Retroprolpulsive landings vs parachutes

[FishInferno]

So, most people here probably know about the whole plan for Spacex to land the lower stages of their rockets/the dragon via a powered descent with engines. But wouldn't using parachutes be cheaper? No rocket fuel to spend money on.

[N_las]

Maybe: No parachutes to spend money on?

[Guest]

Parachutes are not free either, and not weightless too.

[hugix]

PArachutes may be the safest most tested system known now.

But you need to carry that extra mass your entire mission. Not only the prarachutes, but the drogues and all the subsystems.

While a propulsive landing may cost extra fuel. The systems needed are already on board. You already have the engines on your craft. Why not use them again during landing?

[N_las]

Also, most people overestimate the fuel requirements for a propulsive landing. Try it in KSP. It isn't an accurate simulation, but it will give you a rough idea. You just have to burn a little bit before touchdown. decelerating a nearly empty stage is much easier than accelerating a heavy full stage.

[xenomorph555]

But people underestimate the amount of fuel and precision needed to fly a booster from the middle of the ocean back to land.

[zarakon]

Powered descent allows for a much softer landing, which means less wear-and-tear for re-usability.

As far as I know, all large spaceflight equipment that we've recovered by parachute (shuttle SRBs, various capsules) without propulsive assistance have needed to land in the ocean. I would presume that this is because the size of parachute needed to land safely on the ground is prohibitively large/heavy. Landing in the ocean is undesirable because then you have to deal with seawater getting into everything, again bad for re-usability.

Re-checking and re-packing parachutes is probably time-consuming and expensive compared to adding a bit more fuel during refueling.

Propulsive landing also gives them much more accuracy, so they can eventually land their parts right where they need to be for re-use instead of needing to haul them back from the ocean or the middle of some desert.

[DaveofDefeat]

The soyuz capsule also has to use retrorockets when landing, the parachutes alone do not slow it down enough.

[Albert VDS]

Pros of powered descent on Dragon v2:

- Lower landing speed

- Control of were it's going to land

- No need to fish it out of the ocean

- 8 thrusters makes it possible to have multiple thruster failures and still land

- 3 Backup parachutes

- Ensure reusabillity

[StrandedonEarth]

Dragon v2 will still have parachutes as a backup, especially in case of an abort, as the SuperDracos used for landing also serve as the abort rockets. While the 'chutes are still being hauled around everywhere, they will still be there for the next mission, unlike Orion's LES (Launch Escape System?) which is jettisoned every flight (wasted money).

[crubs]

Why not use a hybrid approach? That's how I landed on eve...

[ZedNova]

It allows it to land accurately, so it can be refueled, reloaded and reused quickly without waiting for someone to pick the crew up in the sea. It still has parachutes in case the rockets fail.

[Airlock]

Quick note: Dragon V2 will initially land relying on parachutes, using its thrusters to cushion touchdown: http://www.nasaspaceflight.com/2014/08/dragon-v2-rely-parachutes-landing/

"As has now been revealed by Dr. Reisman, Dragon V2 will initially conclude missions under her parachutes, prior to a later switch to a fully propulsive landing."

"This will allow of an incremental approach to transitioning the V2 toward the eventual goal of landing exclusively under SuperDraco power. However, the new thrusters will still find a use during the final few seconds of landing.

“We land on land under parachutes and then use the SuperDraco launch abort system to provide cushioning for the final touchdown,†noted the former Shuttle astronaut to Future In-Space Operations (FISO) Working Group this week."

[magnemoe]

Powered descent allows for a much softer landing, which means less wear-and-tear for re-usability.

As far as I know, all large spaceflight equipment that we've recovered by parachute (shuttle SRBs, various capsules) without propulsive assistance have needed to land in the ocean. I would presume that this is because the size of parachute needed to land safely on the ground is prohibitively large/heavy. Landing in the ocean is undesirable because then you have to deal with seawater getting into everything, again bad for re-usability.

Re-checking and re-packing parachutes is probably time-consuming and expensive compared to adding a bit more fuel during refueling.

Propulsive landing also gives them much more accuracy, so they can eventually land their parts right where they need to be for re-use instead of needing to haul them back from the ocean or the middle of some desert.

Another very serious problem is that you could only land something like falcon 9 first stage with parachutes if it was pretty much perfectly calm, try landing an lander with two orange tanks on top of each other with an 2-4 m/s horizontal speed, now add that the wind will continue to push you sideways after touchdown.

This also aplies to the popular hybrid landing who is common in KSP and the Russian pods uses.

This is not an problem for pods who is as wide as they are high, would probably work with the dragon pod, not with stages.

Airlock stated that the dragon 2 pod would start doing an hybrid landing.

Edited October 24, 2014 by magnemoe

[Streetwind]

A big problem with parachutes is that you can't predict precisely where it will land - especially not ahead of launch. As a result, you need large areas (many square miles) of target area that is completely devoid of any obstructions that may damage the craft as it comes down, and also devoid of things that may be damaged by the craft coming down on them. In case you didn't immediately grasp what this means - it translates directly to "not anywhere near the launch site, or anything else manmade for that matter". You then have to bring expensive, heavy equipment into that fairly remote piece of landscape in order to retrieve your craft... or you have to fish it out of the ocean, where the craft itself will have suffered corrosion damage from the salt water while it was waiting for pickup. In some cases, this may mean that it's cheaper to just build a new craft than try to repair the old one, meaning you have no reusability, and launch prices will remain high. Also, if the craft has a passenger in need of medical assistance, you lose a lot of precious time waiting to get to it before you can help.

A retropropulsion system on the other hand can actually target the launchpad the craft was launched from - or, even more convenient, the yard in front of the maintenance hall where it will be refurbished. You save all that heavy retrieval equipment and the effort involved in reaching the capsule in its remote landing zone, you save renting the landing zone itself, and you save the personel involved. That's a good cost reduction. You can also ensure that the craft remains undamaged and easy/cheap to refurbish, and that ground teams are able to assist passengers within a minute of touching down.

It's not that parachutes are a bad landing system... there is in fact nothing wrong with them. It's just that the other factors around the landing process benefit from a well-targeted propulsive landing. Even if parachutes are cheaper and less complex than retropropulsion, you save more money and complexity after the touchdown than you lose from switching to propulsive landings.

Edited October 24, 2014 by Streetwind

[Nuke]

for unmanned stages id probibly accept a lower rate of landing success and dispense with the chutes all together. i would however use a small drogue chute to aid initial deceleration. if everything goes right, it would be jettisoned when the thrusters take over (after ignition and idling, but before throttle up). for manned stages you might have additional backup chutes as well if you have engine trouble. you would attempt to light the engines at a safe altitude and let them idle till you need them. if for some reason the engines dont throttle up, dump their fuel and release the chutes.

you might not even need the drogue if your ship is large and low in density. in ksp i like to use chutes to help decelerate during a duna landing, i usually cut them and use engines when i get to below 500m. its one of those fuel saving things i do because im cheap.

Edited October 24, 2014 by Nuke

[Sky_walker]

Quick note: Dragon V2 will initially land relying on parachutes, using its thrusters to cushion touchdown: http://www.nasaspaceflight.com/2014/08/dragon-v2-rely-parachutes-landing/

Yep.

Dragon V2 is basically a Soyuz on steroids.

And as for the topic - Parachutes got one important advantage: They're much safer than retrorockets. And that's pretty much the only thing that matters when it comes to the landing.

[Guest]

It all depends on what you want to land. A rocket stage is hard to land with parachutes, because it's heavy and you've got a very fragile thing pointing down - namely, your engine. SSRBs jettisoned their nozzles just before the spalshdown, to avoid damaging the casing. With a liquid rocket, you'd have to ditch the engine... which is pretty much the whole point of recovering it in the first place. Tanks and SRB nozzles are cheap, engines aren't. Also, with a parachute-only descent, you can only splash down. In Soviet space program, all capsules that landed with crew used landing rockets. The exception was Vostok, which had another cushioning system - pilot's own legs (he/she ejected halfway through terminal descent). Voskhod (which landed with it's crew inside) had landing rocket attached to parachute lines, and all Soyuz variants have solids in the capsule bottom. When landing a rocket stage, you only lose money in case of a failure, not to mention you already have an engine with lots of excess TWR by that point. As such, parachutes can be forgone and the landing made with the rocket only.

On the other hand, if we're talking a capsule, parachutes are much safer and therefore, a better choice. Rockets are still needed if you don't splashdown, but landing without them needs to always be possible.

[tomf]

What about landing liquid stages upside down with parachutes? Most of the expensive stuff on a stage is at the bottom, while I expect the top end when empty will act as a huge crumple zone.

[Alchemist]

Landing liquid stages on parachutes? That was developed for boosters of Energiya (but never tested, both flights they had some additional telemetry equipment packed there instead).

The landing sequence was:

1) booster separation - they are separated from the central block in pairs, then the boosters separate from each other.

2) reentry - nose-first with some RCS for attitude control

3) chute deployment - drogue and main chutes deployed from the tail. still pointing down

4) reorientation - chute attachment and the tail is released, the second attachment point is near the center of gravity. the booster gets realigned horizontally.

5) landing - deployable landing legs and solid-fueled retrorockets at both ends of the booster

[Nuke]

i wonder if you could lithobreak liquid engines and boosters by using the tankage as a massive crumple zone to minimize the damage to the engines. it would require a drogue or something to ensure the thing lands nose first and at a survivable velocity.

[Nibb31]

There would still be a risk of damaging the engines.

[Nuke]

if the probability of engine survivability could be high enough, it might be worth it.

[Frozen_Heart]

I think the future is retro-propulsive but still keeping a parachute as backup. It allows much more pinpoint landings right back at the launch sight so the spacecraft can be reused.

[rdfox]

Parachute-only landing is *not* limited to splashdowns. For unmanned single-use spacecraft, a "heavy" landing (typical parachute descent speeds at impact are <10 m/s, equivalent to roughly 20 mph/32 kph) isn't a big issue, as the hardware can take more of a beating than squishy human bodies. For manned spacecraft, it's more of an issue, but not a critical one--all of NASA's capsule-type spacecraft were designed to make land ("contingency") landings without significantly injuring the crew. (They would have bumps and bruises, but no open wounds, broken bones, or concussion from the impact.) This was done because, under certain wind conditions, a launch abort off the pad or in the first twenty to thirty seconds of ascent could, theoretically, see the spacecraft blown back onto land by the wind, in addition (in Apollo) to providing a "wherever they come down" landing capability in the event of a post-TLI abort on a lunar mission requiring a turnaround rather than a lunar swing-by a la Apollo 13.

The method used was by using crew couches designed for shock absorption, mounted on struts specifically designed, should there be excess accelerative loadings on impact, to crush and thus provide cushioning to it. (The struts were actually quite similar to the crushable shock absorber structure in the LM landing gear.) A water landing was the preferred option, by far, but a land landing would be survivable, even factoring in the need to exit the spacecraft and survive until the SAR forces found you.

The one flown exception to this was Apollo 7, where the Block 2 Apollo CM used had to be flown with the Block 1 CM's crew couches, as the Block 2 couches wouldn't be ready until Apollo 8. Testing after the AS-204 fire found that the Block 1 couches were marginal, at best, in a "contingency" landing; as a result, Apollo 7 was the only Apollo mission to have a special mission rule that put more restrictive limitations on the winds at launch, to ensure that an abort would see the spacecraft land in the ocean.

NASA has acknowledged that they did NOT want to deliberately target a land landing with a pure parachute design, at least for a round parachute design, because it was likely to beat up the astronauts and they couldn't *guarantee* that there wouldn't be serious injuries. (The initial impact would not be a problem; however, if the spacecraft tumbled down a slope, the crew might well be injured.) This is why, during the period when Orion was expected to eventually transfer to landing at Edwards AFB instead of the ocean (to reduce refurbishment requirements for reuse of the capsule), the plan was to carry a Soyuz-type retro-rocket to cushion the landing.

Of course, the Gemini spacecraft was originally planned to be the great exception, using a Rogallo parafoil wing to glide to a landing on a runway at Edwards, requiring no impact cushioning at all due to the use of lift to touch down as gently as the Space Shuttle eventually did. (The parafoil was only dropped for time reasons; development had hit snags, and a circular parachute was substituted to keep Gemini's support of the lunar program on time.) Given that such parafoils are now widely used by skydivers, and the US military is testing combining them with GPS guidance systems to allow precision airdrops of supplies and equipment, use of a parafoil for a runway landing may end up being the preferred primary option in the future, with a circular parachute system as a contingency option...