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sevenperforce

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Everything posted by sevenperforce

  1. Not to mention that slipping launch dates are equally punishing. If a GTO comsat company is losing a few thousand bucks a day for every minute their bird isn't in GTO, then they'll be likely to pay a premium for a launch date that they can be confident about, even if total cost and circularization time are higher.
  2. Even if you're trying to go with a pure nuclear-thermal turboramjet, you're still going to need a coolant loop to run things. Probably an expendable one, given how nasty NTRs can be. So ISRU is still important. The "reverse staging" approach has its own problems, as I noted above. However, we should definitely use an airbreather to some degree. One of the reasons I like using a turboramrocket (whether it has tilting nacelles or is a "pure" VTVL) is that it allows for some neat storytelling. A pure NTR or even a ramrocket NTR will turn the landing site into a flaming radioactive crater with no reasonable chance of safe egress, but if the landing can be done on fan thrust alone (although this will still need the coolant loop), then touchdown can leave the landing zone relatively intact. Should be easy enough, since the lander will be virtually empty at touchdown. This also allows for high-speed extractions; you can drop in, pick up your people, and then immediately take off again to head for a safer zone to refuel and launch. This gets interesting in the story. Suppose you're coming in for a landing when you realize your landing zone has been compromised and there are enemy troops waiting in ambush. Why, all you have to do is come in like you normally would, then kick the NTR into gear and flood it with propellant a few hundred feet above the landing site. You pull a couple of gees and blast away to safety while the enemy combatants find themselves staring into the tail end of a nuclear thermal turborocket on full afterburner...which is not a particularly safe place to be. This is why I prefer rotating nacelles or dedicated landing engines to allow a belly landing.
  3. I like this idea too, but the catcher stage is going to need to be better than an SSTO all in its own right. Even if the lander stage can somehow supply 50% of the required dV to orbit, the catcher stage is going to have to burn retrograde to 50% of orbital speed in order to make the "snag" and then burn prograde again, dragging the lander back up to orbit with it. Total propellant expenditure makes a pure SSTO seem simple by comparison. Plus, even if the colony ship has plenty of fissile fuel, propellant reserves are fairly limited in comparison. An SSTO lander that uses ISRU is much better. Haas 2CA wouldn't work, as it has no TPS. But surely even a water-based NTR would be able to beat out the Haas concept. Honestly the most far-fetched part of this is the whole "they still speak English after thousands of years" thing. One way to justify having a less capable SSTO dropship is to posit that the dropships were constructed on Earth, much earlier, with no intention of being used for any sort of military purpose.
  4. Yeah, it gets complicated. In cases where the government subsidizes the launch, that subsidy isn't available to every potential customer. For example, I doubt a US-based comsat company with a 3.5 tonne LEO bird would be able to get the $15M price for a Polar Satellite launch. And things are different based on what sort of payload you're delivering and where you're delivering it to: Price/kg, pressurized cargo to LEO Price/kg, hardened cargo to LEO Price/kg, comsat to LEO Price/person, passengers to LEO (short-term shuttle) Price/person, passengers to LEO (long-duration) Price/kg, comsat to GTO If you want to launch a comsat to GTO, you don't care about price/kg for comsats to LEO, and you certainly don't care about price/kg for hardened cargo runs or for passenger flights.
  5. Yeah, any comsat will have its own maneuvering bus already, so asking the bus manufacturer to squeeze in some extra dV at the expense of a slightly smaller terminal payload would allow GTO payloads, albeit while sacrificing some of the rapidity of the launch platform while you spiral out. Using various configurations of strap-on COTS SRBs would be an even better way to squeeze some variable performance out of the launcher. If it didn't have a vacuum-optimized nozzle on the core stage, this wouldn't work, but it does, so...why not?
  6. Cool table, but it's not quite right. Falcon 9 with reuse can only manage about 15 tonnes to LEO. And the $62M launch price assumes that SpaceX reserves propellant for recovery. Expendable launches will cost more than the standard $62M sticker price, going forward. I feel like Echostar-23 was probably expendable at the standard price as a courtesy to Echostar due to launch delays but that's not going to always be the case. The $44M estimated launch price is the estimated previously-flown booster discount, which isn't the same as the difference between an expendable and a reusable launch.
  7. And they have no intention of pursuing crossfeed at this time. Wow. EDIT: I noticed that the SpaceX pages still show black landing legs, even though the legs have been white ever since they were first installed. I wonder why. Think this is somehow part of the planned Block 5 upgrade that has been in the works forever? As an example, consider Elon's favorite analogy, the airplane thing. "If you threw away an airplane every time you flew, airline tickets would be astronomical." This analogy doesn't work at all for Blue Origin, because they haven't been selling "tickets" this entire time. It's a completely different world.
  8. In the OP's concept, the landing is virtually empty, and the vehicle has enough time for ISRU...a day or two, maybe. They simply pump water into the tanks, check systems, and take off.
  9. Agreed on the "technology collapse" aspect. Honestly, we ourselves aren't necessarily a "spacefaring society" yet.
  10. To the OP: for the constraints of the story, what actual capabilities does the drop ship need? Delivery and extraction of a dozen Marines? Two dozen? Cargo? Downmass is a lot easier to do than upmass, obviously, but needing a cargo bay complicates things. What kind of persistence does it need? Can its occupants count on getting back to the colony ship within hours of reaching orbit, or does it need a few days of on-orbit consumables? Does it carry any armament or can it rely on ground support? The OP specified the need for ISRU with a minimum turnaround of at least a day. So it is much more realistic than that. I suggested turboramrockets so the craft could do a dry landing and thus avoid scorching the landing site into a radioactive crater. Landings would invariably be near-empty. And though NTRs might not be flying now, they have been tested in the past and continue to be component tested today. We know their specs with a high degree of confidence. So definitely near future.
  11. Well, this is a little different. NTR based SSTO is actually quite achievable if you can use altitude compensation and you aren't bothered by a bit of nuclear pollution. Plus, orbit-to-surface-and-back reusability is easier in a lot of ways.
  12. If I were you, I'd crunch some numbers and look at what kind of performance you might be able to expect. Most speculative or development-stage NTRs used liquid hydrogen, but if you know the density of liquid hydrogen and water, a little math will allow you to calculate the correct TWR. For a ramrocket, a simple duct increases static thrust by 15%, going up to 50% at Mach 0.5 and staying there until about half the maximum useful speed, after which thrust begins to drop roughly linearly. Add a single-stage fan and the static thrust boost increases to around 60%, up to 80% with ram compression at Mach 1 and then dropping to match the simple ramrocket by Mach 3. Unless your fan has some kind of folding blade system, the blades will melt around Mach 5.
  13. Right. So the faint red glow I'm seeing in the image corresponds to about 500-550 degrees C.
  14. A pure nuclear air-ramjet has to be virtually unshielded, which is not a good idea. See Project Pluto. Of all the missiles ever envisioned, SLAM has got to be the worst by far. An air-augmented NTR can certainly operate at far higher speeds than a hydrogen scramjet, which can itself operate at higher speeds than a hydrocarbon scramjet. The limiting speed of any vehicle which uses air as part of its working mass is, not surprisingly, the actual exhaust velocity of its propellant. If the airstream is moving faster (with respect to the vehicle) than your exhaust velocity, then trying to mix the exhaust with the airstream slows it down rather than speeding it up. So, for a water NTR, you're looking at around 4 km/s, or Mach 12. Of course, you might want to go out of airbreathing mode at lower speeds since the sustained heating at that speed will be ridiculous and the drag will make your thrust balance very poor.
  15. And the color I'm referencing is definitely a noticeable dull red.
  16. VTOL turbojets are difficult because jet engines have ridiculously poor thrust-to-weight ratios. You need a very heavy multistage compressor to get the air packed into a tight space, and then you need duct length to mix the fuel with the compressed air, and then you need to have more duct length to combust the fuel-air mixture, and then you need a heavy aft turbine to drive the front turbine. Plus, higher-speed turbojets need even longer ducts because the airflow is moving so fast (e.g., the gorgeous but oh-so-heavy J58-1 on the SR-71). A turboramrocket needs none of these things, and an NTR less so. You don't have to collect, compress, or mix your oxidizer; you simply pump your working fluid through your NTR and let it blast out the back end. You need a duct to mix the airflow with the working fluid, and you need a single-stage fan to force air into the duct, but that's it. The air doesn't even need to be significantly compressed, because the rapidly-moving NTR exhaust will take care of that. For lightweight chemical rocket engines, adding the duct alone runs about 3-5x the mass of the engine and a single-stage fan would drive that up to about 6x. But for a heavier NTR, it might double the mass of the engine, maximum. And you'd be increasing the thrust by about 60% at a standstill and up to 80% at speed. When you're dealing with a water NTR, this is really critical because your specific impulse is not nearly high enough to make up for the lower TWR of the NTR (even though your NTR TWR will be better than, say, an LH2 NTR). As far as the form factor is concerned, I was thinking of something like the Skylon, but not nearly so large and with even stubbier wings. Getting your crew in and out of a tailsitting nuclear rocket would be a pretty serious nightmare. Getting cargo out would be impossible. Landing tail-first on a nuclear rocket WILL melt the ground and set everything on fire for some distance around. But since you'd be landing with empty tanks, having a turbofan you can use to set down gently would really be an advantage.
  17. Incandescence begins around 500 C for most metals and Al doesn't melt until 660 C. But I suppose Al would be hella weak at that temperature.
  18. Okay, then there wouldn't be any change in pitch from pressure gradients. Multiple transitions won't change pitch if the start density is the same as the final density. Water and glass will refract light, but the light coming through a flat windowpane isn't visibly refracted because transitioning from the air to the glass is cancelled by the following transition from the glass to the air. The reason voices sounded high-pitched is probably because of differential attenuation. High frequencies and low frequencies are attenuated differently; the sound-through-mask effect probably attenuated low frequencies more than higher frequencies, making the remaining voices sound tinny.
  19. In that case, you're going to want to go with a turboramrocket solid-core NTR that runs on water and uses a toroidal truncated aerospike nozzle. Simple, easy to refuel, and thrusty enough to get you off the ground easily. You'll need the turbomachinery in order to increase your in-atmo specific impulse enough to get you into orbit; the vacuum isp of a water NTR is simply not good enough. With water as a coolant, you can make your NTR bimodal rather easily and it avoids the need for solar panels that need to be extended and retracted. So you already have a power source if you want to add ion propulsion for going to and from the mothership.
  20. It's a tossup, but I think it's the former. The light is brighter closer to the stage wall, which is what we'd expect if it was radiating away heat but not if it was reflecting something at the bottom of the stage. The color temperature is also a lot warmer than the exhaust reflection would be.
  21. Soundwave phase transitions from a high-density medium to a low-density medium cause the frequency to decrease. The reverse case causes the frequency to increase. That's why breathing helium makes your voice sound high-pitched; your vocal cords are vibrating in low-density helium, which increases in frequency when it hits the denser surrounding air. Was this in a sub? Is the EAB pumped at a different mixture or pressure than the surrounding atmosphere?
  22. Elegance counts for nothing; if hanging Chick-fil-A sandwiches on the sides of rockets decreased drag, rocket engineers would make CFA runs before every launch. Pulling off an expendable SSTO for smallsats is a noble goal, if the money works out. And that means they are either stupid or dishonest. If they don't know that their vehicle doesn't have this capability, they are flatly stupid. If they do know that this vehicle doesn't have this capability, they are trying to trick their investors, which is dishonest.
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