RCgothic

Members
  • Content Count

    361
  • Joined

  • Last visited

Posts posted by RCgothic


  1. 1 hour ago, Spacescifi said:

     

    Just accept the effect as a plot device, shielding the vessel from gravity. It can be turned on or off. Which makes it useful for reaching space, and then the ship can reach orbital velocity and shut it off.

    Or don't turn it off and just hover. If the device uses less energy to run than it would take to accelerate to orbital velocities, you just wouldn't.

     

    And if it does use more energy than it takes to get to orbital speed, might not be worth having it at all. The ascent is a relatively small part of the energy requirement for accelerating to orbital speed.


  2. UK radio today reporting that tourist flights to the international space station were to be offered for £28,000. The news was lacking in detail, but that's insane. Even starship with a capacity 100 would struggle to loft a person into LEO for so little.


  3. If a ship can avoid experiencing gravity it can maintain an orbital altitude without an orbital velocity. In fact, it *needs* to experience gravity in order to orbit, because without gravity any velocity is escape velocity unless you apply constant radial thrust. A gravity manipulating ship doesn't have to orbit unless it wants to. It levitates.

    So there's no reason its entry to the atmosphere needs to be great other than reducing transit time. If you can sustain re-entry heating, come in hot. If not, walking pace or slower will do.

    Without gravity all urgency goes away.


  4. On 4/23/2019 at 4:38 PM, mikegarrison said:

    I wonder if steam locomotives were intentionally designed so that the front plate was the weakest point in the pressure vessel? I mean, you don't want them to explode, but if they do explode, then that's probably the safest way for them to explode.

    Well first off they were usually designed with redundant steam relief valves to stop that happening. Of the fifteen boiler explosions in the UK in the 20th century, only two were failures of the boiler barrel due to incorrectly assembled relief valves (overpressure).

    Thirteen were due to weakening of the firebox caused by low water level, which is not quite so spectacular. Low water level is caused by either operator error or faulty water level gauges.


  5. So both second stages are undersized. The core stage is oversized but too anaemic in terms of thrust. And the boosters don't burn long enough. :rolleyes:

    If core and boosters were able to put a sizeable payload in orbit without a second stage the problem wouldn't be half as bad. That would make it mission agnostic, just change what you stick on top of it.

    If I were designing a SHLV today it would probably look a lot like SA-513 that launched Skylab with flyback side boosters. Two and a half stages to orbit and whatever you like on top.


  6. 2 hours ago, sevenperforce said:

    There's absolutely an upside. Gravity is an incredibly harsh mistress. Gravity drag is the only thing that directly, linearly robs you of endpoint dV; if you have 100 m/s more gravity drag, then you stage at 100 m/s slower, and you reach final stage burnout (regardless of how many stages you have) at 100 m/s slower. Cutting gravity drag on the first stage provides a 1-to-1 boost to your final stage burnout velocity.

    Gravity drag comes in units of m/s just like any other source of dV and is the product of gravitational acceleration (g) and time. Every second the rocket spends in its boost phase is 9.81 m/s of dV lost. If a higher TWR means you can shave 1 second off your ascent, you stage at 9.81 m/s faster. If you can shave 5 seconds off your ascent, you stage at 49.05 seconds faster.

    During the first test flight, Falcon Heavy staged at T+373 seconds. If they can uprate the engines and increase TWR by just 2.5%, then they can stage at T+364 seconds instead, meaning they've shaved nine seconds off their ascent, which means around 92 m/s of dV gravity can't steal from them. The relationship between dV and mass fraction (all other things being equal) is logarithmic, so if the upper stage is trying to push a payload into GTO, starting with an extra 92 m/s is a significant advantage.

    They hope to recover this center core, but since they haven't recovered a center core to date, they are building a new one for the next flight out of an abundance of caution. If they recover this core they will probably break it down and inspect it heavily before flying again. It is essentially a brand new rocket.

    This is not quite the while story, as you only get robbed of the full 9.81m/s per second if you're burning purely vertical. Once you start burning sideways gravity drag has less and less effect.

    This is a trigonometric effect. Your thrust makes up the hypotenuse. Gravity drag is vertical. Your desired velocity vector is the difference.


  7. SLS won't be building LOP-G though. It can't co-manifest substantial enough payloads prior to block 2. 

    So we're talking about 3rd party boosters anyway. Falcon can send both the payloads and the crew, separately, within a short enough space of time to be useful. Dragon2 will be able to go to the moon.


  8. Extended deep space operations are pretty pointless if the booster is incapable of taking crew anywhere that requires them.

    The only difference between extended deep space operations and extended LEO operations is risk to crew.


  9. We do need to launch on the timescales Apollo did because that enables rendezvous mission architectures.

    Without those we are never going beyond the earth moon system and we are never establishing a significant lunar presence.


  10. We never *will* be ready for the moon or mars or even just significant orbital presence unless and until we have a booster capable of doing the job. Of lifting substantial payloads and performing the required rendezvous mission architectures.

    Want to forget the competition? Fine. They're forgotten.

    SLS is *still* not the booster that advances manned spaceflight. Its most advanced form can barely recreate Apollo at 1/4 of the flight cadence.


  11. SLS fails on its own terms, nevermind any competitors. It was supposed to take humans to Mars and it never can, not even in its most ambitious block 2 format. It can barely manage the moon. It simply costs too much and can't fly often enough.

    It fails as a stopgap because it doesn't stop the gap.

    Better to cancel it now and focus on designing and building a clean slate big dumb booster with a decent launch cadence and have another 10 year gap, than drag this charade on for another 5 years before realising that cancelling it and starting again is what should have happened in the first place.


  12. SLS is a dead end and any money spent on it is a waste. It's a dead program walking. The only thing keeping it going is sunk cost fallacy.

    There's nothing SLS block 1b or 1b can do that a handful of launches from an alternative extant booster couldn't do for a fraction of the price. Even Block 2 will never have Mars capability, and that is the purpose for which this booster has been touted. If Block 2 ever flies I'll be truly astounded.

    And the gap in human spaceflight capabilities is not ok. It will have been nine years since the shuttle least flew, and everyone knew that retirement date was coming. "Only 50% longer than the Apollo-Shuttle gap" does not make it ok. 6 years then wasn't ok either, although at least that produced a groundbreaking vehicle. Where shuttle went wrong was keeping it in service for 25 years after it became clear it wasn't going to acheive its required cadence.

    They should have done something else. The opportunity cost of the last 20 years had been crippling.


  13. There's a difference between a custom fairing to cover Orion and its LES at about 5m diameter at around Falcon's normal fairing size, and flaring out an interstage to cover EUS at 8.4m. The EUS is the same diameter as the SLS core.

    The Orion service module (built by ESA) is narrower than Orion.