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Everything posted by sevenperforce
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Or if you could find a way to use a supergun of some kind to produce a persistent, coherent plasma beam which the payload could ride into space. Like a rainbow bridge.
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Playing around with my KSP version -- it does look like they have done a decent job of solving the quintessential "point one way on the way up and the other way on the way down" problem.
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I made it in KSP. Looks pretty good. Works all right. One of the issues is that you can't do the separation until you're out of the atmosphere, which leads to some inefficiencies. Beck said that the upper stage being in tension makes it the lightest stage in history. Obviously it's not the lowest-mass stage ever; he probably means it has the best mass ratio for a methalox stage. I believe Centaur has a dry mass fraction of around 9% and F9US has a dry mass fraction of around 4% so if they can get to ~5.5% or so, they would be doing a really good job. That prompts another question, and some more maths. They say they can deliver 1.5 tonnes to Mars or Venus, each which require about 3.55 km/s for the transplanetary injection. I'm going to go out on a limb and say that's probably flying in expendable mode (although if it's not I'm sure the maths will tell us soon enough). We don't know the dry mass of the stage and we don't know the staging velocity. However, the rocket equation does tell us that you need a propellant fraction of around 64% to get 3.55 km/s with 355 s of isp, so for an interplanetary injection, the residual props in LEO are 64% of the total mass in LEO. Using our 5.5% number, we can estimate that the total mass at staging was 18.2 * dry mass + 1.5 tonnes. Similarly, with an expendable launch to LEO, the total mass at staging was 18.2 * dry mass + 15 tonnes. We can assume that staging velocity is the same because a difference of 13.5 tonnes is going to be negligible in comparison to the total mass of the upper stage and the dry mass and residuals of the lower stage. If we take the required Δv from staging to LEO as ΔvLEO, then the rocket equation looks like this in each situation (where m is dry stage mass): LEO expendable: ΔvLEO = 3483 m/s * ln( ( 18.2*m + 15000 kg ) / ( m + 15000 kg ) ) TPI expendable: ΔvLEO + 3550 m/s = 3483 m/s * ln( ( 18.2*m + 1500 kg ) / ( m + 1500 kg ) ) Conveniently, you can combine these two equations to eliminate ΔvLEO and solve for m. 3483 m/s * ln( ( 18.2*m + 15000 kg ) / ( m + 15000 kg ) ) = 3483 m/s * ln( ( 18.2*m + 1500 kg ) / ( m + 1500 kg ) ) - 3550 m/s ln( ( 18.2*m + 15000 kg ) / ( m + 15000 kg ) ) = ln( ( 18.2*m + 1500 kg ) / ( m + 1500 kg ) ) - 1.0192 And using Wolfram to solve (because I hate logarithms) we find that the dry mass of the stage is 4.6 tonnes. Algebra gives us the following projections: Stage 2 dry mass: 4.6 tonnes Stage 2 propellant: 79.1 tonnes Stage 2 methane: 17.6 tonnes Stage 2 LOX: 61.5 tonnes Stage 2 tank volume: 41.5 cubic meters CH4 + 53.9 cubic meters LOX = 95.4 cubic meters total Does this pass the sniff test? Yes. A spherical tank with an internal volume of 95.4 cubic meters would have a diameter of 5.66 meters, which fits well within the visual range of what we see from the small upper stage tank inside the 7-meter fairing. It also allows for a nice sporty upper-stage TWR of about 1.2 gees. These numbers suggest that for an expendable launch, ΔvLEO = 5.63 km/s. For a reusable launch, where the payload is only 8 tonnes, ΔvLEO = 6.91 km/s. This gives us yet another piece of information: staging velocity in an expendable launch is 1.28 km/s greater than in a reusable launch. These numbers also allow us to estimate expendable payload to GTO, which is 4.5 tonnes. It cannot make GTO flying reusably (which is probably why he kept talking so much about constellations). So what else do we know? If total upper stage wet mass is 83.7 tonnes and the total vehicle mass is 480 tonnes, then the lower stage obviously has a wet mass of around 396 tonnes. Using the rocket equation, the reserve propellant for boostback and landing is around 31.6% of the total mass of the stack at separation (since burning it would deliver an extra 1.28 km/s). Burn time for the upper stage is around 248 seconds, plus throttled flight time, but since throttled flight time doesn't really factor into gravity drag, we'll ignore that. The T/W ratio is good and so we can estimate that the area under the curve for gravity drag (accounting for centrifugal acceleration) is about 1/3 of the total time * 1 gee, or about 810 m/s. So if the upper stage needs 7.8 km/s to reach orbit and is carrying 6.91 km/s and loses 0.81 km/s to gravity drag, it must stage at 1.7 km/s.
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Hanging the upper stage on the quadrisymmetric petal fairing reminds me of hanging the Soyuz core on the four Korolev cross boosters. Time to do some math. What do we know? 8 tonnes to LEO reusable 15 tonnes to LEO expended 1.5 tonnes to Mars or Venus (assumed reusable) 480 tonnes GLOW 5.96 MN liftoff thrust 7.53 MN peak thrust 1.11 MN second-stage thrust 320 seconds SL Isp We can draw some inferences from that. First of all, the 5.96 MN liftoff thrust cannot be the full SL thrust of all 7 engines. SL engines grow in thrust as they ascend because underexpanded engines increase in specific impulse as the external pressure drops, but a ratio of 1.26:1 is vastly too high; that would put the vacuum specific impulse of the SL-optimized engines at 404 seconds which is obviously way higher than you can get from methane. I'm not sure why they would launch without full thrust, though. If we divide the 7.53 MN peak thrust by 7, that gives 1,076 kN per engine on the first stage, slightly less than the stated thrust of the upper-stage vacuum engine, so that checks out. Beck said that Archimedes is a 1-MN engine, and while that may be a round number, let's do the math anyway. Growth from 1,000 kN to 1,076 kN for the SL engine and 1,110 kN for the vacuum engine means the SL-engine gets 344 seconds in vacuum and the vacuum engine gets 355 seconds. That math checks out. The Aeon 1 and Aeon 1V are also GG-methalox and are expected to get 310 s and 360 s, respectively. The closed expander methalox M10 (vac) is projected to get 362 s, and Raptor's FFSC gives it 330-350 for SL and 380 for vacuum. The SL Raptor only gets a 6% increase from SL to space, lower than the 7.6% increase I'm estimating for Archimedes, but that's to be expected because the SL Raptor has such high chamber pressure that it has less to gain.
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Yes, yes it is. Although it does sound a little like the idea of a seeded Bussard ramjet.
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Hahaha, yes, that cracked me up. I do rather like the design. Keeping the fairing on the first stage means more dV allocated to the first stage, though, because you need to get further out of the atmosphere. I wonder what the speed at boostback is like. I imagine that knowing reusable vs expendable payload gives us some idea. Gas generator methalox is a solid choice. Lots of digs at both SpaceX and Blue, I thought. I can’t tell if the upper stage is a Rutherford or an Archimedes. Going with a Rutherford would mean better prop density.
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totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
Are you familiar with the concept of... ... ... ...float? -
........and did we just reinvent the space fountain?
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There are a couple of ways to use ground infrastructure to amp up the Black Horse concept. For example, you could design a long, angled electromagnetic or steam catapult on the side of a mountain, thus eliminating the need for (a) GTOW-bearing landing gear, (b) takeoff propellant, and (c) a high subsonic L/D ratio. So that brings your dry mass down. You can also add LOX drop tanks, both to reduce the mass to orbit and to further take advantage of your catapult. The airplane would take off with mostly-empty fuel tanks (reducing the amount of lift needed) and climb on turbine-based combined-cycle (TBCC) engines (operating in turbojet mode) to altitude, where they would meet a KC-135Q and fill up the fuel tanks. Firing the engines in sc/ramjet mode takes it up to Mach 5 and then burning the remaining fuel with LOX in an vacuum-optimized nozzle takes you nearly to orbit. Drop the LOX tanks and circularize with hypergolics.
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Coil guns have diminishing returns and put some rather significant constraints on the magnetic properties of the projectile. Traditional artillery has such high gee-loading that it eliminates anything other than a solid-fueled rocket. Railguns are more versatile than coil guns but have the annoying requirement of extreme forces on the rails and requiring metal-metal contact the entire time. What you're describing sounds not unlike Black Colt. A fairly small airplane with modest jet engines takes off carrying payload and COTS kick stage in its payload bay. It carries LOX in its tanks and just enough JP-5 to get off the ground and up to about Mach 0.8, which is where it performs aerial refueling from a KC-135Q and its tanks are filled with JP-5, JP-7, or RP-1. It then lights a rocket motor, burns to Mach 8 or so, releases its payload and kick stage, and then coasts back to base. So it's really more like a 3-stage vehicle but everything is reusable apart from the kick stage.
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The Tintin Challenge
sevenperforce replied to sevenperforce's topic in KSP1 Challenges & Mission ideas
Only visual mods allowed. Any physics- or part-altering mods make the challenge unequal since this is so razor-thin on margins. -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
Notably, BEFORE the two-launches-per-month cadence. That is, in fact, the entire point. Making your math rather irrelevant. -
I struggled to come up with a worse way. I’ve always liked the idea of Black Horse. If the SR-71 needs to refuel immediately after takeoff then why not do the same with a much higher-energy aircraft? I wonder if that idea could be modernized with a hybrid engine. The challenge was always the use of HTP, which is just a very dense oxidizer but not a very high-energy one. But a lot of people assume that getting high is enough to do away with the first stage. It isn’t. The first stage lofts the upper stage quite high, yes, but it also provides it with a significant amount of dV, enough that the upper stage doesn’t need high thrust any more.
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The Tintin Challenge
sevenperforce replied to sevenperforce's topic in KSP1 Challenges & Mission ideas
If you space a saboteur I'll add a flourish on the mission badge. -
I'm sure most of us are at least partially familiar with the Tintin comics and one of Hergé's best storylines, Destination Moon and Explorers on the Moon. In the comic, Tintin et al. ride to the moon and back on a nuclear-powered single-stage rocket. The comic was written in 1950, well before we had a good idea of what orbital launch vehicles would actually look like, but Hergé did his best to make the launch vehicle as accurate as possible to the imaginations of the time. The rocket was a tailsitter, launching and landing vertically on three fixed landing legs (which apparently also contained auxiliary fuel tanks) and used two inline main engines: a chemical rocket engine with a whopping amount of thrust (enough to make the crew black out during ascent) as well as a nuclear rocket engine for the transfer to the Moon. The chemical rocket engine may have been annular, since it appeared to fire through the same aperture as the nuclear engine. It had three fixed RCS thrusters near the base as well: (There's some discrepancy between the French and English versions of the comic; in the French version the rocket's nuclear engine is a brachistochrone which fires continuously at a noticeable acceleration, completing the trip in a matter of hours, while in the English version the trip takes about as long as you would expect from a standard Hohmann transfer, but for our purposes we'll go with the English version because it's the only one that's realistic for KSP.) The chemical rocket had enough throttle range for a gentle moon landing, and its crew hatch was low, with a ladder that reached all the way to the surface: The rocket had prodigious payload capacity, even able to bring along a "Moon tank" for exploring: In the Earth-Moon system, the dV for such a trip is prohibitively high -- in excess of 18 km/s, not even counting the huge braking burns you'd need to re-enter and land from cislunar space without a heat shield. You would need an open-cycle gas-core nuclear thermal rocket with a LOX afterburner to even hope for that kind of thrust and efficiency. But in the Kerbin-Mun system, it's within the realm of possibility. On Kerbin, a pure-chemical SSTO powered by a Mammoth, Mastodons, or Vectors can reach orbit (about 3.4 km/s with gravity and aerodynamic drag losses) with a payload fraction (payload/mf) of ~48% or so, and getting from LKO to the surface of the Mun and back to Kerbin entry interface is under 2.7 km/s. With the LV-N's 800 seconds of specific impulse, you only need about 30% of your mass-in-orbit to get over 2.7 km/s, so if you can fit the dry mass of LV-Ns, LF tanks, a crew capsule, auxiliary structure, and landing propellant into the remaining 18%, you're golden. Of course, the challenge is pulling it off. Can you? The Tintin Challenge Your mission, should you choose to accept it, is simple. Take Jeb (and whoever you want to accompany him) on a round-trip from the surface of Kerbin to the surface of the Mun and back. You must use a single-stage rocket which takes off and lands vertically on a single axis. You must use a stock vehicle (no, don't use the Tintin mod) and you cannot use propellers, airbreathing engines, or ion engines. NO EXCEPTIONS -- chemical rockets and nuclear rockets only. If you need some sort of ground infrastructure to help support the vehicle at launch, that's fine, but the rocket needs to be able to land by itself. Part clipping is fine -- in particular, you'll probably want to clip your engines together to help ensure a single thrust axis -- but don't abuse it too much. The Tintin rocket may have had comically undersized propellant tanks, but if you do this in KSP then it's just going to become a question of whose computer can handle the highest part count, which is a much less interesting challenge. You can pick any launch site you want and you can use drag-avoidance tricks. Scoring As usual, there are scoring rules: The Basics. Getting Jeb from Kerbin to Mun to Kerbin in one piece: 200 points. Keeping Company. Sending additional Kerbals along for the ride: 10 points per Kerbal, up to a max of 10 Kerbals total. Form and Function. The rocket lands on three fixed landing legs: 30 points. It's the Climb. Kerbals climb down to the surface of the Mun and back up on a ladder from the main body of the rocket: 15 points. Rock Solid. The main engines are fixed with no gimbal and all pointing is provided by RCS: 35 points. Reaction Control. All RCS is clustered at the base of the rocket, near the engines: 10 points. Blackout. The rocket exceeds 5 gees during launch: 40 points. Nukular. Use only nuclear engines for the Mun landing: 30 points. Combat. The main body of the rocket has the traditional ogive bullet shape of the V2 and the original Tintin rocket. Wheelie. Bring a deployable "Mun tank" with three seats to the Mun: 60 points. This challenge has razor-thin margins so I don't think anyone will be able to score all the points. But I'm excited to see what you can come up with!
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I'll also point out that an airbreathing SSTO framework makes more sense in comparison to launch architectures like Soyuz, Delta IV, Atlas V, and Ariane 5, but less sense in comparison to architectures like New Glenn, Proton, Electron, and Falcon 9. With the former launch architectures, you have (relatively) briefly-burning boosters which are used to accelerate a large, high-efficiency core stage which provides the bulk of the dV for orbit, lofting a small, low-thrust upper stage which provides only a small amount of dV for circularization and whatever else is needed for BLEO. Recovering the core stage would be almost as challenging as recovering it from orbit altogether because of its high energy. Since you have a very large, very high-energy core stage which fires all the way from the surface and gets within spitting distance of orbit, it sort of makes sense to imagine replacing the boosters with horizontal takeoff and jet engines and then just accepting the payload percentage loss. Then you have the challenge of recovering it, which is quite difficult but not significantly harder than if you had tried to recover the core stage from the original architecture. There are even some advantages, like being able to coast around the world before re-entry so you can land at your launch site instead of having to create a far-downrange recovery infrastructure and then try to figure out how to get back to your launch site. But in comparison to a "balanced" TSTO like New Glenn, Electron, and Falcon 9 (or a balanced 3STO like Proton), an airbreathing SSTO makes much much less sense. These vehicles allocate the dV nearly equally among their stages, so there's no single high-energy core stage which you can imagine converting into an SSTO. Plus, recovering the first stage is much easier because it has lower energy and so it can either be recovered a short distance downrange or boostback to the launch site.
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totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
What I've seen from news organizations is a bunch of nonsense about how Raptor is too difficult to build properly, when in reality the problem is nothing to do with Raptor -- it is well developed at this point -- but rather manufacturing cadence. It's interesting to get a feel for how much money SpaceX is pouring into Starship. Starlink is definitely not giving major profits yet. There appears to be a bottleneck for the satellite dishes given that I was supposed to receive mine in late 2021 and now it has been pushed out to mid-2022. Although I'm curious to know whether there are any issues with the performance of the current satellite constellation which are also slowing things down. -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
Au contraire, most deep space missions circularize first. You launch from your designated launch site at an instantaneous launch window and then coast to the appropriate inclination, which is where you do your departure burn. We don’t typically have this issue in KSP because the KSC is at Kerbin’s equator, so if you want to launch directly into a desired inclination at a desired instantaneous launch window then you can do so. But since the US does not have any launch sites at the equator, you need to perform a partial orbit first for phasing before you do your injection burn. -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
Oh, I wasn’t saying that DSCOVR was a higher-energy orbit than DART. DART is definitely the highest-energy mission that Falcon 9 has done. I was just giving the example of the two other Falcon 9 launches that went to “deep space” (although TESS doesn’t really count as deep space because it’s still sub-cislunar even if it’s well beyond GTO). DSCOVR is in a heliocentric orbit, sitting at the very edge of Earth’s SOI. Technically it is in an oscillating halo orbit around L1, which means it is never actually inside Earth’s SOI. However, it’s still sort of accurate for McDowell to say it “did not leave the Earth-Moon system” because its heliocentric orbit is constrained by Earth’s gravity. No new block upgrades are planned for F9 or Merlin to the best of my knowledge. F9 will fly until it's no longer economical for SpaceX to maintain production. There'll come a critical point where enough of F9's customers fly on Starship and they'll shut F9 production down. They would never put an RVac on Falcon 9. For one thing, it is very nearly too large. In addition, it would mean plumbing the same pad for both methane and RP-1, something SpaceX doesn’t want to do. Finally, because methane is less dense than RP-1, I believe that a Raptor-based swap-in for the Falcon 9 upper stage wouldn’t actually improve payload that much. -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
TESS went to a high eccentric orbit well beyond GTO (but slightly lower than TLI) and DSCOVR went to Earth-Sun L1 which is technically outside of Earth's SOI. -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
Well TESS was much higher than GTO but still within the Earth-Moon system so I’m not sure if it counts as deep space. -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
Chance of weather scrubs always remains higher than reasons for any other scrub, so there's that. But yeah, it's an instant window, albeit with another instant window at a slightly different time the following day. And it will be an ASDS landing for the booster. Dragon 2 (both the crew and the cargo variant) are heavier than Dragon 1 and so they always reserve more performance, necessitating a droneship landing. In fact the last pad landing for Falcon 9 was also the last Dragon 1 launch. -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
I wonder…with our current state of tech, what’s the worst-case potential impact event that would be just barely preventable? Obviously a Ceres-sized dwarf planet screaming toward Earth with mere weeks to impact is not something we can stop, no matter how much Bruce Willis has a weird bromance relationship with Ben Affleck. But also that’s not going to happen, because there is only one Ceres-sized dwarf planet anywhere close to us, and it’s Ceres. The more likely scenario is a city-killer long-period comet that we don’t see until it’s relatively close to the sun. What’s the longest pole? Deciding whether to use nukes or kinetic impactors? Trying to find a way to get sufficient dV to get an intercept? Getting a large enough impactor into orbit? Choosing between EOR for a large impactors and multiple small impactors? What’s the most challenging challenge that’s still preventable? -
totm nov 2023 SpaceX Discussion Thread
sevenperforce replied to Skylon's topic in Science & Spaceflight
Here’s my social media riff on DART: and if it doesn’t embed https://www.tiktok.com/@justlydeserved/video/7033926796202478895?_t=8NbUJXDhixT&_r=1 -
So believe it or not, hydrolox shines the best when it comes to putting really honking huge payloads into LEO. If you get a big beefy hydrogen upper stage like S-II or EUS, it weighs very little for the amount of energy it carries and so it is very well suited for providing the enormous surge of dV required to reach orbit in the first place. Once you’re in orbit, you’re halfway to anywhere, and the specific impulse of your propellant becomes much less important than mass fraction. Simply put, being able to carry a lot of fuel is a good thing.