• Content count

  • Joined

  • Last visited

Community Reputation

364 Excellent

1 Follower

About sevenperforce

  • Rank
    Senior Rocket Scientist

Recent Profile Visitors

1096 profile views
  1. I don't think he realized the periapsis velocity would be superluminal. We are used to classical orbital mechanics, where the orbital velocity at any point is always smaller than the escape velocity from that point. Obviously this is not the case when dealing with black holes.
  2. Specific impulse is inversely proportional to thrust-specific fuel consumption. Thus, if you can increase thrust without increasing propellant consumption, your specific impulse goes up. An air augmentation system increases the thrust without costing more fuel over a selected range of speeds and altitudes, but reduces the TWR of your engine since you're carrying extra weight. The range of speeds in which adding an afterburner increases specific impulse is rather low. At low speeds, the airstream isn't compressed enough by ram effect to burn efficiently; at high speeds, the airstream is moving so fast that trying to slow it down and burn it makes it super draggy. Thrust is increased in both cases though. In the SSTO galore thread, I show that a Raptor-derived VTVL SSTO using basic air augmentation could achieve a payload fraction over 3% with full and rapid reusability, with a GLOW of only 100 metric tonnes. Adding some hot-gas methalox landing thrusters that could also boost launch thrust would push GLOW up to about 120 tonnes and increase payload fraction to 4.5% or so. With a PSTO configuration you can get payload fractions as high as 8-10% for under 200 tonnes GLOW.
  3. There is a critical trifecta: parallel staging (with or without crossed), air augmentation, and altitude compensation. Using all three, SSTO-like operations should be readily achievable.
  4. Paradoxically, the best way to stop a nuke from going off might be to shoot it with a gun or hit it with a sledgehammer. Of course you're dead, either from the implosion charges going off or from radiation poisoning as the core ruptures, but prompt supercriticality will be stopped before it starts. Modern two-point implosion nukes are designed specifically so that if there is even a fraction of a second of difference in the timing of the implosion charges going off (e.g. due to an impact rather than an intentional trigger), the fissile core will be smushed into an irregular shape and scatter. It will probably kill everyone within a block or two, but at least it doesn't go all Trinity. Thermonuclear weapons use an extraordinary specific design (the Teller-Ullam model) to use the x-ray flux of a fission nuke to trigger thermonuclear fusion. Some of these use a fissionable tamper to further increase the yield. If an Orion ship had a premature detonation in the "fuel bay", it's possible the neutron flux from the first nuke might trigger some additional fission in any fissionable tampers in surrounding bombs. But increased energy from this would probably be less than the energy released by the conventional implosion charges.
  5. Two questions. First, what kind of performance do you think this could provide? Do you have any specific numbers you've put together? Second, whatever SSTO-performance-enabling technology you might posit, how would an SSTO be able to beat a TSTO using the same?
  6. Au contraire. It vastly increases specific impulse.
  7. Your friend is correct. A nuke requires an instantaneous supercritical mass to form under specific conditions.
  8. Unlikely. It is hard enough to land with precision on a swaying deck already.
  9. Heh. But, all other things being equal, which elements are typically the most sensitive to throttle rates? For example, let us say that a given staged-combustion engine can throttle down to 20% of its max stated thrust. What would fail first below that throttle setting?
  10. Good question, actually. The answer is that the space around a black hole is so aggressively curved that you would have to be traveling at the speed of light in order to orbit anywhere near the event horizon. Recall that the speed of a circular orbit increases as you move closer to the center of a body. For black hole physics, the speed of a circular orbit exceeds c well above the event horizon. Calculating orbits near Earth is straightforward enough, but orbital mechanics goes haywire near a black hole. The reason stable orbits exist around the Earth is that Earth's gravity curves space just enough that a "straight" line closes into a loop. But a black hole curves space until there are no closed paths. If you were just outside the event horizon and tried to accelerate away, you would experience space literally stretching out away in front and behind you. The event horizon itself is where gravity drag goes to infinity. Finally, relativistic mass appears only from an outside reference frame. You do not experience yourself becoming more massive as you gain speed, because you are at rest relative to yourself.
  11. I decided to go ahead and crunch the numbers for a small SSTO launcher using a production version of the 1000 kN dev Raptor and a basic air-augmentation ejector shroud. The math actually comes out pretty nicely. The Raptor is supposed to have a better TWR than the uprated Merlin, which boasts 180:1, so let's set the mass of a production dev Raptor at 550 kg. Rule of thumb on an ejector shroud is that it will be 3-5 times the mass of the engine; I'll guess at 2 tonnes. With the ejector shroud giving a static thrust increase of 15%, the pad thrust will be 117.3 tonnes. Set GLOW at 100 tonnes, slightly less than the mass of the Falcon 9FT expendable second stage. Base specific impulse for the SL Raptor is 334 seconds; underexpansion pressure will cause it to climb to 361 seconds as altitude increases. At the same time, the ejector shroud will boost the effective specific impulse, starting at 15% at zero airspeed and climbing to 50% at Mach 2. Starting around Mach 4.4, ram drag due to the increasing airspeed will start to sap the efficiency boost; the boost will drop to zero around 3.4 km/s. However, the ejector shroud will still be able to increase the expansion ratio slightly...probably to around 375 seconds. This is still less than the specific impulse of the Vacuum Raptor. Working these numbers iteratively gives 11.2 tonnes of payload+vehicle+residuals in LEO. Reserving 500 dV at SL for landing leaves 9.59 tonnes. Fuel consumption is 90.41 tonnes. The ITS Tanker has a structure+tankage ratio of 97.4% including TPS and auxiliary thrusters; adjusting for some square-cube losses, I'll place the ratio here at 96%, for a tankage+airframe+TPS mass of 3.77 tonnes. This provides a vehicle dry mass of 6.32 tonnes, for a total payload of 3.27 tonnes. 3.27% is a fantastic payload fraction to begin with. Even better for a fully reusable vehicle smaller than the Falcon 9 second stage.
  12. Is the limiting factor in deep-throttling a pump-driven liquid-fueled rocket engine the pump, the chamber, or the nozzle? Or does it vary from system to system? For example, Raptor can downthrottle to 40% of max rated thrust. Does the 40% minimum have to do with combustion instability in the chamber, or flow choking in the nozzle, or flow within the turbopump and preburner?
  13. Gorgeous! Differential yaw between those two boosters would be really problematic IRL but I love how it works in KSP!
  14. Took a closer look, and I see how it works now. Nice concept!