Boeing's Alcoholic Eighty-Eight

[Kibble]

RS-88 is the rocket engine Boeing selected as the primary (pusher) launch escape system for CST-100, as well as its OMS. It burns ethanol and liquid oxygen. But part of the commercial crew contract is that the vehicle can stay on-orbit for the duration of the expedition, which is several months, and liquid oxygen is mildly cryogenic, and will boil off over time. That's why Cold War-era missiles (which had to be kept in silos for months at a time) quickly switched to (storable) solid or hypergolic fuel! Is the boil-off rate just low enough to be within tolerance for a deorbit burn following a nominal mission?

[SomeGuy12]

Why does the launch escape system need to function after launch?

SpaceX uses the same engines and fuel used for launch escape for in orbit maneuvering and plans to use them for even landing. However, the old school way to do it, there's an escape rocket, and you jettison it after there's no more need for it. (it doesn't help you escape anything in orbit)

Are you sure they plan to keep the rocket attached? It would make more sense that when they separate from the upper stage of the launch rocket, they would leave that pusher launch escape system behind as well.

[sgt_flyer]

@Someguy12

'Pusher' escape systems, if jetisonned at the end of the ascent phase would have been deadweight throughout the flight. as an escape system is a lot of delta-V by itself (they need it to propell at 5+ Gs a capsule above the rocket) - by keeping the escape system and throttling the engine down for OMS manoeuver, give you a big amount of delta-V for orbital manoeuver, without needing to add additionnal engines and tanks to your capsule's service module. (so either the rocket work and you make it almost to orbit - and then you use the pusher LES as a OMS for circularization / orbital manoeuver, or you use it as a launch escape system if there's a problem with the rocket during all the phases of ascent. (and if, at the end of the launcher's ascent, your LES engines don't work, as you have not circularized yet, you'll fall back on earth anyway, so they just detach from the service module and simply fall back to the ground.)

[Shpaget]

Engines can be throttled?

[magnemoe]

Engines can be throttled?

Obviously dragons engines can throttledas its planned to be used both for escape, orbital operations I think and soft landings. This increase weight so an standard escape tower is probably lighter but lack the other functions.

[YNM]

Dragon V2's SuperDraco are not for orbital operations AFAIK, though it can be one theoretically (say, trunk explosion ?)

The PES (what, pusher escape system, or the game of the same name ?) engines is intended to be the escape system on launch as well as being the orbital engine. If you look at the rendering of CST 100 in Wikipedia you'll see four nozzle below the trunk which is the RS-88. Not sure for LOX boiloff, SuperDraco uses hypergolics, maybe RS-88 is intended to be fuel-rich so loss of oxygen can slightly be tolerated ?

[magnemoe]

Dragon V2's SuperDraco are not for orbital operations AFAIK, though it can be one theoretically (say, trunk explosion ?)

The PES (what, pusher escape system, or the game of the same name ?) engines is intended to be the escape system on launch as well as being the orbital engine. If you look at the rendering of CST 100 in Wikipedia you'll see four nozzle below the trunk which is the RS-88. Not sure for LOX boiloff, SuperDraco uses hypergolics, maybe RS-88 is intended to be fuel-rich so loss of oxygen can slightly be tolerated ?

Ok so the superdrako will damage the trunk? trunk is needed if nothing else for solar panels. I was aware they was not used for normal orbital operations.

Still the idea of using the escape system as an 3rd stage makes lots of sense.

[KSK]

Engines can be throttled?

Yup. The Lunar Module Descent Engine was the first example I think. The Space Shuttle Main Engines were throttleable too - listen to a recording of the last Challenger flight sometime - before the explosion you'll hear the 'Go for throttle up' command.

In fact (according to Wikipedia) most rocket engines can be "throttled by a factor of two without much difficulty." Deep throttling is another matter and KSP style 0-100% throttling is (so far as I know) impossible.

[Bill Phil]

Yup. The Lunar Module Descent Engine was the first example I think. The Space Shuttle Main Engines were throttleable too - listen to a recording of the last Challenger flight sometime - before the explosion you'll hear the 'Go for throttle up' command.

In fact (according to Wikipedia) most rocket engines can be "throttled by a factor of two without much difficulty." Deep throttling is another matter and KSP style 0-100% throttling is (so far as I know) impossible.

Well, aside from on-off engines. You can pseudo throttle them, applying thrust for a certain amount of time, stopping for a certain amount of time, and repeating that cycle.

[Streetwind]

It's expected that the CST-100 expends most of its onboard fuel relatively soon after docking with an orbit-raising maneuver (pushing the ISS up). Boeing has specifically advertised this capability, which right now (after the final ATV left) only the Progress spacecraft can perform, if I recall correctly. So it wouldn't need to store a whole lot of liquid oxygen for long periods of time.

For deorbiting, the RCS system is probably sufficient in most cases, maybe supplemented with some remaining engine fumes. SpaceX for example uses only RCS for deorbiting on their cargo Dragon spacecraft and plans to do the same for the crew Dragon.

[SomeGuy12]

Cryogenic coolers do exist and they do work in space. I found a NASA paper where at one of their research centers, they made a cryogenic tank and kept it cool using commercial Cryomech cooling devices. Just saying - I have the impression that Boeing wouldn't risk something not used for decades like this, but they could actively cool their oxygen tank.

[YNM]

Ok so the superdrako will damage the trunk? trunk is needed if nothing else for solar panels. I was aware they was not used for normal orbital operations.

Meant to say that probably can be turned on in space for really bad occassion, like loss of trunk section by any means (say, micrometeorite, explosion, or fuel leak kind of in the film gravity). Or getting to the surface of Mars and back unmanned.

Anyway, CST 100's trunk is quite small - you don't expect that (remaining ÃŽâ€V) to last long...

[magnemoe]

Meant to say that probably can be turned on in space for really bad occassion, like loss of trunk section by any means (say, micrometeorite, explosion, or fuel leak kind of in the film gravity). Or getting to the surface of Mars and back unmanned.

Anyway, CST 100's trunk is quite small - you don't expect that (remaining ÃŽâ€V) to last long...

I was thinking more of an event like loosing an first stage engine leaves them short on dV. Using some of the superdracko fuel to finish the burn would be relevant if the other option was mission abort.

Yes this would result in no powered precision landing, either spashdown or Soyuz style landing.

I assume it can be used in an mission abort setting where they will drop trunk anyway.

[Aghanim]

Yup. The Lunar Module Descent Engine was the first example I think. The Space Shuttle Main Engines were throttleable too - listen to a recording of the last Challenger flight sometime - before the explosion you'll hear the 'Go for throttle up' command.

In fact (according to Wikipedia) most rocket engines can be "throttled by a factor of two without much difficulty." Deep throttling is another matter and KSP style 0-100% throttling is (so far as I know) impossible.

Depends on the thruster technology though. You can expect a cold gas thruster can be designed deeply throttled, as it is basically a gas tank connected to a nozzle by a valve. I have no idea for ion drives though.

[Kibble]

It's expected that the CST-100 expends most of its onboard fuel relatively soon after docking with an orbit-raising maneuver (pushing the ISS up). Boeing has specifically advertised this capability, which right now (after the final ATV left) only the Progress spacecraft can perform, if I recall correctly. So it wouldn't need to store a whole lot of liquid oxygen for long periods of time.

That makes sense, except won't it be facing the wrong way? PMA-3 is (will be) on the R-Bar, and PMA-2 faces against the V-Bar!

[Shpaget]

Interesting. I was under the impression that they were really bad at it.

[Armchair Rocket Scientist]

Ok so the superdrako will damage the trunk? trunk is needed if nothing else for solar panels. I was aware they was not used for normal orbital operations.

Still the idea of using the escape system as an 3rd stage makes lots of sense.

The superdracos are deeply throttleable IIRC, but even firing only two engines at 10% throttle would give you a TWR of almost 0.2, which is more than you actually need for operations in LEO (the Shuttle's OMS only had a TWR of about 0.05). Having such a high minimum thrust isn't that good for precision maneuvering (such as lining up an orbital rendezvous), and is only really useful for large burns like a TLI (which the Dragon doesn't have enough dV for anyway).

Also, the SuperDraco has a relatively small nozzle, and is most likely optimized for atmospheric operation (an abort could be required anywhere from on the pad to first-stage burnout), so it should have worse vacuum ISP than the Dracos. Using them in orbit would just waste fuel.

[xenomorph555]

I should note that kerolox was used for the OMS of Buran, which was designed to last at least a month in space. Cryo-coolers for space do exist and they can last a long time however they require more power.

[magnemoe]

The superdracos are deeply throttleable IIRC, but even firing only two engines at 10% throttle would give you a TWR of almost 0.2, which is more than you actually need for operations in LEO (the Shuttle's OMS only had a TWR of about 0.05). Having such a high minimum thrust isn't that good for precision maneuvering (such as lining up an orbital rendezvous), and is only really useful for large burns like a TLI (which the Dragon doesn't have enough dV for anyway).

Also, the SuperDraco has a relatively small nozzle, and is most likely optimized for atmospheric operation (an abort could be required anywhere from on the pad to first-stage burnout), so it should have worse vacuum ISP than the Dracos. Using them in orbit would just waste fuel.

You are right, I was mostly thinking of improvised 3rd stage operations where you need an decent burn anyway, however this is something they need procedures for before launching.

And the nozzles are probably undersized for atmosphere too, they need high twr and don't have the room for larger ones.

trying to use an 10 TWR engine for orbital operations in ksp is hard and they can be throttled 0-100%

[Capt. Hunt]

I was thinking more of an event like loosing an first stage engine leaves them short on dV. Using some of the superdracko fuel to finish the burn would be relevant if the other option was mission abort.

Yes this would result in no powered precision landing, either spashdown or Soyuz style landing.

I assume it can be used in an mission abort setting where they will drop trunk anyway.

It's probably likely that they will mission abort in any case where second stage does not have enough dV for orbital insertion. They wouldn't want to cut into their safety margin, which continuing to orbit on the Super Dracos would do.