sevenperforce

Hey, no one is saying it wouldn't work, per se. Eight Merlins arranged around a central plug nozzle with an air-augmentation shroud to kick up the thrust would SSTO easily enough.

It's a common proposal, and it's not a terribly bad one. Now, towering up to 36,000' is a bit overly optimistic. The infrastructure investment is just so high that we'd never be able to get it set up when you can just put your orbital stage on top of a first stage and launch from anywhere. Not to mention that you're limited as to inclination.

The interstage is between the first stage and the second stage; the trunk is between the second stage and the Dragon capsule. The trunk stays with the capsule on orbit and is jettisoned shortly before re-entry.

It won't be going at 200 km/s until it's way, way below Mercury's orbit. A gravitational slingshot per se won't work unless you're already on a hyperbolic trajectory with respect to your target body. Think of a gravitational slingshot like riding a skateboard past a speeding car. If you grab hold of the speeding car for a moment as it passes, you can use its momentum to help slingshot you forward much faster. But if you're already traveling alongside of the speeding car, holding onto it, then you can't slingshot off of it. However, you can certainly boost your final velocity by using the Oberth effect to burn hard at perihelion. Back to our skateboard analogy; if you are holding onto the speeding car and push off of it with your foot, you'll get more of a kick than if you tried to simply push off the ground.

A mass driver on the moon or on an asteroid is a fine idea, sure.

Conversion would be virtually impossible in this particular case. Aerospike/plug nozzles are designed in one of two ways. Either they have a toroidal combustion chamber which opens in a ring around the nozzle, or they have many small combustion chambers arranged around the perimeter. For linear aerospike engines, it's always the latter. Toroidal combustion chamber: Ring of small combustion chambers: Linear aerospike, many small chambers: The Merlin engine has a very strong, lightweight combustion chamber enabling high combustion efficiency and high chamber pressure, leading to good sea-level specific impulse. The combustion chamber would have to be completely redesigned in order to be converted into a toroidal one, as combustion and flow in a toroidal chamber is completely different. Now, it might be possible, in theory, to use the Merlin engines themselves as the smaller thrust chambers arranged around a single aerospike nozzle. But that would be....just ridiculously massive.

"Spleen access port here."

I'm not sure if density is the key factor. IIRC the efficacy of an ablative heat shield is a combination of its heat capacity, its heat conductivity, and its total mass. You want something with a high heat capacity so it takes a lot of heat to ablate a given mass away, and you want something with very low heat conductivity so that each microlayer must ablate away completely before the next layer feels any significant heating.

Yeah, I've been curious as to whether the new titanium grid fins will be square, like the current ones, or switch to something like the diamond-shaped ones depicted for the ITS Booster. Simply placing a cork lip around the base of the second stage should be enough to keep it on track. The TPS panels currently used for the Falcon 9 first stage are cork.

Apparently that's not what NASA thought when they built Gemini.... Gemini 5 nose cone shown.

The specific-impulse-to-airspeed curve for a water-based TaHfC pebble-bed nuclear-thermal turboramrocket looks something like this: Starts a little over 700 seconds, climbs briefly to over 1000 seconds around Mach 4, and then drops gradually to 555 seconds by the time you hit around 5.4 km/s. Honestly, curve probably stays high for longer if you have the right design. Note that this is effective specific impulse, not actual specific impulse; it is based on acceleration. A turbofan is most efficient at cruise, where vehicle acceleration is zero. This is not the direction you want to go for a launch vehicle; you want near-constant acceleration across your flight profile.

Aww, you beat me to it. And me. Asking "precisely how many watts of energy would it take to vaporize Earth" is a bit like asking "precisely how many inches of force would it take to lift a tank?"

Both the Agena and the Gemini capsule launched with a fairing to protect the docking ports. Oh, right. Good catch. There was definitely a parachute on Vostok; it just wasn't enough to keep the person alive so they would eject once the altitude was low enough. There's been at least one time that the braking rockets on the Soyuz failed. I believe there were some spinal compression fractures but at least everyone lived.

To fit the docking adapter on the unmanned Agena docking target.

Just looked him up because never heard of him before. Having a geoscience background i feel unable to comment in a reasonable way :-) There is no reasonable commentary on the radioactive dumpster fire that is Ken Ham.

Better than one of the Russian designs, which had no parachutes on the capsule at all and used the ejection seat as the standard capsule egress route following re-entry.

You need 5.5 km/s just to get into a Mercurian Hohmann transfer.

Presumably there's a slingshot involved somewhere, right? A nice tight retrograde slingshot can get a low perihelion easily enough. Their projected aphelion is somewhere between Earth and Venus, so maybe the idea is to launch on a Hohmann transfer from Earth to Venus, then use Venusian gravity to get into an Earth-retrograde slingshot?

Trying very hard to skirt the edges of a flamewar... ...I find it very amusing that the Creationists (Ken Ham particularly) have taken a hard line that we will find no microbial life anywhere else in the solar system. Even in their silly-as-a-flat-earth models, there's no reason whatsoever why their deity could not have created microbial life elsewhere.

Yeah, that's what I mentioned before. Basically, you take an NTR (my preference is a Tantalum Halfnium Carbide pebble-bed), throw a shroud around it to make it air-augmented, and set up the secondary coolant loop to make it operate bimodally. Since you've already got to gear the bimodal turbine in order to rotate the pebble-bed reactor, there's a low weight penalty for adding a turbofan to the front end of the shroud.

I get that, but we aren't talking about a 15-20% margin difference; we're talking about a 74% decrease in vehicle dry mass. That's insane. Even if you add hefty square-cube losses, you'd only increase total TSTO dry mass to 40% of the SSTO variant. I think the more likely culprit is that they are simply far overestimating how tight they can get the dry mass ratio.

That's not an extensible bell; that's apparently a nozzle that retracts in and extends out of the stage body...presumably to shorten the interstage and protect it during re-entry. That's not the direction SpaceX has gone with the MVac at all. The MVac is useless at sea level. Fire it up, and it will be so underexpanded that flow separation inside the nozzle would tear it to pieces. Even if this didn't happen, the aerodynamic turbulence would shred it; that nozzle is ridiculously thin, to the point that it's practically pressure-stabilized in flight. Watch the S2 startup from one of the SpaceX live launch videos; the entire nozzle flexes during startup transients. And, even if it could do controlled flow separation and could somehow avoid aerodynamic turbulence, the TWR at minimum would be around 12, waaaaay too thrusty for a hoverslam. That's why we need an integrated S2+Dragon!

ARCA projects a million bucks per launch. If we assume a 10% profit margin, and a bit of quick number-crunching gives me a propellant cost of $135,000 per launch, then we arrive at an operating-and-manufacturing cost of $765,000 per launch. I find it hard to believe that a company which can build and launch a single-engine 550-kg rocket for $765,000 could not build and launch a two-engine 143-kg rocket for less than $870,000 (the number to beat, given far lower propellant costs for the TSTO). Even if the first stage still cost the same $765,000 as the whole SSTO, the second stage would only cost $28,000 if you use the same price per kilogram for construction and operation.

Has it figured out yet that staging before burnout is never, ever a good idea? I'm reminded of this....

Sorry for being unclear. I meant that you would simply do the de-orbit burn, wait until you hit the atmosphere, and then push propellant through your engine as gently as possible in order to deflect heating. Or you could pump water through the lower skin and let it exhaust out the engine. Either way. Planning to add a ton or two of water is going to drive up your dry mass by 6-10%; not too bad really. For a lifting body design I'm going to take advantage of a few other things and do an NTTRR with rotating nacelles. That'll be forthcoming.