sevenperforce

I bet you could use end-to-end struts with something that would slide on it, and attach a chair to it. A decoupler would give the initial push. It couldn't be as long as the real one but it should work. Maybe use separatrons to slow the landing.

The MVac would have catastrophic flow separation at sea level. Not to mention that a retrograde burn (even a subsonic one) would shred the thin, ultra-lightweight nozzle extension. It would need to land on auxiliary thrusters for sure. If Elon is serious about landing the second stage, my best guess is that he'll have his people design a mini-Dragon-2 (heat shield, SuperDracos, tanks, and legs) that attaches to the standard S2 payload adapter. Then it would simply be a matter of adding grid fins and a bit of TPS to the second stage body.

I was thinking about this, and for a proposed expendable SSTO, the question is really whether the simplicity of an SSTO actually saves you money. If a TSTO can deliver the same payload for a lower expense than an SSTO, then SSTO makes no sense. If not, it makes sense. So all I need to do is compare ARCA's SSTO to a hypothetical TSTO using the same performance specs and margins. First, I'll verify ARCA's specs. Since we are dealing with altitude compensation I will take the dV to orbit as 9,150 m/s and just use the vacuum ISP of the aerospike engine (314 seconds) for the entire trip. They claim a GLOW of 16,290 kg, a payload of 100 kg, and a dry mass of 550 kg. Plugging everything into the rocket equation, we get 833 kg to orbit, so their numbers (at least) aren't unreasonable. If we use their payload numbers and solve for dV, we get 9,913 m/s which will be our baseline for gravity drag, aerodynamic drag, and impulse losses. So what can we get for a balanced TSTO delivering the same payload? We don't know the claimed TWR or mass of their engine, which will make our estimate a little more guessy, but we can just adopt their overall GLOW/dry-mass (29.62) and hope for the best. Let's split that 9,913 m/s up equally between the two stages: 4957 on the second stage and 4956 on the first stage. Second stage wet to dry ratio needs to be 5:1, so if our payload is still 100 kg, then total second-stage mass at staging needs to be 601.7 kg (481.4 kg of propellant, 100 kg of payload, and 20.3 kg of dry mass). This 601.7 kg becomes the payload on the first stage. First stage wet to dry ratio also needs to be 5:1 (remember, balanced TSTO), so our GLOW needs to be 3,620 kg (2,896 kg of propellant and 122.4 kg of dry mass). This is ridiculous. Propellant costs for a TSTO using the same performance and margins as ARCA's would be a scant 22% of the SSTO, and dry mass would be just 26%. Even if there were substantial square-cube losses, that's not anywhere close. There's no way it would cost more to do a TSTO than it would to do an SSTO. Why do I get the feeling that ARCA simply set the performance numbers up and then back-calculated to get the mass ratio that they needed? Their claimed GLOW/Mdry is just a hair under 30. By comparison, the ITS Tanker proposed by SpaceX comes in at a GLOW/Mdry of 28.8, and that's with the highest-TWR turbopump-driven liquid-fuelled engines in the world, non-pressure-fed tanks, and massive square-cube advantages. Not gonna happen.

There is no material strong enough to "strut" the interior of a planet. A terrestrial planet like our own is a gigantic droplet of lava with big hunks of cooled-off rock floating on the surface. It would be like filling a swimming pool with pudding and then trying to "carve" out a cave inside. I mean, if you just specify unobtainum with a compressive strength millions of times greater than chemical bonds can provide, then sure, you can basically do anything. But you'd be dealing with pressures at the core sufficient to sustain nuclear fusion.

Or, in certain situations, the rocket could simply touch down near a body of water and pump it up, through a filter, and into the tanks. With payload margins like these, the rocket could easily carry extra water up to the mothership, more than it would need for the return journey, so each sortie adds a little bit of propellant to the mothership's reserves. Water has a ridiculously high heat capacity, after all, so you don't really need a lot to use as an evaporative-cooling solution. Alternatively, you could smear TPS all over one side of it and have it come in normal to prograde, though control would be a bit tricky. Might have to make it more of a lifting body.

Back to the VTVL pure-NTR crew shuttle. I forgot to mention: egress is by Pegasus 1 Mobility Enhancers...in other words, an externally-mounted ladder. This design is not very good if you intend to transport wounded soldiers back to orbit. For brevity, I won't reproduce all my calculations here. But it's a fairly straightforward set of equations. I'm going to assume that we want a launch TWR of 1.3 for nice speedy takeoffs. Note that with a requisite 9,150 m/s of dV and an Isp of 555 seconds, our m0/mf is a glorious 5.4:1. I come out with a GLOW of 161 tonnes. Engine mass is 4.67 tonnes, payload allowance is the previously-discussed 15 tonnes, and the ship needs 131 tonnes (which is also 131 cubic meters) of water. Total mass of the ship body, tankage, aeroshell, etc. is about 10 tonnes. Total dry mass is thirty tonnes.

I've moved around a lot in the last couple of years; my family situation has not been great and finding local friends/community has been a crapshoot. I've ended up feeling pretty isolated and it's been consistently tough. I moved to a new city most recently a couple of months ago and I have very little connection here. This forum has been really, really good for me, both as a distraction and as a source of community. I spend most of my forum time on Science & Spaceflight, but all the forums have really great people and I enjoy my time here. So...thanks to all of you.

I have three tats of my own that I got a few years ago and I don't regret them for a moment. But my only recommendation would be to place it just far enough up on your arm that it won't show if you wear a collared, cuffed shirt. The slide rule is an awesome idea, and (in theory) useful. Here's one of mine: It's as many different constants and values from physics as I could squeeze into a single graphic. The spiral is built on Euler's constant and matches the shape of the spiral arms of the Milky Way; viewing it from approximately a foot away matches what it would look like to view the Milky Way from one million lightyears away. The open circle at the center (where the Milky Way bar would be) is 8.87mm, the Schwarzschild radius of Earth. The three straight lines together form the Greek letter lambda, and the thickness of the lines is 1.063mm, the (current) wavelength of the CMB. The vertical line marks pi and tau, and is tilted off the axis of the spiral by 1 part in 137, representing the fine structure constant. Finally, the three dots connected with curved lines represent the relative masses of six of the most important particles in physics. The smallest dot is the mass of the electron, the top dot is the mass of the up quark, the bottom dot is the mass of the down quark. The line connecting the two quarks is an arc which represents the mass of the proton; the line connecting the electron to the down quark represents the mass of the W boson, and the line connecting the electron to the up quark corresponds to the Higgs boson. Finally, the position of the electron against the spiral matches the location of Sol in the Milky Way. The various lines combine to form a star of David.

Hah! The first bar where I bartended had a "pain train" for new employees -- the first time you came in to drink off the clock, the bartender bought you a shot of chartreuse, a shot of fireball, and a shot of Jaeger, and you were supposed to pound them in that order. Chartreuse is disgusting, but I know that swallowing something means you taste it less than if you spit it back out, so I had no trouble.

Anyway, I promised an analysis providing minimum mass for the following two near-future SSTO configurations, the pure-NTR VTVL crew shuttle, and the nuclear thermal turboramrocket tilt-rotor VTVL blended lifting-body crew shuttle. Going to start with the pure NTR. Pure-NTR VTVL crew shuttle For this design, I'm going to use a plain SSTO with no frills, landing and taking off vertically as God and Heinlein intended. For simplicity, I'll use the old-fashioned "nuclear bullet" design seen in fiction like Tom Swift and Tintin. As a further simplifying assumption, I'll take the requisite dV to orbit as 9,150 m/s, following Mitchell Clapp's method of adding Isp losses to dV when assuming altitude compensation. First of all: payload. Bare bones, assuming that a computer handles all aspects of launch and landing, means you need at least two crew members (to program autopilot and fix things that the computer can't automatically do), plus ten passengers. Twelve is roughly double the capacity of a Dragon 2, so for the "payload" I will assume a dry mass of 12.8 tonnes plus 1200 kg for the people themselves. I'll add a tonne of onboard liquid propellant for LES, OMS, and RCS, and we can assume that the crew capsule has its own heat shield and parachutes to act as a lifeboat. The mothership would be responsible for providing the onboard liquid prop. The ship itself would nominally use pressurized steam from the reactor for RCS and OMS; capsule propellant would only be used in an emergency. Total net payload: 15 tonnes. Because the mission profile is orbit-to-surface-to-orbit, rather than surface-to-orbit-to-surface, we can assume the mothership is able to provide ample propellant for the downward journey. That makes re-entry easy enough; just point retrograde, burn through at the lowest possible throttle setting, and let the propellant stream carry the entry heating away. If that's not enough, the base of the rocket can be designed with channels to pump water through in order to act as an additional heat sink. The engine will be a pure NTR pushing water through a liquid-fissile tantalum-halfnium-carbide pebble-bed reactor, using coolant steam to run the turbopump. The guys over at Children of a Dead Earth are insistent that a pebble-bed reactor pushing water can reach 555 seconds of with a bare TWR of over 200, but given that this excludes the mass of heat rejection and shielding I'll ignore their figure and go with the far more reasonable values given by Project Rho: TWR of 20 with LH2, or roughly 45 with water. Don't want to make this too easy on ourselves. Finally, I'll take the mass ratio of the ITS Spaceship as a rough estimate of tankage, reaction thrusters, coolant systems, and aeroshell...which is conservative since the ITS Spaceship contains living space and engines, which I'm treating separately. It's also conservative because water is a lot easier to store than liquid oxygen and liquid methane. The mass ratio is 13:1, so that's what I'm going to use. Tomorrow I'll crunch those numbers and we'll see where they come out.

Indeed. This is one of those things where even if we cannot come up with a situation where you'd need this particular sort of evac, it's still better to have it and not use it than need it and not have it.

Ah, yes, good catch. For those still following along, a vapor core rocket is essentially the same as a solid core, except that your nuclear fuel is a vapor encased inside something that won't melt. This allows it to run hotter than a solid-core rocket, so you don't have to worry about the pile melting down (since it's already vaporized); you just have to keep it contained. A tantalum halfnium carbide casing can go up pretty high, high enough to push a water-based NTR to a specific impulse of over 500 seconds. It also has a better TWR than a solid-core NTR, nominally, but you do have to factor in the whole vaporization and containment side of things. The difference between a vapor core rocket and a nuclear lightbulb is that the former uses heat transfer through the casing to get the propellant going, whereas the latter uses hard x-ray blackbody emissions from the gas core to heat up the propellant.

For a while, unscrupulous persons were setting up these little paid service text schemes (like, $0.99 for your daily horoscope, sports tips, whatever) and going around to cellular telephone outlets and signing up all the sample phones for the service. Eventually the carrier stores caught on and shut that down.

Oh, I wasn't questioning the validity of the plan; it's a good plan. Just thinking about the very small number of instances where it would actually save someone.

Yeah. Recovery and reuse is easier if your vehicle is physically smaller, but SSTO can only get positive payload fractions when it is very, very large.

Yeah, definitely a scam. I was out of work for ages and spent a lot of time job hunting and I got TONS of these.

Bachelor's degree in physics, 2012. Couldn't find a job doing physics with anything short of a PhD, so I work in law.

Yeah, this is the sticking point for me. I'd be interested to see Raptor's ridiculously good specific impulse and TWR being used for smaller payloads, like a fully-reusable mini-ITS that could fly either with an integrated crew capsule or with a cargo bay. A Raptor-based upper stage can use the autogenous pressurization gas with the planned methalox thrusters for a feather-light landing.

If I was designing a next-gen Raptor-based RLV, I'd do a composite body 3.66m in diameter with just four SL Raptors in the base. I think the Raptor has low enough throttle. Upper stage would be oversized, large enough to make orbit on its own if not for the Vac nozzle, and land biconically using pressure-fed methalox thrusters pressurized off the Raptor. Upper stage could fly with an integrated Dragon 3 or with a cargo bay.

Yeah, you're definitely not going to need disposable tanks. Frankly, what you need for a nuclear SSTO is impulse density more than anything else, and plain old water is almost impossible to beat for NTR impulse density. You could go to the trouble of buying a few tanker loads of RP1, but why, when you can just land near a body of water and pump the stuff directly into the tanks? All right, then. As soon as I get a chance, I'll throw together two sets of specs: one for a pure-NTR VTVL crew shuttle (think a nuclear ITS), and one for a blended lifting-body NTTRR VTVL crew shuttle. The latter will have crossrange capability and loitering/surface-ferry capability; the former will not. Not sure which one will come in at a lower dry mass.

The dropship needs to be able to land virtually anywhere, so wings or a blended body are nice for crossrange capability but it still needs propellant for vertical landing. I don't think so; it's near-future tech. Airbreathing is one thing, but there's no heat rejection system anywhere on the horizon that allows a vehicle to compress (let alone liquify) atmospheric air from an intake without massive expenditure of coolant. But a bimodal nuclear-thermal turboramrocket can, in the right configuration, hover almost indefinitely as long as it can circulate its coolant over a large enough area to keep it from overheating. And even if it is simply venting its coolant/propellant, it will still last a lot longer than if it was using that coolant as reaction mass.

Eh, the rocket equation is just math, and the math checks out. If ARCA is ignoring anything, it's the demonstrated limitations of structural mass fractions. I'm skeptical of their ability to fit a linear aerospike engine, a pressurant system, fairings, and pressure-fed tanks for a 16.3-tonne-GLOW rocket into 550 kg of dry mass. Unrelated note: I wonder if it would be possible to use liquid hydrogen or liquid methane as a pressurant. Obviously you'd still need liquid helium for your oxidizer, but if you could get the same performance out of LH2 or CH4, and your engine could handle changes in fuel flow density, you could end up with a de facto tripropellant rocket that converted to a higher-isp fuel once you were nearing orbit.

Ah, yes, you're absolutely right. My mistake. I was thinking of a wavelength shift and I had photons on my brain, so I was still associating wavelength change with frequency change.

As others have stated, the point is getting your desired payload into your desired orbit as cheaply as possible. This changes things. For example, that's why cost per kilogram isn't always the most accurate determiner. If Company A charges $10M to put up to 500 kg into LEO and Company B charges $6M to put up to 150 kg into LEO, then Company B is twice as expensive per kilogram...but if my bird is only 120 kg, I'm sure as heck not spending an extra four million bucks just for the fun of it. Same with expendable vs reusable launchers. An RLV is only cheaper if the cost of recovery, refurbishment, and recertification, plus manufacturing costs amortized over the expected lifetime, is less than the cost of a completely new ELV. Recovery, refurbishment, and recertification costs all get passed on to the launch purchaser. An RLV must reserve payload margin for recovery (TPS and legs/gear, and either parachutes or additional landing propellant). So, if you assume the same engine performance and tankage ratio, a single-stage RLV will be larger than a single-stage ELV for the same net payload. This drives up vehicle cost and fixed operating costs. For example, consider an ELV that can put 100 kg into orbit for $1M. Let's say half that ($500K) is fixed operating costs, profit margin, and fuel, while the other half is the cost of the expendable vehicle. Then consider an RLV with the same performance and structural ratios. If it needs to reserve 30% of its payload capacity for reuse, then it needs to be 50% larger. This means the manufacturing costs and operating costs also go up by 50%. So now it costs $750K to operate and $750K to construct. Let's estimate refurbishment, recovery, and recertification at 20% of the cost of a new LV, or $150K. Now the fixed operating costs for each launch are up to $900K. In order to beat the ELV, the $750K sticker price to construct the LV will have to be amortized over at least eight launches with no major part replacements in order to break even. However, an RLV program is more expensive to develop than an ELV program, so the launch company needs additional profit margin to recoup its investment. That's how an ELV SSTO can beat out an RLV SSTO. Now, ARCA seems to think that the complexity of staging drives up the costs of an ELV, and they're partly right. There are fixed manufacturing costs associated with building a stage that are more or less independent of the stage's size, up to a point, which is why it is cheaper to build a single stage with larger engines and larger tanks than it is to build two distinct stages. You only need one engine, one avionics package, and so forth. Doesn't give me any confidence in ARCA's ability to pull it off, but the configuration seems sound...if they can meet their structural mass ratios, that is.

Fuel for entry and descent, and landing really is going to depend a lot on the ship configuration. I know that the OP didn't really want to lose drop tanks every time the bird flew a sortie, since the mothership is kind of strapped for resources. That, it seems, was the primary driver for needing ISRU; the mothership can't spare propellant. Entry fuel is probably minimal or nil; near-future TPS should be plenty efficient. Descent got me thinking...what is the inclination of the mothership? Because it's not enough to get into orbit; you actually have to get back to the mothership. A polar orbit or a highly-inclined orbit allows the dropship to reach virtually any point on the surface once every calendar day, but this also means that returning to orbit can only be done in a virtually-instantaneous launch window, and the ship doesn't have the benefit of the planet rotation kick. A near-equatorial orbit provides much less time-sensitive access, but higher or lower latitudes are inaccessible unless you have crazy crossrange capabilities or extended in-atmo cruise. A blended lifting-body SSTO would have excellent lifting-reentry crossrange capabilities, permitting it to reach distant latitudes without much trouble. Getting back to an equatorial orbit is tough, but a nuclear-thermal turboramrocket can have a pretty efficient cruise...maybe not enough to fly directly into orbital rendezvous from the 45th parallel, but definitely enough to get down to a lower latitude and refuel. You just don't want to be in orbit already and realize you need to make a plane change. Unrelated note from the CDE forum: heavy water has a noticeably better specific impulse than light water, probably because deuterium hugs its electrons just a little more tightly, weakening the oxygen atom's stranglehold and giving heavy water a lower disassociation temperature. This could be a plot device; the ship has enough performance to make orbit on ordinary light water, but if it has time to use an onboard centrifuge to spin itself some heavy water, its performance goes way up and it can return heavy payloads to orbit. Fine... lets say... 10 people and some equipment for them, 100 kg each to make it easy, so 1 ton for 10 people. Also of course some basic life support and seating for them. That's the payload to get to and from orbit. Okay, great. That gives me a starting point.