche application for distant destinations once they get orbital refuelling worked out. They absolutely do not want to be expending these things.

    But it kind of makes sense as an easily acheivable early milestone. Personally if I were Elon I'd like to do it just to smash the "double Saturn V to LEO" marker.

  8. 1 hour ago, Mikenike said:

    Still, you are correct, how in the heck would you hide something that is 1000 sq ft in LEO, or even HEO.

    If it had to be in LEO I probably wouldn't bother with solar, I'd go straight to bomb-pumped. Much smaller, much cheaper. One-shot, but you could have multiple.

    If it absolutely had to be solar-powered, I'd put the solar collector somewhere separate from the weapon and beam the power with a microwave laser or similar.

  9. It'll probably be a dual ASDS landing for the side cores with centre core expended.

    15t is comfortably within FH's GTO expendable capability of 26.7t, but enough more than the 8t reusable threshold that being on a sub-GTO trajectory probably won't make enough of a difference.

    I wonder if using an extended fairing comes with much of a performance penalty?

  10. 8 minutes ago, CatastrophicFailure said:

    Now, where I am that’s actually extremely competitive, I pay Comcast around $180 for cable and broadband. Local phone company has options from $60-$90 but, well, they suck too. Did your buddy get the same “mid to late 2021” estimate?

     

    Same for you, eh? :( Maybe that timeframe is just general and they’ll actually be shipping to early orderers sooner?

     

    As I understand, and I could be wrong, StarLink should not really be affected by weather much, as the frequency it operates at can get through clouds/moisture, and the phased array antenna can compensate a great deal for any wind jostling of the dish. 

    As well as picking an alternative satellite in an alternative direction, once the constellation is mature, which should give some protection from passing storms.

  11. 3 minutes ago, RuBisCO said:

    Called my step-father up, told him I want to get him starlink for the family farm, he is paying $180 a month for 2 phones lines and 20 Mbs DSL and he wants to get rid of the phone lines and replace them with Voip (he has been saying he wants to replace them with Voip for years now, needs a kick in the butt to do it). Minnesota is high enough latitude we should get good early coverage. 

    That's exactly the sort of situation Starlink is optimal for.

  12. With 1 engine out allowed on ascent and landing, at 2% chance of failure per burn the Starship upper stage would have a 99.4% chance of successful ascent (6 failures in 1000) on 6 engines and a 99.9% chance of a successful landing on 3 engines. That's 7 failures in 1000 due to engines.

    With 4 engines out allowed on ascent, the Superheavy booster, the Superheavy booster has a 99.98% chance of successful ascent. That's 2 failures in 10,000.

    Loss of crew due to engines would therefore be 72 in 10,000 missions.

    The booster landing on 4 engines would land successfully (up to 1 engine out) 99.77% of the time. ~25 lost boosters total ascent and landing per 10,000 flights.

     

    If the engines are more reliable than 2% chance of failure then things improve a lot. Merlin 1D has flown 110 (990 engines) missions with 2 engine failures on ascent. Failures during static fire/landing are unknown and therefore excluded. That's 0.2%.

    If Raptor can achieve Merlin level reliability of 2 failures in 990, then Superheavy might be expected to fail 24 in a million landings and suffer no failed ascents. The Starship upper stage would suffer critical engine related failures on 72 of a million flights.

     

    That's not a very long way off where it needs to be TBH. In 1960 Boeing alone suffered over 40 accidents per million flights. Sure, engines are not everything that can go wrong with a rocket, but they are the most critical.

  13. 2 hours ago, Meecrob said:

    You forgot the inclination. The reason SSO's work is because you never pass over the poles. This makes it so you are never equidistant from the increase in concentration of mass due to the oblateness, as you would be if passing directly over the poles. Thus, the satellite will always be pulled towards the equator directly north or south of its location as there is a "close" and "far" node. For Saturn, to reduce the increase in precession when compared to Earth, they would set the inclination closer to 90* to decrease the difference in the gravitational pull of the close and far nodes.

    I didn't forget it, it just wasn't relevant to whether a planet can have SSOs or not.

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