sevenperforce

Unlikely. It is hard enough to land with precision on a swaying deck already.

Simply glorious.

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?

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.

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.

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?

Gorgeous! Differential yaw between those two boosters would be really problematic IRL but I love how it works in KSP!

Took a closer look, and I see how it works now. Nice concept!

SpaceX achieved multiple retropropulsive liquid engine restarts as far back as September 2013 and controlled propulsive touchdown of the F9 first stage as far back as April 2014, but New Shepard was the first liquid rocket to be launched, MECOd, restarted, and recovered intact, a month before ORBCOMM-OG2. EDIT: Sniped by the goat.

AFAIK the smallest orbital vehicle ever launched was the Lambda 4S, with a GLOW of 9.4 tonnes. The SS-520-4 was far smaller, at just 2.9 tonnes, but it failed to reach orbit successfully. So even though there has never been a successful orbital launch of this size*, a ~4 tonne expendable launch vehicle with a vacuum-optimized nozzle could most likely put a cubesat in orbit from a 100 km apogee with negligible horizontal velocity. Blue Origin could do it as a demonstration if they wanted to, and if they were willing to work with a COTS upper stage. Then New Shepard would become an orbital booster rather than a suborbital one. *Falcon 1 does not count; the Kestrel-powered stage 2 had a downrange velocity of 2.8 km/s. Random: Did Grasshopper and F9dev restart in-air? Because if not, that's something unique for NS.

But they didn't, and that makes all the difference. Even the difference between *almost* to orbit and TO orbit is a world of difference, because one stays up and the other doesn't. And on the same note, you could also just slap 3 Falcon 9s together and have a Falcon Heavy. And yet somehow it's not there, despite being worked on for the better part of a decade. Turns out that in aerospace, seemingly simple things have a way of not being so. Yeah, that's basically my thought. If they used the NS to launch an orbital payload, then the NS would be a reusable VTVL orbital booster. They have not done so; thus, it isn't. If they did, it would be. Not saying they can't. But capacity is not the point. SpaceX had the capacity to relaunch its very first recovered VTVL orbital booster all the way back in December 2015. Also, New Shepard might have more trouble with an orbital launch than you'd think. The booster only travels 20 km from the launch site, so horizontal velocity is really negligible. In terms of requisite orbital energy, there's a huge difference between going straight up at 1,300 m/s and heading nearly flat downrange at 1.8-2.6 km/s. New Shepard has not performed a gravity turn and does not re-enter the atmosphere in the same way. Could New Shepard launch something to orbit with its current ascent profile? Sure, probably, though it's suboptimal. It might even be able to do a proper gravity turn; I don't know. But that's the bar.

Looks great, but I can't tell -- do the orbiter's engines fire from the very start? If not, it's not a PSTO.

Hey, no one is saying that New Shepard is "barely reusable". I was just responding after Corel noted that some caution must be taken in comparing the achievements of SpaceX with the achievements of Blue Origin. Is the distinction between "suborbital sounding-rocket class" and "orbital-class first stage" meaningful? I mean, I suppose you could call New Shepard orbital class. It's notionally possible to launch a payload to LEO from a starting velocity of 1.3 km/s and a stage+payload mass of 4.5 tonnes, but you're cutting it pretty close. Falcon 1 staged a similarly-sized payload at twice the velocity of New Shepard.

Yes, and Grasshopper involves those aspects as well. I mean, Grasshopper and the F9dev didn't reach suborbital spaceflight, and ordinarily I wouldn't consider them to be part of the conversation, but if we can't see F9 as a different class than NS, then we can't really consider NS to be in a different class from Grasshopper and F9dev.

Depending on how early the Vacuum Raptors can be ignited without dangerous levels of overexpansion, the optimized approach is either nine SL Raptors or seven SL raptors and two balanced Vacuum Raptors. Even so, the best it can do as an SSTO is probably around 55 tonnes of fuel to LEO. But that's just much, much less than what it can do on the top of the ITS booster. Another option would be to give it a parallel booster system, either with COTS solids, FH-style asparagus staging, or a launch skirt as with the Saturn-1D concept.

Yes, this is true; there are no first stages which reach orbit because nobody flies SSTOs. But this fact does not negate the difference between a glorified sounding rocket like New Shepard and an orbital-class first stage. Also, CRS-8 was LEO. GTO missions like JCSAT-14 staged at 2.32 km/s. You can also look at it in terms of payload. On escape, the Aerojet Rocketdyne CCE-SRM develops 70,000 lbs of thrust, accelerating the BO crew capsule at a peak of 7 gees. Thus, the crew capsule masses around 10,000 lbs or 4.5 tonnes. So the New Shepard propulsion module delivers 1.3 km/s to a 4.5 tonne payload. In contrast, the Falcon 9 first stage on GTO missions delivers nearly twice that velocity to a 120+ tonne payload. This isn't apples and oranges; it's grapes and watermelons.

From here: http://spaceflight101.com/spx/its-spaceship/ "2,500 metric tons of propellant are carried by the tanker for use during its own mission plus 380 t of propellant upmass that can be transferred to the Spaceship." That may be an error, of course.

In the specific case of CRS-8, which is the booster which will be launching SES-10 next Monday, staging took place at a velocity of 1,850 m/s at an altitude of 68 km. 1850 m/s is twice the kinetic energy per unit mass and experiences nearly three times as much peak heating.

Each tanker in a full-stack TSTO launch launches with 2880 tonnes of propellant. It burns about 2480 tonnes to get from staging (2.4 km/s) to orbit, transfers 380 tonnes of propellant to the target vessel, and then uses about 20 tonnes of residuals to deorbit and land. There are no tankers left depleted in orbit.

Well, that also changes the math. Nine SL Raptors can develop 2,798 tonnes-force, enough to lift 2,543 tonnes off the pad at a 1.1 T/W ratio. That's 2,453 tonnes of propellant, meaning the tanks would be about 85% full. That first 2 km/s (to get to 1 km/s) will cost 1,163 tonnes of fuel. But since there are no higher-efficiency, higher-thrust engines to ignite, this will kick the additional gravity drag up to 0.5 km/s, meaning we need 7.3 km/s more. The vacuum specific impulse of the SL Raptors is 361 m/s, which means orbit is reached with 85 tonnes of residuals. Allowing about 20 tonnes of fuel for deorbit and landing means 55 tonnes (of fuel) can be delivered in a single SSTO launch, as opposed to 380 tonnes in a single TSTO launch. I guess they'd rather do five TSTO launches than 40+ SSTO launches. Especially since any decrease in performance or increase in the amount of fuel required for the landing could instantly double the number of SSTO launches required.

I probably was unclear. The idea is to have a pure liquid rocket engine at the center with a small shroud packed full of solid fuel. The liquids combined with the solids get you off the pad, and when the solids burn out there's enough forward airspeed for air augmentation through the shroud to boost the thrust of the liquid engine. Allows you to use a smaller liquid engine than you'd otherwise need. TWR would be horrible, I think.

The ITS booster needs 3.3 km/s of dV for its boostback and return, which requires m0/m1 of 2.7; an orbital vehicle would need less due to not needing a boostback burn. I know the Falcon 9 goes transonic off drag alone, so the dV for the landing wouldn't be more than 500 m/s. The ITS tanker carries 2,880 tonnes of propellant and has a projected dry mass of 90 tonnes. But since only three of the engines can be used at SL, with a combined thrust of 9,150 kN, achieving a TWR of 1.1 limits launch mass to 848 tonnes, which requires that the tanker launch with its tanks 74% empty. An SSTO launch would burn nearly straight up in order to ignite its higher-ISP engines as soon as possible; I'm guessing it will be able to do so at around 1 km/s but will incur 1 km/s of gravity and aerodynamic drag in order to get there. At this point, fuel reserves have dropped to 350 tonnes. Adding another 0.2 km/s of gravity drag means we still need 7.0 km/s, but those 350 tonnes of fuel can only get us 5.8 km/s of dV even with the higher-ISP Vacuum Raptors. So I don't see how the ITS tanker can achieve SSTO at all. If Elon plans on swapping three of the Vacuum Raptors out for SL raptors in an SSTO demonstrator, then launch thrust would jump to 18,300 kN and the ITS tanker could launch with 1,605 tonnes of propellant. Using the same numbers, the tanker would reach orbit with approximately 37 tonnes of residuals, which gives it 1.1 km/s of SL dV for a landing.

Not sure what you mean. The ITS Tanker is already going to need to reenter from orbit controllably.

They don't even have to be two spaceplanes. Skylon is supposed to employ a kicker stage in its cargo bay for BLEO missions like GTO comsats. In this instance you have a two-stage vehicle (Skylon and the kicker stage) but orbit is reached in a single stage. Then again, Skylon could potentially carry a larger payload in a suborbital hop, but give its payload a high enough apogee to circularize. In this case it would be back to a TSTO (two stage to orbit), since orbit isn't actually reached by the first stage. As Jane said, recoverability is just an element of this challenge, not anything necessary to the definition of a PSTO. Not every engine has to be ignited at launch, and engines ignited on the pad don't necessarily have to be used for orbital insertion; technically, it's enough that some of the engines ignited on the pad reach orbit. The Space Shuttle and the R-7 were both PSTOs; the Atlas D was right on the cusp of being a PSTO depending on what you consider an orbit. A crossfed Falcon Heavy without its second stage could easily launch a Dragon into orbit as a PSTO, but without enough residuals to recover the core. During the Saturn V era, recovering the four F-1s on the skirt would have been more feasible with midair helicopter catch than by propulsive landing. I don't know when the first inflight-restartable kerolox engine was designed, but that tech was in its infancy if it existed at all, and it would not have been possible to downthrottle the F-1 low enough. Nor, for that matter, was there sufficient computer technology for autonomous propulsive landing a la Dragon 2 or Falcon 9. A simpler option was to simply chute them down and let them drop in the ocean. The F-1 was a pretty hardy beast and could have been refurbished and reflown more easily than the SSMEs were, even after a dunk in the Atlantic. A Saturn 1D-based PSTO launcher with 4/5 recoverable engines would have been able to outperform the Shuttle system in cost, frequency, and pretty much every way other than downmass. Good lord that is beautiful! How large are the prop tanks on the orbiter itself (and, for that matter, where are they located)? I can see from your gallery that staging happens around 1.7 km/s so there has got to be pretty substantial fuel reserves on board. Are the wings wet (i.e., fuel-carrying)? Using recoverable onboard solids to boost launch TWR is a nice trick! I'm guessing the losses from carrying the empty solid boosters up and down are minimal. One of the ideas I like is a blended air-augmentation-and-solids launcher, where a liquid booster is shrouded by an annular solid-fueled rocket ignited at launch; the solids burn away to leave behind an air augmentation shroud.

If you think about it, the Shuttle SRBs are in some ways closer to the F9 first stage than the Shuttle itself is. Recovered and relaunched first stages. The F9 S1 is just a lot easier to reuse.