Why can't the SSME be air started?

[kraisee]

With so many informed people on here, can anyone actually dispel the speculation and rumours as to why the SSME can not be air-started as the ESAS originally planned?

Does anyone on the board here know the real technical and/or budgetary reasoning which killed this concept?

Ross.

[Jim]

kraisee - 25/3/2006  3:07 PMWith so many informed people on here, can anyone actually dispel the speculation and rumours as to why the SSME can not be air-started as the ESAS originally planned?Does anyone on the board here know the real technical and/or budgetary reasoning which killed this concept?Ross.

The SSME is a head started engine, meaning there is no start cartridge or turbine spin up system.  Just the pressure (head) in the tanks.  Because of this the start box (temp and pressure tolerances) is very small.  The SSME goes thru 4 purge sequences to bring it to the start conditions.  This are tightly monitored by the ground software before it give the go for main engine start.  As a second stage engine, it could be conditioned until T-0 by ground commodities but during the 2 minute first stage burn, some combination of liquid and cold gasous He, N2, H2 and O2 would have to be used.  This would be non propulsion mass that would have to be carried.  

Additionally, the SSME was designed for a sea level start.  The pressures at altitude may not allow start box conditions to be achieved.  Modification to start at altitude may not be cheap  or actually create a "new" engine.

[kraisee]

Thanks for the info Jim.

It does seem strange though, that throughout all the trade studies the ESAS did, that it was a 'relatively simple modification', until it, errr, wasn't.

This is especially perculiar as on page 406 of the ESAS report, they reference two different studies done in 1993 and 2004, by NASA, into air-starting the Blk II SSME.

Surely *someone* should have figured out before now that it wouldn't be easy...

Ross.

[lmike]

I think what they meant by 'relatively' was making the SSME a single air-start engine 'relative' to making it a multi-restart engine (as in the second stage application).  Tank ullage re-pressurization between re-starts, turbopump spin-up issues are more severe in the latter case, AFAIK.

[MATTBLAK]

Reasons? Principally: Time and $$$$

[Hotol]

MATTBLAK - 26/3/2006  1:48 AM

Reasons? Principally: Time and $$$$

A bit like why the methane was taken out of the ESAS. Time and $$$$ for new engine developement was a no no.

[MATTBLAK]

Deleting the methane is a mistake they will regret later.

[Propforce]

There are several problems in modifying SSME to be able to air start, as far as I can see.  

1)  The existing SSME start box (range of pressure & temperature of propellant at inlet) is too high.  The 1993 study did indicate one can lower the inlet pressures (which lower the tank ullage pressure which could reduce tank wall thickness, etc.).  But that was a PAPER study.  To prove this would encompass a new engine development and certification program.  A technical and programmatic risk that NASA is apparently not willing to take on, in addition to cost.

2) On the ground, you have 1 G gravity which will give you the start box pressure "head" needed (NPSH) for engine start.  As a 2nd stage engine, after the SRB is cut off and the whole vehicle acceleration SLOW DOWN, you will not have the 1 G acceleration available to achieve the head pressure required for engine start.  To start the SSME while the SRB is burning is a very high and dangerous risk.

3)  Existing ground start software needed to be encoded into SSME engine controller which will be a major upgrade.  This would effectively replace the existing Honeywell controller with a brand new one, which could be very costly.  How to do engine start/abort "red line" logics will need to be completely re-visited as aborting a 2nd stage engine start is a very bad idea, and making it a very bad day for the astronauts.  This issue alone will require a complete development and certification program for the SSME which is very expensive.  More importantly, it introduces a schedule risk and a technical risk that apparently NASA is not willing to take on the CLV.

4)  Safety issue.  Existing SSME has leaks that include gaseous hydrogen (GH2), oxgyen (GO2) and hydraullic fluids all at the same time.  This is not so much as an issue for the existing shuttle orbiter where they are vented to open air.  This is a serious issue when SSME is a 2nd stage engine, as the interstage is enclosed and having a mixture of GH2 and GO2 in a confined space presents a potentially explosive hazard.  The Russians use open grid interstage design but this is beyond the US launch vehicle experience.  It's another risk that NASA is apparently unwilling to take on.

5)  Eventually the CLV 2nd stage engine will be the EDS engine for the CaLV.  This will require a "re-start" capability which will be almost impossible for the SSME.  The SSME preburners burn fuel-rich mixture (MR=1) that produces steam with water vapors coated on turbine blades as well as injectors.  These water vapors will turn to ice after engine is shutdown.  It would take lots of heated helium to purge the engine for a long time, too much heliumm and too long of a purge, that can be accommodated in between first and second burns for the EDS mission.

[ADC9]

MATTBLAK - 30/3/2006  12:32 AM

Deleting the methane is a mistake they will regret later.

All because of cost, not the the fact that it's a very difficult process?

[mkirk]

Propforce - 30/3/2006  1:53 AM

4)  Safety issue.  Existing SSME has leaks that include gaseous hydrogen (GH2), oxgyen (GO2) and hydraullic fluids all at the same time.  This is not so much as an issue for the existing shuttle orbiter where they are vented to open air.  This is a serious issue when SSME is a 2nd stage engine, as the interstage is enclosed and having a mixture of GH2 and GO2 in a confined space presents a potentially explosive hazard.  The Russians use open grid interstage design but this is beyond the US launch vehicle experience.  It's another risk that NASA is apparently unwilling to take on.

Here is a picture of the notional 4 Segment/SSME configuration of the CLV prior to going to 5 Segment/J2...as you can see they had no choice but to go with the open grid.

Mark Kirkman

[Propforce]

mkirk - 30/3/2006  12:34 PM

Here is a picture of the notional 4 Segment/SSME configuration of the CLV prior to going to 5 Segment/J2...as you can see they had no choice but to go with the open grid.

Mark Kirkman

Yeah, but for some reason they switched back to the enclosed interstage shortly afterward, before switching to J-2.

[mkirk]

Propforce - 30/3/2006  3:06 PM

Yeah, but for some reason they switched back to the enclosed interstage shortly afterward, before switching to J-2.

Thanks, I didn't know that.  

I thought the change to the interstage occured after the switch to J2.  I guess it would have been doable if you ran a purge through the interstage or used a vent system similar to what we do with the orbiter's aft compartment.  Open grid for SSME makes more sense to me since the engine itself would have to run prestart purges and conditioning.

Mark Kirkman

[MATTBLAK]

ADC9 - 30/3/2006  8:49 AM

MATTBLAK - 30/3/2006  12:32 AM

Deleting the methane is a mistake they will regret later.

All because of cost, not the the fact that it's a very difficult process?

Yep; cost AND schedule.

[MartianBase]

MATTBLAK - 1/4/2006  4:09 PM

ADC9 - 30/3/2006  8:49 AM

MATTBLAK - 30/3/2006  12:32 AM

Deleting the methane is a mistake they will regret later.

All because of cost, not the the fact that it's a very difficult process?

Yep; cost AND schedule.

Do you think the Chinese will build a Methane Engine, one problem with NASA is that it seems to be insisting that it doesn't need Russians or Europeans to help its VSE. China on the other hand doesn't seem to have an problems sticking its flag into the Moon or Mars with some international help, the Chinese could be very close to a methane engine for Mars.

[Jim]

MartianBase - 2/4/2006  1:33 PM

MATTBLAK - 1/4/2006  4:09 PM

ADC9 - 30/3/2006  8:49 AM

MATTBLAK - 30/3/2006  12:32 AMDeleting the methane is a mistake they will regret later.

All because of cost, not the the fact that it's a very difficult process?

Yep; cost AND schedule.

Do you think the Chinese will build a Methane Engine, one problem with NASA is that it seems to be insisting that it doesn't need Russians or Europeans to help its VSE. China on the other hand doesn't seem to have an problems sticking its flag into the Moon or Mars with some international help, the Chinese could be very close to a methane engine for Mars.

What are you basing that on.  Just conjecture?  They are still using hypergols for their LV's, why would they go to methane?

[simonbp]

MartianBase - 2/4/2006  1:33 PM
Do you think the Chinese will build a Methane Engine, one problem with NASA is that it seems to be insisting that it doesn't need Russians or Europeans to help its VSE. China on the other hand doesn't seem to have an problems sticking its flag into the Moon or Mars with some international help, the Chinese could be very close to a methane engine for Mars.

It's not so much that developing a methane engine is difficult or very time consuming (IIRC, an RL-10 ran on methane perfectly fine a couple of years ago), but rather whether you want to got the trouble of having cryo systems to store the LOX and LCH4. The cryo system is less than you need for liquified Hydrogen, but still more than just using hypergolics. Basically, it's the design philosophy behind the move to J-2X/5-seg; if you're starting from scratch, work with what you have to build the simplest and most reliable configuration possible to do the job, and then when that works, look at upgrades...

Simon

[ADC9]

Is there no way this can happen, to be clear?