STS-131 Shuttle Ascent OMS Burn?

[Firov]

I was watching this (excellent) video showing the launch of the space shuttle Discovery on STS-131 when I noticed that, at around T+2:30, shortly after SRB separation (~5 minutes into the video) the announcer indicates that the OMS engines are also active to provide additional thrust, but this is the first I've ever heard of this. I was under the impression that the OMS engines weren't used until MECO and external tank separation, no matter if it was a direct-ascent or a two-burn ascent.

So did some shuttle missions actually necessitate firing the OMS engines during ascent? If so, why? And for how long? The announcer seemed to indicate that they'd remain firing for around 1 minute and 44 seconds but it seems like with it's already limited OMS system Delta-V budget (~300 m/s) that they'd want to hold off on using the OMS engines until they'd exhausted the main engine fuel supply.

I looked up the STS-131 payload, and the total mass came to just over 15 metric tons, which is well below the maximum payload capability of the shuttle ~24 metric tons, so I don't think unusually high payload mass is the reason here. Any ideas?

[SFJackBauer]

Its called an "OMS assist". By burning OMS fuel while the SSMEs are working, the overall mass decreases and therefore more dV is extracted from the remaining external tank propellants, while also providing a small amount of thrust. The amount of OMS burned would not affect orbital ops since, like you mentioned, the orbiter payload was below than max.

Then why they carried that extra fuel you may ask? One factor is that it was easier to just fill the OMS tanks to capacity on the ground rather than partially. Also, they would better be fully-fuelled if any emergency during ascent arised, and they arrived at a less-than-nominal orbit, for example.

[tavert]

Evidently it was first performed on STS-90: http://en.wikipedia.org/wiki/STS-90

When you have multiple propellant types, it's more efficient to use the lowest-Isp propellants before the higher-Isp propellants, or in this case use the OMS as early as it makes sense to. For missions that aren't expected to require all of the OMS' fuel capacity, and if everything goes nominally during launch, then it's probably good for another few hundred kilos of payload as compared to an otherwise identical mission that brings the excess OMS fuel back from orbit.

Edited December 20, 2013 by tavert

[Firov]

That's fascinating. Thank you both. I had no idea that they would use their OMS engines during ascent.

It still seems odd to me that they'd burn off their reserve OMS fuel unless they absolutely had to, since if something goes wrong once in orbit that's really their only effective means of getting home. All it would take is one OMS engine failing in orbit and they'd lose half their available delta-V, since, as far as I know, the OMS pods aren't linked.

Surely they weren't operating with such a razor thin margin? After all, the shuttle was rated to lift up to 24 metric tons and generally the ISS components weren't nearly that heavy.

[K^2]

Burning OMS early gives you more OMS fuel once in target orbit. So it actually increases safety margin. The better question is why they don't always use OMS assist.

[SFJackBauer]

That's fascinating. Thank you both. I had no idea that they would use their OMS engines during ascent.

It still seems odd to me that they'd burn off their reserve OMS fuel unless they absolutely had to, since if something goes wrong once in orbit that's really their only effective means of getting home. All it would take is one OMS engine failing in orbit and they'd lose half their available delta-V, since, as far as I know, the OMS pods aren't linked.

Surely they weren't operating with such a razor thin margin? After all, the shuttle was rated to lift up to 24 metric tons and generally the ISS components weren't nearly that heavy.

Well, guess what? The pods can be crossfed. Also they sometimes used only one OMS engine in orbit for small burns to reduce wear on the other engine by the restarts. So a deorbit could be done in one engine only.

Look at the Execute Packages for the STS-131, on Flight Day 2, page 15. It reports 37% of the tanks filled, after they were in a 333x255km orbit, with 107m/s availabe in the OMS. Then compare with the 15th and last day, page 11, listing the tank levels before and after the deorbit burn.

Don't miss the Sunday Funnies too on day 15th, hillarious stuff!

[rdfox]

The OMS and RCS could crossfeed, too, and even if both OMS engines failed, they could deorbit using the RCS instead.

The big reason that they used OMS Assist on ISS launches is that the rated maximum payload for the Shuttle that you see is the maximum payload *to a 28 degree inclination orbit*... in other words, launching due east from the Cape. Due to the location and alignment of the Russian launch complexes and missile ranges, the ISS is in a 67-degree inclination orbit. This requires launching in a direction well NORTH of east from the Cape, costing quite a bit of extra delta-V to reach the same orbit due to the loss of assist from the Earth's rotation. (Think about the difference in d-V required to launch on a heading of 45 degrees versus 90 degrees in KSP...)

[Pappolus]

On ‎20‎/‎12‎/‎2013 at 4:34 AM, rdfox said:

The OMS and RCS could crossfeed, too, and even if both OMS engines failed, they could deorbit using the RCS instead.

The big reason that they used OMS Assist on ISS launches is that the rated maximum payload for the Shuttle that you see is the maximum payload *to a 28 degree inclination orbit*... in other words, launching due east from the Cape. Due to the location and alignment of the Russian launch complexes and missile ranges, the ISS is in a 67-degree inclination orbit. This requires launching in a direction well NORTH of east from the Cape, costing quite a bit of extra delta-V to reach the same orbit due to the loss of assist from the Earth's rotation. (Think about the difference in d-V required to launch on a heading of 45 degrees versus 90 degrees in KSP...)

Actually, the ISS is in a 57-degree inclination orbit, that is a compromise between the location of KSC (Kennedy, not Kerbal ) and Baikonur

[Red Iron Crown]

This is a bit of an old thread to dig up to correct a typo, no?

[Ten Key]

Now that is fascinating-- I never knew they did that with the OMS.

[Pappolus]

On ‎07‎/‎11‎/‎2016 at 5:42 PM, Red Iron Crown said:

This is a bit of an old thread to dig up to correct a typo, no?

I don't see why... the thread might be old, but the ISS is still flying, so it fits. And besides, you are not forced to read it if you don't like it. Bye

[Starhawk]

Removed a couple of posts.

Please don't accuse each other of violating Wheaton's Law.  It violates Wheaton's Law!

Happy landings!

[magnemoe]

On 20.12.2013 at 2:56 AM, K^2 said:

Burning OMS early gives you more OMS fuel once in target orbit. So it actually increases safety margin. The better question is why they don't always use OMS assist.

Because you might need it later. Think of monoprop in KSP, its smart to have more than you need, some docking are harder than others, however if you find yourself in an narrow dV budget you might want to use rcs in addition to main engine to both increase trust and to lighten the ship, yes its reduced your mono prop reserves, but not so much as having to use it for circulation 
 

[kurja]

Looking only at delta v, it's always beneficial to spend the lower isp propellant first.

[KerikBalm]

that only applies when there is no staging event

10 units of monoprop in the command pod can give you significant dV with nothing else attached to the pod (ie after decoupling), whereas it won't reduce the dV much of the stage before it because the mass is rather small compared to the rest of the dry mass.

So... yes it has lower Isp, but if that tank masses too much, its better to spend dV after dropping that tank mass.

[kurja]

10 hours ago, KerikBalm said:

that only applies when there is no staging event

10 units of monoprop in the command pod can give you significant dV with nothing else attached to the pod (ie after decoupling), whereas it won't reduce the dV much of the stage before it because the mass is rather small compared to the rest of the dry mass.

So... yes it has lower Isp, but if that tank masses too much, its better to spend dV after dropping that tank mass.

Staging to a final monoprop stage ofc changes things, but if that's the case, in KSP you get more dv in that stage (AND less weight!) with liquid fuel and a pair of spider engines. Just did some back of the envelope calculations and I ended up with a ~60 m/s advantage using spiders versus 2 of 4 way rcs consuming ~300kg of propellant (smallest inline monoprop tank or a toroidal lf/ox tank) added to a 2 ton pod.

Sometimes one wants to have monoprop, for docking maybe, but strictly looking at delta v, I can't think of a case where monoprop would be better? Could you give an example? Hmm if you only have rcs fuel in a pod so you get that tank mass "for free"...?

Edit - with adding only rcs blocks the added weight is not much, but still, the dv to be had from the 10 units of fuel in a small pod only amounts to ~40m/s for a two ton craft. Pretty sure the total dv will be higher if omitting rcs and monoprop, than by adding rcs to use those 10 units in a capsule.

Edited November 11, 2016 by kurja