RCgothic

Members
  • Content Count

    553
  • Joined

  • Last visited

Posts posted by RCgothic


  1. You can't just compare masses of airliners and rockets. (By the way 737max is roughly 1/4 payload at max takeoff mass).

    Falcon 9 is the single most popular launch platform worldwide bar none. The F9 upper stage masses approx 112t wet including 16t heaviest payload actually flown. It stages high (80km) and fast (Mach 6) so that in likelihood an aircraft-launched upper stage is going to have to work much harder

    So a reusable upper stage less than half the mass of F9 upper stage is going to have less than half the payload and tighter margins, even before you include the effort of making it reusable (F9US is not reusable). Tighter margins means the vehicle has to work harder, which makes it less reusable.

    It's no coincidence that the two new reusable craft on the drawing board - starship/superheavy and New Glenn are both going larger than Falcon9.


  2. 29 minutes ago, Dragon01 said:

    Not true. In fact, you want to make your nuclear reactor reusable. TWR issues are not common to all NTRs, NERVA (which is what most people think about when they hear about it) was an orbital engine. It's like judging chemical propulsion by what the RL-10 can do. It's relatively straightforward to make a reactor which can survive most possible accidents without a radiation release, but the real problem is that a reactor is friggin' expensive. A disposable nuclear reactor of any kind is wasteful, because these things can run for years.

    A simpler way would be to have a nuclear near-SSTO (can be something like Ajax), and attach a small kick/maneuvering stage to the payload. We already have rockets that are nearly single-stage, but use a small, solid or hypergolic upper stage to complete orbital insertion. It's efficient, doesn't leave too much junk in orbit, and it's easier from the mission planning perspective, especially if it's a high precision hypergolic stage.

    I didn't say not reusable. I said "not repeatedly flying through the atmosphere and repeatedly intersecting the ground." Reuse nuclear propulsion stages as much as you like. Just keep them away from Earth.

    I am very much in favour of doing useful things with nuclear propulsion and building a bajillion reactors for clean energy. I am very much not in favour of doing risky things with nuclear propulsion.


  3. You don't really want a nuclear reactor on anything that has to repeatedly launch or land because sooner or later there'll be an accident. Also nuclear propulsion has poor thrust to weight and the exhaust can be problematic for the launch and landing sites.

    Nuclear propulsion is best left to deep-space only. Things don't start getting really radioactive until being turned on for the first time, so the initial launch isn't too much of a risk. But it's not suited to repeated trips through the atmosphere.

    We currently don't have any air-breathing engines that work well at high hypersonic speeds. You can get higher, faster, with better payload fraction and use less propellant by using conventional rocket engines.


  4. You're right that SSTO is effectively pointless as it can't carry any meaningful playload, and that two stage to orbit is much better from that perspective of you can make both stages reusable.

    But I think nuclear propulsion and wings is overcomplicating. 50t from suborbital is much less playload than a Falcon9. Why not 1200t reusable craft on top of a mammoth reusable booster?


  5. Some more maths - at 2MN and a sea level ISP of 330s a single raptor has a specific propellant consumption of approx 620kg/s.

    A single raptor can lift about 165t at a TWR of 1.2. If the dry mass is 120t+ then the propellant mass is <45t. That's 72s of full throttle or less.

    Technically the acceleration is an inverse function of thrust divided by time-variant mass, but as the mass fraction is low for an estimate I save myself a lot of trouble by assuming acceleration varies linearly between ignition (0.2g) and burnout (0.7g). This overestimates the acceleration mid-flight by about 2.5%.

    a = 2.1 + 0.061*t   m/s/s

    v = 2.1t + 0.031*t^2 m/s

    Velocity at burnout (72s) is 312m/s. Coasts to 0m/s in a further 32s.

    The altitude at burnout is:

    s = 1.1t^2 + 0.01t^3 m

    Or approx 9.4km. From 312m/s it'd coast another 5.0km before gravity brings it to a stop.

    That's an absolute flight ceiling of 15km expendable on a single raptor, ignoring air resistance and being really generous with the dry mass. Spitballing I'd guess 5km max altitude including reserves for landing.


  6. 2 hours ago, RCgothic said:

    Screenshot-20200331-183936-Drive.jpgFeel free to do some maths. Extra-long 22m version also available apparently.

    The trapezoidal volume of revolution of the given points is 678m3, or 917.5m3 for the 22m version assuming it's the 8m section that's stretched not the curve.

    Space shuttle had a cargo volume of 10600cuft or 300m3.

    SLS Block 1 cargo has 286m3.

    SLS Block 1b cargo has 537m3

    SLS Block 2 cargo has 988m3 assuming an 8.4m fairing.

    So SLS Block 2 cargo has more volume than even a stretched starship and could potentially be larger still with a 10m fairing, but let's not start pretending SLS Block 2 isn't anything more than a paper rocket, lol.

    6 minutes ago, RealKerbal3x said:

    I love how Starship can get more payload to the lunar surface than SLS can get to LEO :P

    With refuelling.


  7. 46 minutes ago, mikegarrison said:

    Every cargo vessel is both weight and volume limited. We're only seeing the weight limit here, but I wonder what the volume limit is? And of course there will be a door-passing size limit, probably.

    Screenshot-20200331-183936-Drive.jpgFeel free to do some maths. Extra-long 22m version also available apparently.


  8. 9 minutes ago, zolotiyeruki said:

    What's not clear to me is where/how the canards are hinged.  They follow the curve of the nose, but there's no indication of where the hinge axis is.  Or is it expected to simply be faired very cleanly?

    Unclear!


  9. 2 hours ago, Ultimate Steve said:

    My personal opinion is that the idea of a lunar station itself is far from pointless, but Gateway's location is kinda pointless, mostly where it is because Orion never got a better service module post constellation. 

    SLS can't boost Orion plus a better service module.


  10. Probably partially expendable. The reusable payload to GTO is only about 8 tonnes, and not only is TLI more difficult than that but dragon plus 5 tonnes is about 17 tonnes. The fully expendable payload to TLI is probably in the region 20-21 tonnes.

    With the reusable side boosters being only about 10% payload penalty it really doesn't make sense to expend them if you can avoid it.