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sevenperforce

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Posts posted by sevenperforce

  1. 10 minutes ago, magnemoe said:

    Wonder if it would be practical? I imagine you would have an outer niobium skin then a gap before the steel tank. using pins to connect the skin to tanks. 
    Think its more an question if it manages the heat. Its used on the cone of the falcon 9 vacuum engines but reentry is even more demanding. 

    Now I thought spaceX has thought about it they use it in engines after all, don't think it would be heavier than the tiles as it would not be structural. 
    And its so much simpler than the tiles. 

    A niobium heat shield for Starship would be vastly heavier than the tiles. The tiles are extremely lightweight. Also, heat management would still be a problem.

    The limiting factor for Starship is the temperature on the back of the heat shield, where it radiates to the steel. Niobium radiates and conducts heat very well, which makes it great for radiatively-cooled engine nozzles in a vacuum but not so great for a thin heat shield attached to steel. 

  2. 2 hours ago, kerbiloid said:

    All spaceplane projects of 1960s were requiring a niobium shield in front.

    And exactly zero of those projects (none of which were ever constructed, mind you) were intended to perform aerobrake circularization. The heat shields were for re-entry.

    This is, to date, the only vehicle to have ever performed multi-pass aerobrake circularization:

    NASA-MRO-Mars-640x353.jpg

    You will note the conspicuous lack of a niobium heat shield anywhere.

  3. 4 hours ago, kerbiloid said:
    21 hours ago, StrandedonEarth said:

    How much of a heat shield is necessary for a stainless steel ship to aerobrake down to LEO?

    In the glorious early days of the spacenautics allof them required a niobium shield at some distance from the hull itself.

    Niobium was very expensive. If they could, they would never want it.

    It would be understandable if the Starrship was made of a post-industrial nanosilicarbon, not of same steel which needed niobium a half-century earlier...

    What on god's green earth are you on about?

    No vehicles have ever performed multi-pass aerobrake circularization into LEO. There has never been any niobium-shielded stainless steel re-entry vehicle.

    The only vehicle to perform multi-bass aerobrake circularization was the Mars Reconnaissance Orbiter, which did it at Mars, and quite famously had no heat shield at all. 

    6 hours ago, Beccab said:

    It's alive!

    Well @tater it looks like you were right and I was wrong. I couldn't think of any reason that the OLT would need to have QDs to the individual engines, since the Raptor igniter is so small, but I forgot about the need for spin-up gas. 

  4. 8 hours ago, tater said:

    I was looking into this, and I'm unsure. I think if you are open to longer time periods (multiple passes) it can probably be done. The trick I think is that something like LSS can return with a decent amount of residual props, just not enough to propulsively circularize in LEO without aerobraking. So while at some point with almost no props left the problem might become orbital period vs heat dissipation (load up with heat at perigee during aerobraking, then need time to radiate that heat away before the next pass), if you can scrub off just enough the remaining props can do the job, then it's fine.

    I think the challenge is not re-radiation of absorbed heat, but peak heating during the passes. The highest-velocity passes produce the highest dV savings but also have the highest peak heating. 

  5. Placement of the depot is anticipated to be LEO, right? It’s a little tricky, I think. It’s a shame to burn propellant to move a  heat shield and flaps and such into lunar orbit when you don’t need them there. But a heat shield and flaps and such are the best way to get back into LEO. You want to spend your propellant in as efficient a way as possible.

    In the long-term, what if they put the fuel depot into an Earth-Moon cycler trajectory? Each tanker mission would need to boost to TLI in order to match the fuel depot’s trajectory, but then could return for EDL easily. The depot would perform the NRHO insertion, which it can do more efficiently than the tankers because it has less mass to contend with. And then once (nearly) empty, it could perform Earth Interface Injection, returning it to the cycler trajectory but never needing to brake back into LEO propulsively.

  6. 1 hour ago, tater said:
    1 hour ago, mikegarrison said:

    Any high school physics student should be able to tell you what the result of an inelastic collision is.

    DART is set to impact at something like 6.6 km/s. Recent asteroid missions have shown them to be perhaps loosely bound conglomerates, so the result of the impact in terms of dv delivered might be expected to be whatever it is (cm/s?), the impact can also send a bunch of material out into a cloud of ejecta... should be interesting.

    Indeed.

    An inelastic collision may be simple enough…as long as it remains a two-body problem. But a three-body problem has only discrete solutions. And a 999,999-body problem? Well that’s a new bird entirely. 

  7. EDIT: I ran the numbers. An 84-tonne lunar Starship requires 5.79 km/s for a landing demonstration mission if it skips NRHO. That’s 333 tonnes of props. Lunar Starship’s LEO residuals plus a single tanker mission will get it to 321 tonnes of props, so if it can shave off some added mass (perhaps by only carrying a mock-up crew capsule and no cargo) it’s doable. 

  8. Moving this discussion with @tater to the correct thread….

    Suppose we have an 84-tonne lunar Starship that leaves LEO, picks up crew from Orion in NRHO, goes to the lunar surface, returns crew to Orion, and then returns propulsively to GTO (that's the max burnout mass we can achieve without a tank stretch). If a Starship tanker (which we will notionally say has a dry mass of 85 tonnes and carries 30 tonnes of landing propellant) can deliver 150 tonnes of propellant to LEO, then it can deliver 33 tonnes of propellant to the lunar Starship (assuming a 2.27 km/s GTO burn at 380 s isp).

    This gives our lunar Starship, in turn, 1.2 km/s of dV, which it can use to lower its apogee significantly. The next tanker will only need to burn 1.07 km/s out of LEO to meet it, meaning it arrives with 86 tonnes of propellant, giving the lunar Starship enough propellant to reach LEO with 43 tonnes of remaining residuals.

    It will need just under eight tanker missions to refill its tanks for the next sortie.

    On its initial launch, it would have reached LEO with 171 tonnes of residuals, requiring less than 7 tanker missions to refill.

    So you need a total of 10 launches for the first mission (1 lunar Starship + 7 tanker missions for the outbound journey + 2 tanker missions for the return journey) and you need 10 launches for each subsequent mission (8 tanker missions for the outbound journey + 2 tanker missions for the return journey).

    On the other hand, if you are only going to make it a one-way trip (LEO to NRHO to the lunar surface to NRHO), that same 84-tonne lunar Starship will need only 7.8 km/s of dV, meaning it needs only 598 tonnes of propellant in LEO. Accounting for the 171 tonnes of residuals that lunar Starship will have when it reaches LEO on its initial launch, you only need 3 tanker missions and you'll have 127 m/s of extra margin...hopefully enough to get the now-derelict spacecraft clear of the Lunar Gateway.

    So that's 10 launches per mission for a reusable lunar Starship or 4 launches per mission for an expendable lunar Starship. This all assumes the minimum dry mass of a naked Starship is something like 45 tonnes (based on Elon saying that a naked Starship without SL Raptors is 40 tonnes) and that ~39 additional tonnes is enough mass budget for landing legs, paint, solar panels, landing engines, landing propellant, crew capsule, fairing, and cargo.

    A landing demonstration mission with no NRHO stopover and no lunar ascent (except perhaps a hop) could probably get away with only 1-2 tanker missions, though I haven’t run the numbers. 

  9. 47 minutes ago, tater said:

    I think I did the math with a propulsive return all the way to LEO. Yeah, lower with only NRHO by a bunch.

    I still see propulsive return to LEO as a pretty long pole. If it weren’t for stupid Orion hanging out in NRHO, it would be easier, but the NHRO layovers make the whole trip a whopping 12.5 km/s. At 380 seconds for the RVacs (not accounting for any of the SL vac burns at lower efficiency), the required propellant fraction is 96.6%. Starship’s dry mass would need to drop to 42 tonnes to do that, with no payload at all. A tank stretch would only provide incremental improvements.

    A propulsive return to GTO, where Starship tankers could easily meet it and give it the additional props to return to LEO, is a little easier. You’d need 10.2 km/s, which comes to a propellant fraction of 93.5%, giving you an allowable dry mass of 84 tonnes, which is closer to something useful.

    45 minutes ago, tater said:

    More on topic, it would be nice if BO would stop screwing around and build all the things. I want to see a 7m, reusable BO vehicle flying. I want all the things flying.

    Agreed.

  10. 36 minutes ago, Beccab said:

    The rest of the post is speculation, but this is incorrect. It is what BO claims, but that's not even the biggest lie on the infographic

     

    30 minutes ago, tater said:

    With 100t of residuals and the nominal 1200t of props for SS, then you get 13 launches for SS. 1 for the depot, 11 more to fill it, then the 13th is the LSS itself. You could use the LSS as the depot, and remove 1 launch. Stretching the LSS tank slightly makes sense from math we've done here, which might add a launch to fill.

    If SS can do more like 150t, then 9 launches to fill the stretched tank., leading to 10-11 in total.

    Am I missing something?

    Elon has claimed that it will only take 8 refueling launches max, citing 150 tonnes of residuals per launch and speculating that it may even take fewer launches if Lunar Starship is lightweight enough (lacking flaps and a heat shield) to make the trip without full tanks. Indeed, if they can get the dry mass of Lunar Starship down to 60 tonnes and if we assume 40 tonnes of crew cabin and payload, it can do the trip from LEO to NRHO to the lunar surface and back to NRHO starting in LEO with only 970 tonnes of residuals in its tanks. With that approach and 150 tonnes of residuals delivered per launch, it would only need 5-6 refueling launches, since it would reach LEO with 135 tonnes of residuals for its own launch.

    But nevertheless, I was citing the “12-15” number because that’s what Blue Origin was claiming and so that’s the argument they would have made to the court, whether true or no. 

  11. What would a New Armstrong configuration have looked like? The Blue Origin descent element for the National Team lander was planned to have used two BE-7 engines. But if they were proposing a single-element, integrated reusable lander, they might have gone in a different direction.

    The BE-3 can deep throttle to around 18%, and Bezos has talked about the closed dual expander BE-7 having similar capabilities. If we assume ad arguendo that the open-expander version of the BE-3U has similar deep-throttling ability, then it could conceivably throttle as low as 128 kN in a landing engine configuration. Padding it to around 135 kN to allow margin for hover, etc., then a single-element integrated lander using a single BE-3U as its landing engine would weigh in at around 83 tonnes at landing.

    For safety reasons, of course, you wouldn't want to be limited to only a single engine. But that's where Blue could borrow from the old Soviet LK lander design. The BE-7 produces 44.5 kN, so placing four of those around the BE-3U would give the lander a nice clean abort mode as well as alternative landing engines. The BE-3U engine bell is around 2.5 meters across and the BE-7 is less than a meter wide, so they could all fit easily in a quincunx within a 7-meter circle, with space for landing legs as well.

    A lander needs 2.6 km/s to get from NRHO to the lunar surface, and the same for the return trip. Assuming ~445 s of specific impulse for the open expander BE-3U, you'd need to leave NRHO weighing in at around 151 tonnes and you'd get back to NRHO after the mission with a burnout mass of around 45 tonnes.

    What do we know about the New Glenn upper stage? Well, estimates of its dry mass put it around 16-18 tonnes, and it is believed to carry around 175 tonnes of propellant.

    If "New Armstrong" was to be Lunar Starship knockoff based on the New Glenn upper stage, then it would be able to launch on the reusable New Glenn first stage, reaching LEO with around 17 tonnes of propellant residuals. After three New Glenn refueling launches with hydrolox propellant transfer, it would have 150 tonnes of propellant on board, enough to perform its own TLI and insertion at NRHO with 40 tonnes of residuals. 

    With that approach, New Armstrong would need to take on about 66 tonnes of additional propellant at NRHO in order to get down to the lunar surface and back. As luck would have it, a New Glenn second stage which is fully refueled in LEO can perform its own TLI and insert at NRHO with 67 tonnes of residuals. That same New Glenn second stage would have reached LEO with 45 tonnes of residuals, requiring just three New Glenn refueling launches to be ready.

    Obviously there would need to be some dry mass added for maneuvering, docking, prop transfer, and the like. But I'm probably sandbagging how much propellant New Glenn can put in LEO with each launch, so I think it still closes just fine.

    So Blue Origin could have proposed a single-element "New Armstrong" integrated lander which would require only 8 New Glenn launches (rather than the 12-15 Superheavy launches required for Lunar Starship), based on the New Glenn upper stage. It would have about 28 tonnes for a crew cabin, additional structure, power generation, propellant management, landing legs, and lunar surface payload. And the hatch would have been 11 meters closer to the lunar surface than Starship's.

    Blue would argue that the 100-tonne+ capability of Lunar Starship is overkill and that cutting the number of required launches almost in half reduces complexity and schedule risk. If they did make an argument, it would definitely explain how the Court described their protest:

    Quote

    "Oh. That's what the agency wanted and liked best? If we had known, we would have instead submitted a proposal that resembled the successful offer, but we could have offered a better price and snazzier features and options."

     

  12. 40 minutes ago, RCgothic said:

    The hypocrisy is running attack ads "This is ridiculously complex and unsafe!" whilst proposing simultaneously behind closed legal proceedings to do exactly the same thing.

    Agreed.

    The redacted portions seem interesting. There aren’t many redactions overall in the opinion. I wonder if they were hiding a “New Armstrong” architecture. 

  13. It’s hard to explain just how hilarious it is to see this damning of a statement in a memorandum opinion….

    “Blue Origin has not submitted any contemporaneous documentary evidence to support its allegations of its alternative architecture.”

  14. 5 hours ago, RCgothic said:

    Roasted.

    Some interesting excerpts:

    "The United States has moved to dismiss . . . under RCFC 12(b)(1) for lack of subject-matter jurisdiction. . . . The defendant argues that Blue Origin does not have standing to bring its challenge because Blue Origin did not have a substantial chance of award, even assuming its allegations were true, and Blue Origin's alternative proposal is speculative."

    "To establish standing under this court's bid-protest jurisdiction, a protestor must be an "interested party." The Federal Circuit has interpreted this term to require a protestor to have alleged facts, which if true, establish that it . . . possesses the requisite direct economic interest."

    "[T]he court must decide whether those alleged facts show the protestor was prejudiced by the alleged errors. . . . To establish prejudice for standing purposes . . . the protestor's complaint must show that there was a substantial chance it would have received the contract award but for the alleged error."

    "[E]ven assuming its allegations to be true, Blue Origin cannot establish that it would have had a substantial chance of award but for NASA's alleged errors."

    Sounds familiar. I seem to recall saying something about that:

    Spoiler
    On 11/9/2021 at 7:32 PM, sevenperforce said:

    Blue Origin is definitely objecting to "a proposed award or the award of a contract" and so the only way to challenge this rests on the first part of the sentence: "an action by an interested party." A challenge to subject matter jurisdiction is often a challenge based on standing, and so my guess is that the MTD on Rule 12(b)(1) is a challenge to BO's standing.

    Here's what Blue Origin had to say in their Complaint about why they think they have standing:

    "Blue Origin is an interested party with standing to pursue this protest action because it was an actual offeror for the HLS Option A Contract and was next in line for award. Thus, Blue Origin's direct economic interests are affected by the award of this contract to SpaceX. Blue Origin is prejudiced because but for NASA's erroneous and flawed evaluation actions and conduct, including waiver of a critical and mandatory solicitation requirement for SpaceX, there was a substantial chance Blue Origin would have received a contract award."

    And that's where they lost. The federal court must, by law, give deference to factual determinations of the administrative agency for the purposes of evaluating questions of standing. The GAO (likely) said that the Blue Origin was NOT second in line because they did NOT qualify for the award and would NOT have received it but for the award to SpaceX, due to the limited budget authorized by Congress. Thus, Blue Origin did not have standing and so the court did not have jurisdiction under §1491.

    On 11/10/2021 at 2:32 PM, sevenperforce said:

    The facts established by the GAO demonstrated that Blue Origin was not second in line for the Human Landing System award and would not have received the award regardless of whether SpaceX was involved, because the National Team submitted a non-qualifying bid. Thus, EVEN IF all BO’s criticisms of SpaceX and the award process were 100% true, it still wouldn’t matter because BO lost independently of SpaceX’s involvement and so BO doesn’t have any legal right to interfere.

    More from the opinion:

    "The Court finds that Blue Origin does not have standing because it did not have a substantial chance of award."

    "Considering the funding shortfall, even if the Court found that either NASA's evaluation of SpaceX was improper or SpaceX's proposal was unawardable, Blue Origin cannot show that it would be in a position for award. . . . NASA could not have awarded Blue Origin the contract at its proposed price or anything close to it."

    "Blue Origin alleges that SpaceX's proposal was not complaint with the terms of the solicitation and unawardable. In contrast, Blue Origin alleges that its own proposal was next in line for award because it was compliant and awardable. . . . This argument is fatally flawed because NASA determined that Blue Origin's submitted proposal also was not in compliance."

    The full memorandum opinion can be read here.

    Also, more details about Blue Origin's alleged, hypothetical, secret alternative plan:

    "Blue Origin asserts that it too would have proposed a single-element Integrated Lander, [redacted]. This alternative design would 'take full advantage of Blue Origin's [redacted].' The details of Blue Origin's alternative approach are unclear, as is the precise nature of what Blue Origin would have proposed; Blue Origin has not submitted any contemporaneous documentary evidence to support its allegations of its alternative architecture.

    "Despite Blue Origin's allegation that it has been [redacted], it would have had to start from scratch with NASA. Blue Origin's alternative proposal is purely speculative, including hypothetical pricing and hypothetical technical ratings.

    "Blue Origin is in the position of every disappointed bidder: Oh. That's what the agency wanted and liked best? If we had known, we would have instead submitted a proposal that resembled the successful offer, but we could have offered a better price and snazzier features and options. Blue Origin cannot use its speculative alternative proposal to establish that it would have had a substantial chance of award but for NASA's alleged evaluation errors."

    Now I want to know what the redacted part was. Were they suggesting a BE-3U lander architecture?

    Finally, a little more fun.

    "Even if it had standing and portions of its complaint were not waived, Blue Origin could not succeed on the merits. Bid protests are evaluated under the APA's standard of review. A court may grant relief only upon the finding that either 'the procurement official's decision lacked a rational basis' or 'the procurement procedure involved a violation of regulation or procedure.' 

    "On the record before the Court . . . Blue Origin has not met its burden to show that NASA's award decision was arbitrary and capricious. The Court finds NASA's explanation to be 'a coherent and reasonable explanation' of its award decision."

     

  15. On 11/17/2021 at 1:58 PM, CatastrophicFailure said:

    Hmm... someone mentioned nuclear... could you run a NTR on methane? Or even methane/LOX? Sure, it wouldn't be as efficient as hydrogen, but would surely be better ISP than Raptor could ever do, with the simplicity of using the same propellant(s).

    I was going to do the math on this, casually, but it looks like someone already has:

    http://toughsf.blogspot.com/2021/10/nuclear-conversion-for-starship.html

    They go through the possibility of (a) adding a second bulkhead and using liquid hydrogen with NTRs in place of the RVacs, (b) the same as before, but filling the cargo space with liquid hydrogen too, (c) moving the common bulkhead to increase the methane proportion and pushing methane through simple NTRs, and (d) filling the whole thing (except header tanks) with methane and pushing that through advanced NTRs.

    Hydrogen is so fluffy and the mass ratio plunges so low that the hydrogen nuclear Starship can't even reach orbit. Filling the whole cargo space with hydrogen allows it to just barely reach orbit, without payload.

    Simple methane NTRs can approximately meet the performance of the current Starship.

    Advanced methane NTRs can approximately double the payload to LEO, but payloads beyond LEO are only slightly improved. They conclude that it's not worth it, and I'm inclined to agree.

  16. 6 hours ago, Rakaydos said:

    Not ORSC.

    FFSC, but on the oxygen rich side of schtochimetric. 

    Higher thrust, lower ISP, cheaper and denser propellant mix.

    Raptor already has such a deep throttle range, so the turbopumps must have a pretty large allowable speed range as well. And they run independently. So I don't see why you couldn't vary the mixture ratio already, like the F-1.

  17. 1 hour ago, JoeSchmuckatelli said:

    How do you air-augment a Raptor?  

    Wrap a duct around it.

    Raptor is a fairly decent engine for air augmentation. It has a high O/F ratio so its propellant is reasonably dense, but it has a high specific impulse so it can operate in AA mode up to a theoretical maximum of 3.7 km/s. It has a terrific T/W ratio which helps counteract the extra weight of the duct system. It's just a little oversized for air augmentation purposes. A mini-Raptor with a thrust closer to that of the Merlin 1D would work better.

  18. 4 hours ago, wumpus said:

    Not sure how effective air-augmented methalox is, but unlikely to hurt.  Cutting fuel costs seems a long way out, but the was spacex moves, they might be there by the time the engine design is completed.

    The booster needs higher T/W more than it needs higher efficiency. Air-augmentation can increase static thrust a little but the thrust multiplication really only kicks in once you're high-subsonic or supersonic, and by that time Superheavy will be outside of half the atmosphere and it will have lost about a quarter of its liftoff weight.

    Also, you can't very well just add a duct system to Superheavy as it is. The length of an air augmentation duct needs to be on the order of its diameter in order to allow sufficient space for air-exhaust mixing and expansion.

    What you could do -- I suppose -- is create ram air inlets about halfway up the LOX tank with ducts that penetrate the LOX tank and open at outlets between the inner and outer ring of Raptors. Just like a boat-tail bullet, Superheavy has a region of low pressure at its base during flight, low pressure that will cause parasitic drag and suck plume recirculation up between the engines, robbing them of some of their exhaust efficiency. If you had a ram air duct pushing compressed air out there, it would mix with the engine plumes and increase efficiency, and also reduce parasitic drag. But I am guessing that the added weight and the structural issues in the LOX tank wouldn't be worth it.

     

    3 hours ago, cubinator said:

    It was calculated some time back that the regular propellants in a Starship would give more delta-V with NTR than hydrogen, due to density. 

    Yeah, the mass ratio is just so so different.

    I wonder what would happen if you moved the common bulkhead down and kept the SL Raptors in the  middle but replaced the RVacs with methane-based NTRs. You would lost some mass ratio but would the methane nukes make up for it?

    3 hours ago, JoeSchmuckatelli said:

    Unless, of course, they redesign the booster with several side mounted / circumferal air augmented boosters replacing the outer raptors for lift and keep three gimbaling raptors for landing? 

    A small spaceplane with air-augmented Raptors (preferably variable-mixture-ratio) would work for an SSTO. All the better if you carried extra methane and transpiration cooling, because if you did it properly the transpiration cooling system could double as a fuel injector for a shock combustion ramjet....

  19. 19 hours ago, tater said:

    FEXW-yGWUAE5OEu?format=jpg

    A definite taste of N1 with the engine-bell-hugging shield, although presumably without the doesn't-actually-work bits.

    I am sure @kerbiloid appreciates. I suppose some design choices are made because they are just good.

    17 hours ago, mikegarrison said:
    On 11/14/2021 at 1:22 AM, tater said:

    The GSE on the launch table seems to be rigged for starting the outer engines. That just the outside ring of fixed engines. Not never restarted, not restarted in flight. They don't gimbal, and once the booster hits MECO, the next time they are used is the next liftoff. The best part is no part, leave the igniters on stage 0.

    I have no idea whether this applies to a Raptor engine, but I will mention that sometimes airplane engines operate close enough to the flameout margin that they turn the ignitors on in flight. It's much easier to restart a flame that partially goes out than to restart an engine after the flame is completely out.

    Raptor uses a blown-plasma augmented spark igniter. Well, two of them. They are quite small and screw directly into the side of the combustion chamber. There is no TEA-TEB being squirted up into the combustion chamber through the engine bell, as with the ground-started Merlin 1D.

    Plus, Elon says there will be additional close-out covers added to the  outside of the engine skirt, so it wouldn't make sense to have ignition from the GSE.

    17 hours ago, tater said:

    For Raptor from what he has said (Tim Dodd interview/tour), it sounds like the Rboosts are simpler because they don't throttle (much?), don't gimbal at all. SL are what we have seen, throttle to ~40%, gimbal. And the vac engines that they have right now that also work at SL (no gimbal, unsure on throttle).

    I think lacking the gimbal mount probably saves weight, but doesn't make any difference to the rest of the engine. The RBoosts have a different, simpler, higher-thrust set of guts because they don't throttle.

    The RVacs can definitely throttle. They have to be able to throttle in order to provide pitch and yaw by differential thrust. 

    4 hours ago, Rakaydos said:

    The majority view on NSF is that The Engine to Surpass Raptor is probably a second-generation FFSC methalox engine, using all the lessons and materials developed for Raptor 2 but unrestricted by Raptor's form factor, which was locked in years ago. Particularly optimizing "thrust per dollar"  with the goal to make it so the cost to travel to mars is within the reach of at least a million people who want to go.

    Also some speculation of Oxygen-rich methalox mixtures.

    They have a very good oxygen-rich preburner so they should have no trouble scaling it up to make it an ORSC engine, and ORSC gives you a slightly denser fuel mix to boot. Perhaps using only a single preburner and turbopump would make the engine lighter.

    But then you lose three main advantages of FFSC: lack of ox/fuel hot seals, variable mixtures and efficiently-sized preburners, and gas-gas combustion efficiency.

  20. 1 hour ago, Lisias said:

    There's another solution: the arm being perfectly balanced without the payload, and the balancing weight being ejected at the same time the payload.

    This will solve the timings and the shock (it will be zero).

    Of course,  now you need something to colect and absorb the impact of the huge Kinect power of the weight (that would go to space itself if not collected), but this is way easier than the alternative.

    But it will demand more time between launches, as one will need to rebuilt the weight collector before the next launch - it will probably be destroyed and, so, you will need to remove the debris, clean all the system from any pulverized debris from the Impact, and put a new collector (and a refurbished weight) on the machine.

    If you use a counterweight that is heavier than the payload, but closer to the center, then you can reduce the energy you have to absorb from the weight, although I believe the momentum remains the same. 

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