Man Rating Atlas V or Delta IV?

[JimMcDade]

There are a number of interviews and articles where astronauts talk about how rough those early space rides were in comparison to Apollo-Saturn and the Space Shuttle. They weren't just talking about g levels, but we generally look at max g as an indicator of astronaut comfort.

Mercury-Redstone boost placed about 6 g on the astronauts. I don't recall the precise max G on a Mercury-Atlas boost, but I think that it was around 5 or 6 g. Space Shuttle max G-forces are around 3 g during the boost phase. GT hit astronauts with a staggering 8 g force at second-stage shutdown. Apollo-Saturn V maxed out at about 4 g.

Ares I will get off of the ground in a hurry. You don't have to wait on an SRB to run up to 100% thrust as you do with liquids.

MA-1 registered 25 g just before it broke up. There are lot more things can go wrong with liquids with all of those moving parts. Solids are extremely reliable and less costly in comparison. I think that is is inevitable that solids will be used as the sole propulsion technology on all almost all future rocket designs. That's just an opinion, but no better than yours. The only cheaper and more efficient way to get into space is to take-off from a runway with air breathing engines and then releasing the orbital stage from altitude. I have seen those numbers. (That was the original Shuttle concept)

Back to man-rating: Man rating is a process that inevitably adds costs, weight,and complexity to a design. NASA looked at man-rating existing launch systems and rejected that idea. Perhaps the cost and weight penalty projections made that option undesirable. I may be that decision makers were given bad numbers, but it's too late to do anything about that. Ares I-X is coming down the river.

[J.McDonald]

Ares I will get off of the ground in a hurry. You don't have to wait on an SRB to run up to 100% thrust as you do with liquids.

I think this is talking at cross purposes. The speed of lift-off (as in the actual acceleration) has got nothing whatsoever to do with how long it takes to spin up an engine, because you wait for it to reach full thrust before releasing the vehicle. So solids give no advantage in that particular case.

There are lot more things can go wrong with liquids with all of those moving parts. Solids are extremely reliable and less costly in comparison. I think that is is inevitable that solids will be used as the sole propulsion technology on all almost all future rocket designs. That's just an opinion, but no better than yours.

My opinion, then, is that it is better to use liquids because the failure modes are usually quite graceful and, in the case of a manned payload, would allow time for an abort system to be activated. By contrast an SRM failure is going to be catastrophic and not give the payload much chance of survival. A related disadvantage of SRMs is that they cannot be practically shut down in a hurry, and that means that the launch abort system must be more powerful than for an equivalent liquid vehicle, which could termiante thrust almost instantly. Another disadvantage of SRMs is that they appear to be something of a 'black art', and getting a design that works reliably with the required thrust profile and total impulse is, well, tricky; once you've got one that works you don't generally want to mess around with it. As well as making them expensive to develop, this also means that incremental upgrades are almost impossible. Contrast with liquid rocket stages which can take advantage of tank stretches and engine upgrades with comparative ease. A further point against the upgradability of SRM-based boosters is tha they have massive roll-out weight; had STS used LRBs rather than SRBs, perhaps Ares-V woudl have sported multiple pairs of them, whereas the massive weight of the SRBs is overloading hte crawlers and cralwerway and impeding further vehicle growth.
Finally, pouring, transporting, and assembling large segmented SRMs is a major health and safety worry. Is it really cheaper than handling a big, inert, empty vehicle, and then tanking it up with kerosene and oxygen?

NASA looked at man-rating existing launch systems and rejected that idea.

Not in OSP they didn't. Maybe STS107 made everybody more risk-averse? And, whilst they may have laid down requirements at the time of ESAS which excluded EELVs as an option, problems with Ares/Orion have forced them to back down from those requirements. It would appear that building a safe and powerful rocket is quite hard, and NASA are not doing as good a job of it as they'd hoped. Luckily they can keep on prattling on with Ares-I for another three or four years and still come out ahead by switching to EELV.

it's too late to do anything about that. Ares I-X is coming down the river.

Ah, is this, at last, the real justification for Ares-IX? Flying a dummy upper stage atop an SRB, roll-control system, and avionics which are all taken from donor vehicles and which are not Ares-I hardware. It makes it look like something is happening; when you say 'first test flight is next year!' does it not seem a bit odd that first operational flight won't follow it for another six or more years??