Hot answers tagged

75

Hobbes' answer focuses on why we might want to build SLS. There are also significant barriers to rebuilding Saturn/Apollo. In addition to the (vast) amount of existing technical documentation on those designs, there's a (probably vaster) pool of knowledge that the individuals who actually built the things collected during the process. Nearly all of those ...


56

It is still way too early to make such a judgement. It's easy to be overly optimistic about the cost of a program. The Space Shuttle was supposed to have dozens of flights each year and be super-cheap because it was reusable. However, you simply don't know the true cost until a program has been in use for several years. After several years of the Shuttle, ...


39

There are several reasons: We can do better these days. Saturn and Apollo were designed in the early 1960s, so the design tools used were mainly pen and paper, with some primitive computer tools thrown in here and there. These days CAD can be used to create a design that performs far better (because you can design parts closer to the strength they need, ...


29

The shuttle stack broke up at ~73 seconds after launch of STS-51L. The Solid Rocket Boosters separated from the other elements and continued flying in a more or less stable manner (surprisingly). Air Force range safety personnel detonated the boosters at ~110 seconds after launch using the self-destruct system built into the boosters. (timeline reference) ...


28

NOTE All dollar values are in present day values accounting for inflation. Another reason we're not reusing the Saturn V is the same reason it was cancelled in the first place: cost. The SLS is supposed to be half the cost per launch. Whether that works out remains to be seen. The Saturn V was expensive. The Saturn V program cost \$47 billion over 10 ...


24

I am not a fan of the SLS. So take my comments with that as a grain of salt. SLS is fundamentally a jobs program. It has no mandate to be affordable. It also has no real mission at the moment. Thus there is nothing really pushing its development other than jobs in states of politicians. Thus logic and rationality do not apply to SLS. It will be ...


20

The usual answer - $. Booster recovery was only marginally worth it for STS - once you witnessed how every screw and nut was taken apart, cleaned, and reassembled, you began to wonder why not just build a new one. That said, it is a wonderful thing for problem resolution if you get the hardware back to look at. Here are some pictures I took at the STS-...


19

Short answer, no. Falcon Heavy and SLS Blocks 1B and 2 aren't even in the same lifting capacity category. The former is a heavy lift LV and latter are super-heavy LV. There also seems to be an increasing interest in SLS from the various scientific communities, for example, this slide was shared during the recent MEPAG (Mars Exploration Program Analysis Group)...


19

What's the reason that they don't use the Delta Heavy for crewed flights to the Moon and beyond? The Delta IV Heavy isn't powerful enough to send Orion beyond low Earth orbit. Orion masses about 26.5 tons in its lunar-mission configuration, which is just within Delta Heavy's LEO launch capability. SLS is much more powerful, and can launch both Orion and a ...


16

SpaceX has decided that reusability is a critically important goal to keep long-term costs down, therefore they've designed in the ~15%-30% performance margin they need to achieve reusability (the lower figure for mid-Atlantic barge landing; the higher figure for boost-back to the launch site). Cost is critical for SpaceX because Falcon 9 has to compete ...


15

SRBs have a very high thrust to weight ratio, even if they have an only modest ISP. That makes it complementary with LH2/LOX systems that have a relatively low thrust. Early on acceleration is almost as important for over all efficiency as exhaust velocity. For the first few hundred m/s of $\Delta v$, a combined metric of ISP and propellant density, called ...


15

Eh, to begin with this statement isn't accurate. SpaceX ... has the same capabilities if not better? Falcon Heavy as stands can't replace SLS and launch Orion on the required orbit without significant modification. (and even if Falcon Heavy could launch Orion, it wouldn't be able to dual manifest Gateway modules) Dragon isn't comparable in capabilities to ...


14

As it turns out, SLS is actually very similar to the original Ares V configuration, proposed around 2002 (I believe). During this time it was known simply as 'CaLV,' or Cargo Launch Vehicle, as a launcher to loft 125 tonnes to LEO for Constellation lunar missions. This rocket featured STS 8.4m tanks on both the core and second stages. Main sustainer ...


13

All the answers are right in their own way. One thing that is not addressed: The Falcon Heavy is not even remotly on par with the SLS in terms of rocket diameter and payload mass. According to Wikipedia (Falcon Heavy, SLS) the Falcon Heavy can launch 63 tonnes to LEO while SLS can deliver a whopping 95 tonnes in the Block 1 configuration which (if everthing ...


12

The initial SpaceX plan for recovering a first stage was parachutes, into the water. After all they had tons of data available from NASA on SRB recoveries (266 of them?). But after the tried on the first few flights of the Falcon 9 1.0 they realized they needed to slow the rocket down, before it hit atmosphere (where parachutes would work) or else they got ...


12

Pork. It is a jobs program not a technology program. The SRB's are built by ATK, but the design ensured they needed to be reworked, so they could not just the existing Shuttle ones. Need an extra segment, which in SRB land means redoing the inner contour to control the burn rate to control thrust profile. The switch between SSME to RS-68 to SSME (Yes, ...


12

A Venus flyby does little to nothing towards the stated goal: a manned landing on Mars. because you get a lot closer to the Sun than on a Mars mission, you need to modify the spacecraft to reject all that extra heat. There goes the commonality in the spacecraft. A flight time of 5 months vs. 6 months (one way) for a Mars mission is not a significant ...


12

Part of it is leveling employment. The government is fond of large projects that require multi-year ramp-up and ramp-down and massive up/down swings in employment need. Suppose in 2019 you're hiring every rocket scientist in town for project X. 2023, you lay them all off because the project is done. Then project Y arrives, a modification on an existing ...


11

A different approach might be to rebuild the Saturn V, with modern techniques, not as a one for one rebuild, but rather take the good parts, and make them better. For example, the F-1 engine of the first stage, (5 used, 1.5 million lbs thrust) is being reconsidered as a more modern version with higher thrust. The nozzle was meticulously assembled in ...


11

The answer is more political than technical. (Will be some opinion here, alas) SLS exists as a booster without much of a mission. Well it has Orion, since SLS is the only booster (except maybe Falcon Heavy once man-rated) that can launch Orion. In Constellation there were several projects. Ares-1 - SRB based manned booster for original Orion design. ...


11

It's always difficult to make apples-to-apples comparisons between the space shuttle and other launchers, because the orbiter is ambiguously part launcher and part payload. This is compounded by the broadness of the term "LEO"; shuttle payloads went to a variety of altitudes and inclinations. However, since the title of the question specifies "mass to LEO" ...


11

I have retrieved via FOIA request the "Space Launch System Program (SLSP), Flight Software Application, Software Assurance Plan (SAP)". It is the core document describing the software development processes for the Flight computer (the bit responsible for on-pad prelaunch, launch, and ascent of the SLS vehicle) and the Green Run Application Software (software ...


11

The engine controller is a computer mounted on the SSME which monitors and controls the engine. It, for example, takes throttle commands as a % of power level from the vehicle and translates them into control commands for the engine valve control loops. The SLS engine controller is a new hardware device and presumably new software as well. The pogo ...


11

That project has been cancelled (or at least put on hold) as of June 2019. But NASA discontinued studies of upgrading the SLS, including study of reviving the F-1. Soon the testing of the F-1 gas generator at Marshall Space Flight Center also came to a halt. The F-1 went silent once more. This article in The Space Review has a good summary of the ...


10

It can but it won't. It was decided that the costs of recovery, inspection and refurbishing the boosters plus increased risk of disaster due to unnoticed wear is just not worth it - the boosters will not be recovered for economical, not technical reasons.


10

Some changes appear to be under consideration for future production of SLS RS-25s: From Wikipedia's SSME page, about the SLS plan: Once the remaining RS-25Ds are used up, they are to be replaced with a cheaper, expendable version, currently designated the RS-25E ('E' for expendable). This engine may be based on one or both of two single-use ...


10

Engines at the size and performance of an SSME are very difficult to develop. The SSME has one of the highest ISP's ever attained in a production engine. (Not some crazy Fluorine involving demo). It is anything but a simple, or even 'old' engine. Its re-manufacture would likely benefit from modern techniques, like the people examining starting F-1 engine ...


10

The top-level answer, as others said, is "because it was decided so". But following that, we have actual design decisions that simply make SLS completely impractical to recover. It focuses on getting the big payload to a high orbit, and through ignoring the necessity for reusability, it follows design principles that make recovery completely impractical. ...


10

The SRBs for the Shuttle and SLS are used to get the stack moving and off the pad, so that the more efficient LOX/LH2 engines can do the hard work. In the case of the Space Shuttle, an amazing amount of thrust is needed. I like using lbs of thrust because it is more inspiring. The SRBs produced 2.8 million lbs of thrust. That is just awe inspiring huge. ...


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