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SLS is to a large degree a continuation of the canceled Ares V, basically the STS without the orbiter. But what changes in design were made from Ares V to SLS? What were the reasons for those changes? Were there for example problems discovered with Ares V that motivated some of the design changes? Or changes in the basic specifications for the launcher, for example its compatibilities with launch pads and industrial capabilities? I understand that the change happened for political reasons, but I suppose that it gave engineers an opportunity to improve on the previous design.

I see on Wikipedia that Ares V was to have had 5 Space Shuttle Main Engines compared to SLS' 4, or 5-6 RS-68 engines which is what Delta IV uses. Is the fewer number of engines compensated for by extending the solid boosters for SLS? Is there any particular reason for the change in number of engines?

Is it correct to say that both Ares V and SLS were designed to be a little more powerful than Saturn V, in order to send astronauts directly from Earth's surface to the Lunar surface without having to leave half the spacecraft in Lunar orbit?

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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 propulsion came from 5 SSMEs, while the upper stage was to use a single J-2X. A pair of 5-segment boosters would augment the stack.

SLS is nearly identical, as it also features 8.4m core stage tankage to reuse shuttle tooling. In addition, the 5-segment boosters have been carried over. Upper stage options have changed, though, now featuring either 8.4m or 5m diameters with Rl-10 engines. The CaLV throw weights are very close to SLS Block 1B because it is basically just that; the Block 1B should throw around 118 tonnes (though this may include upper stage propellant).

This all seems pretty similar, but what makes this a good question is the constant growth the Ares V design had to absorb to keep up with the demands of its mission as Altair and Orion came of age.

CaLV was an inline shuttle-derived vehicle that could meet the needs of Constellation's EOR architecture alongside Ares I ('The Stick'). It was to have 8.4m core and upper stages with 5 RS-25s and one J-2X, respectively. Two 5-segment SRBs would shove it off the pad. With a payload to LEO of 125 tonnes, it was fine for the early 2000s plans... but these were short-lived.

The lunar lander, known as 'Altair,' was its primary payload and was growing in mass. Orion, too, was getting heavier for the TLI burn. As such, a lunar injection mass of over 60 tonnes was needed. Thus, CaLV morphed into Ares V, with a new design packing more power. The engine of choice became the less efficient but more powerful and cheaper RS-68. But being more thirsty, the tanks had to change. The core stage grew to 10m in diameter and 65m in length, and to support this the boosters each received an extra half-segment for greater thrust.

Next to grow was the upper stage, or EDS. Altair hit 45 tonnes, meaning a combined TLI mass of over 65 tonnes. Previously 8.4m, it also widened to 10m while retaining the single J-2X. More mass at the top of the rocket meant the bottom had to adjust, and the core added a 6th RS-68 - in fact, the updated RS-68A was planned. Ares V was, if it ever flew, to end up using the RS-68B, with even greater thrust and a newer ablative nozzle. The core also stretched to 71m in length, leading to a rocket capable of lifting 188t to LEO, 71t to TLI, standing 116m tall and weighing 3,705t on the pad.

Here, problems started to surface. Notably, base heating with the RS-68 cluster and SRB exhaust turned out to be severely problematic to efficiency. The ablative lining could not dissipate heat with nozzles packed together, unlike the RS-25 and its regenerative cooling system. Along with this, pad infrastructure changes and the need for 10m tooling meant money, and lots of it. The Augustine Commission called for a smaller Ares V Lite that would be cheaper to operate, with a 140t LEO payload, 5 RS-68s and the old 8.4m upper stage. This vehicle could also have eliminated the need for Ares I and all of its development costs.

This advice was not adhered to, and Ares V continued to grow up until it's death. Near the end it was a beast of grand proportions; some studies showed 6.0- or even 6.5-segment boosters (!) on either side of a 7-engined RS-68B core. Just like science fiction, mythical giants may seem incredible until it is revealed they are too heavy for their legs to support them. Ares V was almost the same: it could have worked, but it's 'authors' kept writing too long into the night, and when they stood back the 9-billion dollar tag could not keep it afloat.

SLS is similar to the Ares V Lite in that it also sits in the niche of throw weight that allows it to act as the sole vehicle in any EOR mission architecture, rather than the Ares I/Ares V combo. It is perhaps more sensible for keeping to existing shuttle and EELV tooling for its 8.4m core stage and 5m Block I upper stage. Also, it uses the 5-segment boosters that were already tested for Ares I.

The operation of SLS is similar to the Ares with a few exceptions. Both feature hydrolox cores and upper stages flanked by solid boosters. However, the Ares used its core stage as more of a first stage (closer to Saturn V operation than that of SLS); it had over twice the thrust (with less efficiency) and a shorter burn time, meaning it's upper stage was larger because it would have to generate a larger portion of the ascent delta-V. On the other hand, SLS' core stage uses very efficient but less powerful engines that burn for a longer duration - SLS is a lighter rocket by several hundred tonnes. Therefore, the core is really a sustainer stage that fires for nearly all of the ascent, and the small RL-10 powered upper stages only need to give a little push to orbit. In fact, the SLS core stage dropped the fifth engine both to preserve engine stockpiles and to extend its burn time.

To increase payload, therefore, Ares grew its first stage - the core and its RS-68s, and the boosters grew both to lift its greater mass and to line up with the rising intertank distance from the thrust structure. SLS' sustainer remains the same, but Block 2 will increase payload through the use of liquid-fuel boosters because they constitute most of its first stage power.

Key differences, then, are:

  • Core stage diameter (Ares = 10m, SLS = 8.4)
  • Ascent operation (First stage vs. sustainer)
  • Mission requirements (Ares EOR means more TLI mass)
  • Booster type (reflects ascent operation)
  • Vehicle scale and mass (Ares is larger)

It has always been interesting to me when I look back at Ares V as a 'child,' in its CaLV days. The rocket was all there, taken straight from shuttle hardware, with just a bit of stretching, no RS-25 uprating needed, and without the J-2X in the trash can as soon as it had proven itself. CaLV fits into the gap SLS fills, and that Ares V created and could never stoop to; it just did it well before any of them. Which begs the question regarding why twenty years of development and thirty billion dollars have brought about a rocket with one less engine than it started with...

Written on my phone, so please forgive me of any spelling errors or horrendous grammar!

Al.

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Ares V was proposed to be quite a bit more powerful than SLS, putting 188 tons into LEO as compared to 130 for SLS. Besides the first stage engine differences, the Ares second stage is substantially more powerful than SLS.

Ares was designed for a manned lunar landing mission with the Altair lander, but this would be a lunar-orbit rendezvous with an Orion spacecraft, just like Apollo, rather than a direct-ascent mission. The larger rocket is needed to send about six people to the moon for a somewhat longer stay than Apollo.

SLS doesn't presently have a manned lunar landing mission. Single launch LOR with Orion plus a lander is probably out of its reach; it's not significantly more powerful than Saturn V.

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But what changes in design were made from Ares V to SLS? What were the reasons for those changes? Were there for example problems discovered with Ares V that motivated some of the design changes?

The Wikipedia article on the Space Launch System states that the NASA Authorization Act of 2010 conceptualized a space craft that would shape the Ares I and the Ares V into a single launch vehicle resembling the Ares IV. It would be used for both cargo and crew transportation. This means that the SLS was designed to human-rated, while the Ares V design was to purely for lifting power. The reason for this probably being that NASA considered using one vehicle instead of two to be more efficient, cost wise as well.

The payload capacity was also very different. The Ares V was projected to be able to lift roughly 188,000kg to Low Earth Orbit, while the Space Launch System is projected to lift between 70,000kg to 130,000kg to Low Earth Orbit.

I see on Wikipedia that Ares V was to have had 5 Space Shuttle Main Engines compared to SLS' 4, or 5-6 RS-68 engines which is what Delta IV uses. Is the fewer number of engines compensated for by extending the solid boosters for SLS?

Actual no. The Space Launch System is suppose to use a "beefed-up" version of the Space shuttle boosters with five segments instead of four. The Wikipedia on the Space Launch System notes this:

Blocks 1 and 1B of the SLS will use two five-segment Solid Rocket Boosters (SRBs), which are based on the four-segment Space Shuttle Solid Rocket Boosters.


Is there any particular reason for the change in number of engines?

The most likely reason is because the Space Launch System - as I've already said above - is designed to be safe enough for human travel, while the Ares V was not. The reduced power would probably help limit the force of oscillations.

Is it correct to say that both Ares V and SLS were designed to be a little more powerful than Saturn V, in order to send astronauts directly from Earth's surface to the Lunar surface without having to leave half the spacecraft in Lunar orbit?

The Ares V was designed to lift the Altair lander into Low Earth Orbit and, after rendezvousing with the Orion spacecraft, put it on a trajectory towards the moon.

The total mass of the Altair lander paired with the total mass of the Orion spacecraft, would need a considerable amount of force to put the two space craft in a Trans-luanr Injection. More force than the Saturn V could provide.

As for the Space Launch System, it would probably have a hard time matching up with the Ares V, and would fall short. And as @Russell Borogove has already said, it doesn't seem to be that much more powerful than the Saturn V.

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