Depends on when your oath is. You will know that only after your interview. You are still Indian citizen till the oath.
After the oath, travel with Indian passport is strongly not recommeneded to avoid future problems. You Indian passport technically becomes invalid the moment you take the US oath.
You can ask at your interview about any paperwork you may need to complete if are travelling between your interview and oath (most likely just have to answer a question in the back of your oath letter).
Provided your oath is on same day as interview, here are my thoughts -
Travel with original citizenship certificate is allowed (with a lot of security hassles to traveller) by US authorities but very strongly discouraged, since other country (in this case Germany) may not allow you to enter without passport. The example we got at my oath ceremony was for travel to Canada or Mexico, but that was because travel between US and these countries has some special arrangements.
Expedited processing for US passport still takes about 2 weeks.
A Tip: you can opt at the interview to NOT take the oath same day. This will probably add a month or two delay before you can take the oath, but then you will still be an Indian citizen with a passport at the time of your travel.
OR, you can try to postpone your travel
Hope this information helps.